PRACTICAL WIRELESS, June 15th, 1940.

FREQUENCY

A

NEWNES

PUBLICATION

&ma

E J.CAMMVol. 16. No. 404.

page

EVERY

WEDNESDAYJune 15th, 1940.

* PRACTICAL. TtLEVI I 0) N *

Capacity Tester

Frequency Response

Thermion'sCommentary

Improving the S.W.Set

From Crystal to Valve

Remote ControlSystem

Practical Hints

Readers' Letters Choosing a Choke

TESTED IRELESS CIRCUITS A Practical BookBy F. J. CAMM

Modern circuits of every type. Diagrams and instructions for assembling and wiring. including Circuitsfor Battery and Mains -operated Receivers, Adaptors, Units, Portables, Short-wave Receivers, All -waveReceivers, Amplifiers, and a Room -to -room Communicator, This is a complete guide to the construction ofall types of receivers, from crystal sets to superhets, from battery sets to mains sets, from all -wave toshort-wave sets, from amplifiers to a room -to -room communicator. All of the circuits described havebeen built and tested, and in many cases wiring diagrams have been included. NETFrom all booksellers, or by post 3/10 from the Publisher, GEORGE NEWNES, LTD. (Book Dept), Tower House, Southampton Street, Strand, London, W.C.2.

3/6ADVT.

PRACTICAL WIRELESS June 15th, 1940

Practical Wireless

BLUEPRINT SERVICEPRACTICAL WIRELESS No. of

Date of issue. Bidep2iaa.CRYSTAL SETS I

Blueprints, 6d. each.1937 Crystal Receiver . .. - PW71The " Junior" Crystal Set. .. 27.8.38 rw94

STRAIGHT SETS. Battery Operated.One -valve : Blueprints, is. each.Ail -Wave Unipen (Pentode) ... - PW31ABeginners' One-valver .. .. 19.2.38 PW85The " Pyramid " One-valver (11F

l'en) .. .. .. .. 27.8.38 PW93

Two -valve : Blueprint, is.The Signet Two (1) & LF) . 24.9.38 PW 76

Three -valve : Blueprints, Is. each.Selectone Battery Three (1), 2 LF

(Trans))Sixty Shilling Three (D, 2 LF

(RC & Trans)) '..Leader Three (SO, D, Pow) ..Summit Three (HF Pen, D, Pen)All Pentode Three (HF Pen, 1)

(Pen), Pen) ..Hall -Mark Three (SG, D, Pow)Hall -Mark Cadet (D, LF, Pen (RC))F. J. Camm's Silver Souvenir (HI'

Pen, D (Pen), Pen) (All -WaveThrep)

Cameo Midget Three (D, 2 LE(Trans)) ..

1936 Sonotone Three -Four (HFPen, HF POD, Westector, Pen)

Battery All -Wave Three (D, 2 LF(RC))

The Monitor (HF Pen. D, Pen) ..The Tutor Three (Ill" Pen, D. Pen)The Centaur Three (SG, 1). P)F. 3. Canim's Record All -Wave

Three (HF Pen, D, Pell) .The "-Colt" All -Wave Three (1),

2 LI? (RC & Trans))The " Rapide " Straight 3 (II,

2 LF (11C & Trans)) ,

F. J. Caitlin's Oracle All -WareThree (HF, Det., Pen)

1938 " Triband " All -Wave Three(BF Pen, D, Pen) ..

F. J. Camm's " Sprite" Three(HF Pen, D, Tet)

The " hurricane" All -Wave Three(SO, D (Pen), Pen)

F. J. Camm's " Push -Button"Three (HF Pen, D (Pen), Tet)

29.5.37

16.3.35

13.4.35

21.3.3614.8.37

31.10.36

18.2.39

4.12.37

28.8.37

22.1.38

26.3.38

30.4.38

3.9.38

Four -valve : Blueprints, is. each.Sonotone Four (SO, D, LI, 1') -. 1.5.37Fury Four (2 SG, Peu) .. 8.5.37Beta Universal Four (SG, D, LF,

CI. B)Nucleon Class B Four (SG, D

(5(1), LF, CI. B)Fury Four Super (SG, SG, D, Pen)Battery Hall -Mark 4 (111' Pen,

D, Push -Pull)F. J. Canna's " Limit" All -Wave

Four (HE Pen, D, LF, P) - 26.9.36"Acme " All -Wave 4 (HF Pen, D

(Pen), LF, Cl. B) 1-2.2.38The " Admiral" Four (}IF Pen,

liF Pen, D, Pen (RC)) .. 3.9.38

Mains OperatedTwo -valve: Blueprints, is. each.A.C. Twin (I) (Pen), Pen)A.C.-D.C. Two (SG, Pow)Seketone A.C. Radiogram Two

(D, Pow)

Three -valve Blueprints, is. each.Double -Diode -Triode Three (HF

Pen, DDT, Pen) .

D.C. Ace (SG, D, Pen) ..A.C. Three (SG, I), Pen)A.C. Leader (HF Pen, D, Pow)D.C. Premier (BF Pen, D, Pen) ..Unique (HF Pen, D (Pen), Pen) ..Armada Mains Three (11F Pen, D,

Pen)F. J. Camm's A.C. All -Wave Silver

SOUVODir Three (11F Pen, D, Pen)" All -Wave " A.C. Three (D, 2

LF (13C))..A.C. 1031 Sonotone (HF Pen, HF

Pen, Westector, Pen)Mains Record All -Wave 3 (HF

Pen, D, Pen) .. -four -valve : Blueprints, ls. each.A.C. Fury Four(SO, SO, D; Pen) . .

A.C. Fury Four Super (SG, SG, D,Pen)

A.C. Hall -Mark (HF Pen, D,Push -Pull) ..

Universal Hall -Mark (11E Pen, D,Push -Pull) ..

7.1.39

PW10

PW34APW35PW37

PW39PW41PW 48

PW49

PW51

PW53

PW55P.PW62PW434

PW69

PW72

PW82

PW78

PW84

PW87

PW89

PW92

PW4PW11

PW17

PW3413PW34C

PW46

PW67

PW83

PW90

PW18PW31

PW19

PW23

PW2PW35CPW35BPW3GA

PW38

PW50

PW54

PW56

PW70

PW20

PW34D

PW45

PW47

SUPERHETS.Battery Sets : Blueprints, is. each.'

Superhet (Three -valve) -F. J. Caitlin's 2 -waive Super -bet

Mains Sets ; Blueprints, Is. each.AU. Supernet (Tliree-valve) P W43D.C_ £5 Super -het (Tlinv-valve) PW42Universal £5 Superhet <Timm -

valve)F. J. Camel's A.C. Superhet 4E. 3. Canun's Universal £4 Super-

Int 4" Qualitone " Universal Four .. 161.37

Four -valve Double -sided Blueprint, Is. 60.Push Button 4, Battery Model _ 122. ,Push Bottcm 4, _CC, Mains Model .10.38 3'W95

SHORT-WAVE SETS. Battery Operated.One -valve : Blueprint, is.Simple S.W. Otie-waiver .. 23.12.39 PW88

Two -valve : Blueprints, is. each.Midget Short-wave Two (D, Pen)The " Fleet " Short-wave Two

(D (11F Pen), Pen) .. 27.8.38

5.6.37 PW40PW52

Three -valve : Blueprints, is. each.Experimenter's Short-wave Three

(SO, D, Pow) ..The Prefect 3 (D,-2 LF (RC and

Trans)) .. ;The Band -Spread S.W. Three

(IIF Pen, 1) (Pen), Pen) .. 1.10.38

PORTABLES.Three -valve : Blueprints, ls. each.F. J. (.411117D'S ELF Three -valve

Portable (11F Pen, 1), Pete) .

Parvo Flyweight Midget Portab.le(SG, D, Pen) .. .. 3.6.39

Four -valve : Blueprint, is." Imp "'Portable 4 (D, LF, LE

(Pen))

MISCELLANEOUS.Blueprint, is.S.W. Converter -Adapter (1 valve) -

PW4-4PW59

PW60PW 73

PW38A

PW91

PW30A

PW63

PWC,8

PW8.5

PW77

PW86

PW48A

AMATEUR WIRELESS AND WIRELESS MAGAZINECRYSTAL SETS.

Blueprints, 6d. each.Four -station Crystal Set .. 23.7.38 AW4271934 Crystal Set . - AW444150 -mile Crystal Set .. AW450

STRAIGHT SETS. Battery Operated.One -valve : Blueprint, is.B.B.C. Special One-valver

Two -valve : Blueprints, Is. each.Melody Ranger Two (D, -Trans)Full -volume Two (SO det, Pen) ..Lucerne Minor (D. Pen)A Modern TWO-TiliVer

Three -valve : Blueprints, Is. each.£5 5s. S.G.3 (SC, B. Trims)Lucerne Ranger (SO, li, Tres)£5 5s. Three : De Luxe Version

(SG, D, Trans) .. . 19.5.34Lucerne Straight Three (B,

Trans) . .

Transportable Three (SG, D, Pen)Simple -Tune Three (8G, D,Pen).. June '33-Economy-Pentode Three (SO, D,

Pen) .. .. Oct. '33" WM." 1934 Standard Three

(SG, D, Pen) ..£3 3s. Three (SG, D, Trans) .. Mar. '341935 £6 6s. Battery Three (SG,

D, Pen) .. -PTY Three (Pen, D, Pen) .. -Certainty Three (SG, D, Pen) ., -Minitube Three (SO, D, Trans) .. Oct. '35All -Wave Winning Three (SG, D,

Four -valve : Blueprints, Is. 6d. each.65s. Four (SO, D, RC, Trans) .. -2HF Four (2 SO, D, Pen) .. -Self-contained Four (SO, D, LF,

Class B) '33Lucerne Straight

......Aug!iour(SO, D,

LE, Trans) .. . -.£5 5s. Battery Four (13F,D,2 LF) Feb. '35The H.R. Four (SO, SG, D, Pen) -The Auto Straight Four (HF Pen,

Pen, DDT, Pen) .. .. Apr. '30

Five -valve : Blueprints, is. 6d. each.Super -quality Five (2 111,, D, ItC,

Trans) .. . ,

Class B Quadradyne (2 SG, D, LF,Class B)

New Class B Five (2 SG, D, LF,Class B)

AW3S7

AW388AW392AW426WM409

AW412AW422

AW4:35

AW437'W M271W M327

WM337

WM351W M354

WM371W16389W M393W M306

WM400

. These Blueprints are drawn full size.Ooptes of appropriate issues containing descrip-

tions of these nets rasa) in some cases be supplied atIbe lollowing prices which are additional to the cost

-..of The Blueprint. A dash before the Blueprint NuMberindicates that the issue is out of print.

Practical Wireless (issues dated prior to June1st, 1940) 4d. Post Paid

, ' (Issues dated June 1st,1940, and after)

5d. Post PaidAsimteur Wireless ... 40.Wirefess Magazine ... ..._ 1/4The index letters which precede the BluepAnt

Number indicate the periodical in which the des-criptive appears : Thus P.W. refers to PRACTICALWIRELESS. &W. to Amateur Wireless, W.M. to Wire-less Magazine.Send (preferably) a postal order to cover the cost

of the blueprint. and the issue (stamps over 6d.unaoceptable) to PRACTICAL WIRELESS BlueprintSlept-.. George Newnes, Ltd.: Tower House. South-apipton Street, Strand, W.C.2.

Mains Operated.Twuvalve ithteptists, is. each.Clonsoeleetrie Two (D, Pen) A.C.Economy A.C. TWO (D, Trans) A.C.unicorn A.C.-D.C. Two (D, Pen) -Three -valve Blueprints, ls. each.Home Lover's New Ali -Electric

Three (SG, D, Trans) A.C.Idantovani A.C. 'Three (HF POD,

I), Pen) -£15 155. 1936 A.C. RadiogMm

(HF, e, Pen) .. Jail: '36Four -valve : Blueprints, Is. 6d. each. -All Metal Four (2 SC, 1), Pen) .. Juty '33Harris' Jubilee Radiogram (HP

Pen, D, LF, F) May '35SUPERHETS.

Battery -Sets Blueprints, is. '6d. each.Modern Super Senior .. -'Varsity Emir Oct. '35The Request All -Waver .. June '361935 Super -Five Battery (Superhet) -

AW403W16286W1,1304

AW383

WM374

WM4O1

WM320

WM386

WM375WM305WM407WM370

Mains Sets :' Blueprints, is. 6d. each.lleptode Super Three A.C. .. May '34 WM359",W.M." RadiogramSuper A.C.. - WM366

PORTABLES.Four -valve : Blueprints, is. 6d. each.Holiday Portable (SG, D, LE,

Class B) ' . - AW393Family Portable (11F D, RC, -

Trans) A\}447A447Two HE Portable (2 SG, B,

QP21)Tyers Portable (SG,D, 2 Trans) WM267

SHORT-WAVE SETS. Battery Operated.One -valve : Blueprints, is. each.S.W. One-valver for America .. 15,10.38 AW429Rome Short -WaverTwo -valve : Blueprints, Is. each.

' - AW452

Ultra -Short Battery Two (SG, det,Pen) .. .. Feb: '30 NVA1402

Three --valve : Blueprints, is. each.4.W440Home-made Coil Two_ (D, -Pen) ,. -

World -ranger Short-wave 3 (D,RC, Trans) .. - AW355

Experhnenter's 5 -metre Set (I),:Trans, Super-regen) .. 30.6.34 AW438

The Carrier Short -waver (SG,D,P) July '35 WM300Four -valve : Blueprints, 1s. 6d. each.A.W. Short-wave World-beater

(HE Pen, D, RC, Trans) -. - AW430Empire Short (SG, D,t -

Trans) .. WM313Standard Four -valve Short -waver

..(SO, I), LF, P) 22.7.39 WM383Superhet : Blueprint, is. 6d. -

Simplified Short-wave Super .. Nov. '35 WM397

Mains Operated.Two -valve : Blueprints, is. each.Two -valve Mains Short-waver... (D.:

Pen) A.C. 13.1.40" W.M. Long -wave Converter-Three -valve : Blueprint, ls. -Ernigrator (SO, D, Pen) A.C.four -valve : Blueprint, Is. 6d.Standard four -valve A.C. Short -

waver (SO, D, RC, Trans) - WM301

MISCELLANEOUS.S.W. One -valve Converter (Price

AW370 Enthusiast's Power Amplifier (1/6) --, AW421 Listener's 5 -watt Q.C. Amplifier

(1/6) .. . .. -WM331 Radio -Unit (2v) forr. W11,1.392 (If-) Nor. '35

Harris Electrogram battery amwm3.50 Miller (1/-) .. .. .. -WM381 De Luxe Concert A.C. Electro-WM384 gram (1/-) . :. .. Mar. '36

New style Short-wave -AdapterWM404 GM . .. .. -

Trickle Charger (Oh.) .. -Short-wave Adapter (1/-) .

Superhet Converter (1/-).. _ -B.L.D.L.C. Short-wave Converter

(1/-) -' .. .. . - May '36Wilson Tone Master (1/-) .. June '36The W.M. A.C. Short-wave Con-

verter (1/..) .. . -

WM320

WM344

GN M340

AW453WM380 -

WM352

AW32t)WM387

WM392WM398

WM399

WM403

WM388AW462AW456AW457

WM405WM406

ViT11408

HE fourth in the series of Maurice

produce and it is expected that Celia Liptonwill again delight with her singing of" Over the Rainbow," in the -part ofDorothy, the little girl who is whiskedaway on the crest of a tornado, to thewonderful land of Oz.

My Day's WorkSPEAKERS in the series entitled " My

Day's Work," to be broadcast on June15th, will be Albert Jennens, who willdescribe his job in a big glass works in

North-West Worcestershire ; WalterLevick, of Tamworth. 410 will tell howhe keeps the " roads clear in a coalmine; an interview with George Jones(the Warwickshire Miners' Secretary),and H. Van Bylevelt, manager of acycle depot in Birmingham, who willgive some hints to those who, owingto the petrol control, are riding abicycle for the first time.

"Rhapsody in Black"

"THE Music Goes Round-And

Round " came to the end of amost successful run a few weeks ago,and listeners to afternoon programmeshave probably missed this weekly spotof sparkle and sophistication. How-ever, Roy Speer, its producer, andJames'Dyrenforth, its -new -style com-pere, have put their heads togetherand already announce a successorcalled " Rhapsody in Black." Thisseries, -which begins on June 19th, willbring to the microphone the Negro inall his moods-from the splendid sim-plicity and intensity of the spiritualto the amazing rhythms and othermanifestations of Harlem and NewOrleans.

modern portable field radio transmitter and receiver. Notethe aerial -earth arrangement and battery suppltes

June 15th, 1940 PRACTICAL WIRELESS 269

r9]PRACTICAL TELEVISION *

EVERY WEDNESDAY

Vol. XVI. No. 404. June 1510, 1940.

EDITED 8 Y

F. J.CAMMI Staff:W. J. DELANEY, FRANK PRESTON,

H. J. BARTON CHAPPLE, B.Sc.

ROUND THE WORLD OF WIRELESSChoosing a Component

MANY beginners are confused when-looking through radio catalogues

by the wide ranges offered in some singlelines. Coils may be all< classed under oneheading and all modern coils are de-signed to cover a given range, beingwound to inductance values set out bythe Component Manufacturers' Feder-ation. L.F. transformers, however,ore available with various ratios andinductance values and these confusesome constructors. Similarly, there aremany different patterns of H.F. choke,and this is a most important item insome circuits. Accordingly, in thisissue, and also in the next, we endeavourto explain the functions of chokes invarious circuits and how to determinethe type of component for separatepurposes. Resistances also have variousratings and this again offers some con-fusion. The values are, however, fixedaccording to the voltage to be droppedor the purpose for which they are re-quired and the wattage may easily becalculated. It is the same with mostother components, but the choke isundoubtedly the chief stumbling blockand we think the various difficultieswill be easily overcome when the factsare properly understood.

Thirsty Work

TBrown's " Thirsty Work " pro-grammes is to be broadcast to the L.,___

Forces on June 14th. This time, Brownhas taken the B.B.C.'s mobile recording liunit to a village on the borders ofNorthamptonshire and Rutlandshire where .. --- - - - - - . - - -- - - - - - - - he found some excellent singing in the local ; Editorial and Advertisement Offices:inn. This programme will be slightly I " Practical Wireless," George Newnes, Ltd.different from the three previous ones in so ; Tower House, Southampton Street, Strand,'faritwill bepurelylocalhe ... W.c.2. 'Phone : Temple Bar 4363.

Telegrams : Newnes, Rand, London.songs listeners with the Forces will hear i Registered at the G.P.O. for transmission byare of a more general character and one i Canadian Magazine Post.number includes animal noises. The artists ; The Editor will be pleased to consider articles of ain the programme will include a gamekeeper, t practical nature suitable for pablwatton ina forester and farm -labourers. ; PRACTICAL WIRELESS. Sur 11 articles should be

loineamesiade oftle.4etrheosender. Whilst

should

ONE of the richest scores in film' ; tTpinusieli

Second Film Festival - 7 t Editor does not holdse18'steavnetrijedefftn

bef ,trFmade ttosi returnler

years .1 enclosed. Allstamped

intended'ten;opre the.ismusicals of the last fewbelongs to the recently released The ; Editor should be addressed : The Editor, PRACTICAL

. . WIRELESS, George Nwenes, Ltd., Tower House,-Wizard of Oz." It is possible that Jack Southampton Street, Strand, W .C.2.Beaver's radio version of the score was even i Owing to the rapid progress in the design ofmore colourful than the original ; certainly ;

wireless apparatus and to aur efforts to keep ourreaders in with the latest developments, we giveit contributed considerably to the success no y that apparatus described in our

of the broadcast. The revival of the radio i - columns is not the subject of letters patent.version of this odd but successful adapta- ; copyright in all drawitfas, photographs and

i, articles published in PRAcTic AL WIRELESS istion of a classic American fairy tale will.be i specifically reserved throughout the countries signs -broadcast on June .17th,, within three t tory to the Berne Convention and the U.S.A..months of its first production, and will be t Reproductions or imitations of any of these are

the ore expressly forbidden. PRAXTICAL W IRE -fsecond in the impressive list that makes iTEES.

incorporates " Amateur Wireless."

Maurice Winnick and HisBand

MAURICE WINNICK has just finishedhis successful season at the Dor-

chester, where he has won high praise from theconnoisseurs on his particular rendering ofswing ; and on June 16th he will broadcastas the band of the week. This is Winnick'sfirst visit to the particular B.B.C. provincialstudio froth which he is to broadcast.,though probably not his first engagementin the city itself, as he is a much travelledman, who had toured most -of the music -halls of Britain with his own band beforehe was twenty-one years old. Perhaps hedeveloped his taste for travel in his firstjob in a cinema orchestra, and made up hismind as he accompanied with appropriatemusic the exotic scenes of the silver screenbefore him, that he would visit those sceneshimself. Anyway, Maurice took on the jobas band -leader on one luxury liner afteranother, and his job took him three timesround the world. He studied dance musicin New York, and learned there to play theclarinet and the saxophone ; but the violinis the instrument which he prefers, and withthat he leads the first-class band he is con -up " Film Festival." Douglas Mood' will ducting to -day.

270 PRACTICAL WIRELESS June 15th, 1940

5Mfd

Calgseity

00/ JMfd

Vanap Corn:tenser-

7P.ehhonLs

iothitery

,jr 0/ 81133e 7'

/n5 (a teaft-lerarfnae

Battery loWake4a

ana'greaeCon

Break'

Fig. 1.-Wiring diagram of the simple capacity tester.

THE necessity for some simple formof apparatus for capacity measure-ment, such as that described

here, must have been felt at one time oranother by every wireless enthusiast.The time and labour of fault location inservice work on commercial receivers maybe reduced with such gear. To the experi-menter, of course, there are innumerableways in which the ability to measure capa-cities will prove of interest and value.

The bridge described extremelyeffective, employing as far as possibleparts which constructors are likely tohave on hand, or which, at any rate, may bepurchased with very little outlay.

How It WorksThe action of the instrument is very

easy to understand. If an alternating orintermittent potential such as that obtainedtrona a buzzer is applied across the pointsA and B of the bridge circuit, shown inFig. 2, the current may be considered to taketwo paths-one through the point.0 and thetwo condensers in this arm, and the otherthrough D and the two condensers in thisarm. Now it is possible by so arranging thevalues of the four condensers to obtain acondition when the potential at C is thesame as that at D.

In this case no current will flow throughthe telephones which are connected acrossthese points, and no note will be heard.This is actually secured when the capacityAC bears the same ratio to the capacityCB as the capacity AD does to DB. Thus,if AC is .0003 mfd. and CB three times asmuch, AD could be practically any value,and so long as DB was three times thevalue of AD no note would be heard. Bymaking the capacity in AC variable it ispossible to calculate any unknown capacityconnected in AD, provided AC has a suit-ably calibrated scale and the unknowncondenser is within the working range.The range of measurement available bytuning AC from zero to maximum willdepend upon the choice of the value of DB.

The values used here have been chosenby calculation and experiment to give tworanges of capacity measurement most use-ful for receiver test work while using thecondensers which are most likely to be onhand. So long as the circuit arrangementand capacity values are adhered to, itmatters very little as to the actual layoutor form of the unit. The variable capacity

A Simple Capacity TesterA Handy Bridge for Measuring the Values of

Condensers and Other Capacitiesmust have a maximum of .001 mfd. to givea sufficiently wide range, and in the case ofthe original model two .0005 mfd. micadielectric ganged condensers were used,but one of the old straight-line capacitytype of .001 mfd. would be equally suitable.

These are very often to be hadfrom wireless stores.

Use a " mosquito " or high -note buzzer for preference, andone which is not too noisymechanically, otherwise the notein the earphones will be drownedby the vibration of the armature.

ConstructionWiring and construction should

present no difficulties, sincethere is surprisingly little com-

plication about the unit. It should bepossible to assemble and complete thewhole job in an hour or two. When thebridge is ready for work the scale must, ofcourse, be calibrated on both ranges, and

Fig. 2.-Theoretical circuit

while this presents no difficulties, suitablefixed condensers of marked capacity mustbe available. For the first range two orthree or more condensers between .0001

diagram.

mfd. and .004 mfd. are required. Theseshould be connected in tun' to the " capa-city test " terminals, starting with thesmallest capacity and increasing in steps.Switch on the buzzer and set the single -pole doujale-throw switch for the -right range(the .001 mfd. in circuit for the lower range,and the .5 mfd. for the higher), then tunethe .001 mfd. variable for minimum signals.

Each' zero position obtained should bemarked accordingly. For the second rangevalues between .1 mfd. and 2 mfd. arerequired. It should be noted that withthree or four condensers practically thewhole of the range may be covered byseries and parallel connections. Forexample, a .0002 mfd. and a .0003 mfd.will give readings for the individual values,and also for .0005. mfd. when joined inparallel. Two .5 mfd. condensers willgive .25; .5 and 1 mfd. readings, and so on.

The scale may be calibrated directly,that is, the actual capacity values writtenon the scale, or, alternatively, a scale

divided intodegrees can beused and thereadings plottedin the form ofa graph. Thefirst method issomewhat sim-pler and quickerto read, whilethe secondmethod enablesintermediatevalues t o b eestimatedfrom the curvesplotted withthree or fourpoints. Wherethe first methodis used it is agood idea tomake the poin-ter doubleended, in whichcase one half of

the circle may be used for the lowerrange, and the opposite half forthe higher range, thus avoiding confusionbetween the two sets of figures.

LIIIM.11110.1111=011.10111.110.0.111.11..041104110411.110=0.110MNIMNOMIAMMAIIII. 4141111.1.1,4104

A COMPLETE LIBRARY OF STANDARD WORKSBy

r! PRACTICAL WIRELESSENF.CYJCLOAPRIAMEDIA 7/6, by post 8/-.EVERYMAN'S WIRELESSBOOI., by post 5/6.SIXTY TESTED WIRELESSCIRCUITS/36/6,by post t3/10.COILS, CHOKES and TRANSFORMERS 3/6, bypPRACTICAL WIRELESS SERVICE MANUAL 6/-o,

post 3/10.post 6/6.

I WORKSHOP CALCULATIONS, TABLES & FORMUL,4E3/6, by post, 3/10.

NEWNES' SHORT-WAVE MANUAL 5/-, by post 5/4.THE HOME MECHANIC ENCYCLOPAEDIA3/6, 5/,bi postt 43/-1

.WIRELESS TRANSMISSION FOR AMATEURS

OF METALS AND THEIR ALLOYS )57Ros / 0pos 54.

1 PRACTICAL MECHANICS HANDBOOK 6/., byby

postt 6/6.LAl) obtainable from or through Newsagents or from Geo. Nelms. Ltd., Tower House, Southampton St, Strand, W.C.2.

1

I

June 15th, 1940

For The Beginner

PRACTICAL WIRELESS 271

CHOOSING A CHOKE 1Details of Various Chokes, and Their Importance in the Circuit

AMONG the various types of componentswhich go to make up a radio receiver,there is one class which seems to be

rather neglected-namely, chokes. Thismay be due in part to the somewhat insigni-ficant form of its diagrammatic representa-tion, and -in part to the fact that in manycases-though not in all-the exact valueof its electrical properties is not so critical,so far as circuit efficiency is concerned, asthose of, say, a tuning coil or a variablecondenser.

But, however this may be, chokes of onetype or another do play rather importantparts in the receiving equipment of to -day,and when it is desired to purchase one it iswell worth choosing a type which is in everyway suitable to the job in hand and likelyto give long and satisfactory service.

Functions of a ChokeIn order to be able to make a wise

selection, however, it is necessary to under-stand exactly what a choke is, and what areits functions in a circuit, as well as thedifferent kinds of chokes, which have beenevolved for different purposes. To beginwith, then, a choke is, essentially, a coil ofwire, and its principal property, on accountof which it finds application in radiocircuits, is impedance. This at once callsfor further explanation.

You all know that when a direct current ispassed through any piece of apparatus, thevalue of the current flowing is limited bywhat is known as the resistance of theapparatus, resistance being the opposition

11 which the apparatus offers to the flow ofcurrent. If, instead of passing a directcurrent through the apparatus, we apply analternating current, the apparent resistancemay, or may not, be the same as when adirect current was applied. If the apparatus -consists of or contains a coil of any kind,the apparent resistance to alternatingcurrent will be much greater. In fact, it ispossible to design a coil which has a verysmall resistance to direct -current flow, buta very large apparent resistance to thepassage of an alternating current-andsuch a coil is called a choke.

ImpedanceNow why should a coil offer a higher

opposition to alternating current than toL direct current ? The answer is, because it

possesses the property of inductance. Asthe alternating current grows from itszero value to its maximum value, a magneticfield is built up in the coil and its neigh -

p, bourhood, and the growth of the magneticP field within the coil induces another electro-

motive force in the coil, in opposition to thatoriginally applied, and thus tending toprevent the original current from flowing.Similarly, when the alternating current isdying away, a back " E.M.F." is self -

6 induced, tending to maintain the flow.The coil thus presents a different form ofopposition from that due to pure resistance,although its effect is precisely similar,

Li that is to say, it limits the value of thecurrent. This opposition is termed " im-pedance," and it is measured in ohms in thesame way as resistance.

) One point must be made clear-every

choke has, in addition to its impedance,which is only operative on alternatingcurrent-a certain amount of resistance,whiCh is effective with both direct andalternating current. The resistance, apartfrom any increase owing to high -frequency "effects, is unvarying in value, and dependsentirely, upon the length, diameter andmaterial of the wire. The impedance, on

Fig. 1. -T heposition of thechoke in a stan-dard H.F. stage.

H.F. Valve

the other hand, is not constant in value-itvaries according to the frequency of thealternating current, being higher at highfrequencies than at low frequencies. This isbecause the " back E.M.F." depends uponthe rate at which the magnetic field changes,and the rate of change is, of course, greaterwhen the frequency is higher.

It is for this reason that a choke shouldnever be specified as a choke of so manyohms impedance, because although it ispossible to measure the impedance, it isnecessary to state at what frequency themeasurement is made. It is customary,therefore, to specify a choke as of so manyhenries or microhenries inductance, for,knowing the inductance, it is possible tocalculate the impedance at 4 given fre-

HT+

H.F orDetector

valve \Reaction -

circuit

Fig. 2. - AperiodicH.F. coupling includesa choke as shown here.

Fig. 3.-The stan-dard reaction H. F.

choke.

quency. This is, however, seldom neces-sary on the part of the constructor, as inmost set designs the correct inductance isquoted.

H.F. ChokesNow let us see in what ways the special

properties of chokes are employed in radiocircuits. Chokes are used for variouspurposes in both the radio -frequency and.low -frequency portions of receivers, as wellas in power supply units. We will begin

with high -frequency chokes. Their appli-cations are many, but in all cases they areused primarily to " choke back " or blockthe passage of high -frequency currents-hence their name. For example, a choke isfrequently inserted in the anode circuit of .ahigh -frequency valve, between the anodeand the H.T.-I- terminal (Fig. 1), its objectbeing to permit the flow of the mean anodecurrent, but to oppose the flow of the radio -frequency variations, which are by-passedvia the coupling condenser to the tunedgrid circuit of the detector valve. Of course,it is not absolutely necessary to use choke -fed tuned -grid coupling-the older tuned -anode circuit is just as effective, but tuned -grid coupling has the advar tage that themoving plates of the tuning ondenser maybe earthed because the coupling condenserisolates the tuned circuit from the H.T.supply.

Then a high -frequency choke is oftenused in the anode circuit of a high -frequencyvalve without a tuned -grid coupling, insuch sets as portables (see Fig. 2). Itsaction is very similar to the first applicationexcept that the extra amplification obtain-able with the tuned circuit is not achieved.A third use for a high -frequency choke is inthe anode circuit of a detector valve, asindicated in Fig. 3. Here its function is topass the direct current component of theanode current and also the low -frequencymodulation, at the same time, due to itshigh impedance at radio frequency, chokingback the radio -frequency component whichis thus diverted through the reaction coil.

Special H.F. ChokesIt is clear that chokes for any of these

purposes should have as high an impedanceas possible at the frequencies at which theywill be operated. As the range of fre-quencies to be covered in radio reception isvery wide, it has been found impossible todesign one type of choke which can beused indiscriminately on all frequencies.There have, therefore, been developed whatmay be termed " general purpose " high -frequency chokes, suitable for use on eitherthe medium or long broadcast bands. Theseare the chokes usually specified in normalbroadcast receivers. For short-wave work-ing, special short-wave chokes are marketed.

On the other hand, for use in superhetero-dyne receivers, on the intermediate -frequency side, owing to the lower frequency,it is necessary to employ chokes of higherinductance than for ordinary straightbroadcast receivers. For use in the anodecircuit of a detector valve as in Fig. 3, achoke of the standard type is correct.

The first point to make certain whenchoosing a high -frequency choke, therefore,is that it is of a type suitable for thefrequency upon which it will be used-orrather the band of frequencies. This willbe clearly stated by the maker, and you canhardly go wrong on this score if you tellyour dealer for what purpose you requireto use the choke. Practical values forH.F. chokes are given in the table over-

page.Next, we must pay attention to the

design of the choke. In order to obtain(Continued on next page.)

272

CHOOSING A CHOKE(Continued from previous page.)

the necessary amount of inductance, a largenumber of turns of wire have to be woundon the choke. These turns act as the platesof small condensers, so that there is atendency for the high -frequency currentto pass from turn to turn through this self-capacity, thus defeating the object of thechoke, which is to block off one circuit tothe passage of the high -frequency currentand shunt it along another path. Thehigher the frequency the easier it is for theradio -frequency current to take this shortcut past the choke; so another feature ofa good high -frequency choke is low self-capacity-or what is termed low -loss con-struction. This is particularly importantin the case of short-wave chokes.

PRACTICAL WIRELESS June 151n, 1940

The next point calling for attention isthe matter of interaction. It is obviousthat a choke comprising a number of turnsof wire will produce a considerable magneticfield of its own, and the Magnetic effectsmay cause unwanted coupling with otherparts of the circuit, resulting in instability.Conversely, the windine of the choke mayin their turn pick up either by magnetic orelectrostatic coupling, impulses from someother part of the set, which again mightintroduce unstable operation.

Reducing InteractionThe self -field of a choke can be reduced

by winding the coil in binocular form, i.e., astwo coils side by side. This results in amuch more concentrated field, having verymuch smaller external influence. For

many purposes,however, especiallyin modern sets, itis desirable toscreen the high -frequency chokes by

Self D.C.Purpose Inductance Capacity Resistance

Coupling for S.G. valves . . 200,000/500,000 1/3 mmfd. 200/500Standard H.F. Coupling.. 100,000/200,000 2/4 mmfd. 300/800Ordinary reaction .. 50,000/200,000 1/3 mmfd. 200/700

enclosing them completely in metal cansor covers. Here, however, a further riskmay be introduced, for if the screen is sodesigned as to be close to the choke win-ding, the screen and the choke will intheir turn act as the plates of a condenser,and valuable high -frequency energy willbe by-passed to earth and lost. Hence inselecting a screened H.F. choke, chooseone in, which there is generous spacingbetween the windings and the case.

Finally, the general mechanical designof the choke should be sound. We mustusually trust, to the maker to see that all

.internal connections are well made, and thewinding properly insulated between sectionsand, between the wire and the case. But wecan select types which have sensibleterminalS or connecting -lugs, and fixing_holes which are in convenient positions, andwill tape screws or bolts of reasonablesize, and we can see to it, too, thatthe choke we buy is of a general designwhich will withstand normal usage withoutdamage.

Converting or Adapting?Clearing Up a Present-day Problem

By H. J. BARTON CHAPPLE, B.Sc.

IN PRACTICAL WIRELESS, dated May 25th;some interesting practical notes werefurnished concerning the use of a short-

wave converter, and at the same timemention was made of the use of an adapter.There always appears to be a certainmeasure of confusion existing concerningthe exact functions of these two units, andas at the present time there are many peoplewho are turning their attention to the short-wave end of the spectrum it is useful toexamine the position in regard to the twoways in which the home receiver may bemade to function, although the normal rangeis only medium and long waves. There islittle doubt that for the very best results itis preferable to employ a short-wave orultra -short wave receiver designed solelyfor this purpose, but questions of expensearise, and provided the home set wouldnormally be inactive during those periodswhen short and/or ultra -short wave listeningis to be indulged in, then on the face of itthere seems no reason why this set shouldnot play its part.

Different FunctionsRegarding the two units themselves an

adapter is usually a short-wave detectorwhich is designed to cover the short orultra -short wave band, or both, and isplugged into the detector stage of theordinary broadcast set so as to adapt it toits new purpose. The term converter, onthe other hand, is used when the devicefunctions as a result of changing onefrequency into another frequency whichcomes within the scope or range of thehome set. The double combination thenbecomes a superheterodyne receiver, thehigh frequency side of the home set workingas the intermediate frequency amplifier.

With these two facts clearly in one'smind it is now a much simpler matterto see which method is better suited to meetindividual requirements. Turning to theadapter first, since only the low frequencyside of the home set is to be brought intocommission, the quality of the resultsobtained will depend very much on the

degree of audio gain existing, and also thefrequency response. One thing that mustbe guarded against is insensitivity in theadapter unit. If long range working is notdesired then a straightforward detectorcircuit alone, similar to those which fromtime to time have been featured in thesepages, will suffice. A series of plug-incoils may be used to cover the necessaryband, but it is neater and less troublesometo use a multi -range coil, Provided it hasefficient switching incorporated, and isproperly screened. So many of theseunits fail because of poor switching, so besure to obtain a coil from a manufacturerof repute.

Increasing RangeWhen it is felt desirable to increase the

listening range of the adapter, then a stageof high -frequency amplification can be madeto precede the detector valve, and this two -valve combination can then work in con-junction with the audio -frequency side ofthe home set. Give the adapter a goodchance to prove its efficiency by using asatisfactory aerial installation, and ifworking on the ultra -short waves study thenotes published recently in PRACTICALWIRELESS in order to ensure that any dipolearrangement falls within its correct category.Pay particular attention to the dispositionand length of interconnecting leads betweenthe adapter and set, otherwise instabilitywill mar the working. If the home receiveris A.C. mains driven, remember that bycutting out the normal IT.F. and detectorloads the volts from the rectifier unit willrise. This may cause trouble, and if thesame rectifier unit is not employed to feed'the adapter, then it may be found advisableto introduce a dummy load to equal thewatts consumed by the non -working partof the home set.

Converter UnitComing now to the converter unit, it

must be remembered that this functionsby making' the high frequency section of

the domestic set into an intermediatefrequency amplifier of a superheterodyneset. This part of the home set should there-fore be very efficient, and as a general rulethe receiver itself is left tuned at someposition on the long -wave band-the bestsetting is found by experiment-and allordinary tuning is undertaken with theconverter unit. This can be a first detectoronly, or when higher sensitivity is desired,and more distant listening is necessary,then the detector stage can be precededby a high frequency stage. Fcir usefulworking details the reader is referred topage 218 of PRACTICAL WIRELESS, datedMay 25th, where a typical H.F. anddetector converter unit circuit was described.

On the score of cost it is generally foundthat the adapter is cheaper, but when per-formance. is of the greatest importance theconverter is usually capable of providingthe better results. Very useful operatingknowledge of the short and ultra -shortwave bands will be acquired, and this willpave the way to the ultimate desire of con-structing a special complete set which canbe used independently of the one doingduty for domestic listening.

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June 15th, 1940 PRACTICAL WIRELESS 273

ON Y 'VELENGTHCar Radio Taboo

THE Postmaster -General has 'announcedthat a Defence Regulation dated

May 29th provides that apart `fromcertain authorised exceptions, no personshall use -,---or have in his possession; or tinderhis control, any wireless receiving apparatu,sinstalled in any road vehicle. Any wirelessreceiving apparatus, even if it is not fixedin position-for example, a portable set-wills be deemed for the' purpose of theregulation to be installed in a vehicle if itis in the vehicle in such circumstances thatit can be used or readily adapted for use.On May 31st the Postmaster -Generalcancelled all ordinary wireless receiving -licences for the installation and working ofwireless apparatus in road vehicles. Inthe absence of special authority, therefore,all persons who have wireless apparatus'in,motor -cars, or other road vehicles, musttake immediate steps to remove all Suchapparatus, including aerials, from thesevehicles. As the time limit set was June2nd, no doubt all motorists have compliedwith this new regulation by this time.

The regulation applies whether the vehicleis in use or laid up, but the wireless licencesin respect of these vehicles should beretained by the owners. No refund oflicence fees can be made, but the questionwhether any allowance can be made inrespect of their unexpired periods will beconsidered when the time comes to removethe present embargo, and to issue freshlicences for the use of wireless apparatus inroad vehicles.

I am surprised, however, that theMinister has ignored the possibility ofbicycles carrying portable transmitting andreceiving apparatus. Such vehicles shouldhave been included in the Order, in myview. Also, it seems to me that it will behighly dangerous for a member of thepublic to purchase a portable set and carryit home by car. I -do not know whetherthis will restrict the sale of portables,because people will be apprehensive thatthe possession of such an instrument mayland them into trouble. Obviously, apicnic party seen strolling towards. somefields with a hamper and -a portable wirelessset would be suspect.

Reserved Occupations -

THERE seems little likelihood that theschedule of reserved occupations

will impose higher age limits on thoseengaged in the skilled sections of thewireless industry. I mention this becausesome newspapers have been indulging inwishful thinking, and spreading falseinformation about the intentions of theMinistry of Labour.

Licence FiguresTHE U.I.R. (Union Internationale de

Radiodiffusion) recently publisheda table giving the number of libencesfor wireless receivers issued in each country.Here it is : Belgium, 1,148,659 ; Denmark,834,565 ; Dutch East. Indies, 90,385;Estonia, 90,876 ; Germany, 13,945,022';Hungary, 511,410 ; Iceland, _16,755 ; Ire -

By Thermion

land, 169,392 ; Italy, 1,135,000 ; Japan,4,666,058 ; Latvia, 154,106 ; Lithuania,79,081 ; Palestine, 43,777 ; Portugal,90,856 ; Rumania, 319,708 ; Switzerland,605;574 ; Yugoslavia Belgrad, 107,785-;Ljubljana, 22,151; Zagreb, 35,433.

Radio Engineers' ManualTHE " Radio Engineers' Manual " which

we are publishing in pocket -bookform will be issued very shortly. I makethis statement in reply to those readerswho are anxious to obtain a copy. When itis ready the usual announcements will bemade in this journal.

Fourpence a WeekT9E has been a gratifying number

o letters from readers thanking usfor increasing the price to 4d. rather thanadopting the alternative of reducing thenumber of pages. All readers seem awareof the difficulties with which publishers andeditors have to contend at the present time.

An All -dry Receiver -

HITHERTO all -dry valves have notbeen available to members of the

phblic. I understand, however, that theyare now released, and that- the Editor isdesigning a receiver incorporating them.Such a- receiver will be of immense use toour readers in the Army, to air-raid wardens,and to others who for one reason or anotherare without accumulator charging facilities.

Newly -trained Citizen AirmenSOME months ago a Second -Class Air-

craftman arrived at an R.A.F.Initial Training Wing. He was then merelya transported civilian ; to -day he is a trans-formed Service man. He is a finished pro -duet of the " I.T.W.," and as a treadingAireraftman awaits his posting to a FlyingSchool.

Whk, has happened to this man since theday he' walked into the Service ? He haslearned the new and improved foot drillsyllabus. He is no guardsman, perhaps,but he is a smart airman and that, in itself,is a credit both to himself and his instructor.He has completed a physical training coursefrom which he has learned that the businessof maintaining a healthy body can beeffected under the guise of thoroughlyenjoyable exercise.

He has revised his mathematics up tomatriculation standard. He has learned toreceive and transmit morse,code messages

at useful working speed. He has becomethoroughly acquainted with the operationand mechanism of the more widely usedService armaments. He has learned therudiments of air navigation. And, morepractical than any of these, he has com-pleted a course on the visual Link Trainer,and so has a theoretical knowledge of theproper handling of an aircraft.

Moreover, he- has been imbued with aspirit of discipline which has not robbedhim of his individuality, or his initiative,but has brought home to him a keenerrealisation of the trust that has beenplaced in him. At the same time he hasbeen taught Service etiquette and themodus vivendi of. Service Lk. He is notan automaton ; he is one 'cf the thousandsof new citizen -airmen of the British Empire.

Air Force Require Radio MenMEN up to 50 can now serve in the Royal

Air Force provided they have ex-perience in radio work. A new class of entryhas just been created to provide personnelfor the maintenance of Air Force wirelessequipment of various types. The age limitsfor radio trade entrants are from 18 to 50.

Large numbers of pilots, air observers andwireless operator/air gunners are required

at the moment. Youngmen of- good education, with dash andinitiative-especially those in age groupswhich have not yet been registered-areasked to volunteer now.

Application can be made at any Com-bined Recruiting Office ; or to any localLabour Exchange.

Jamming Haw -Hawj SEE that one of the daily papers hasI worked itself into a fine frenzy overthe amusing broadcasts of Lord Haw -Haw.The daily paper seriously suggests that thetime has come for the ruthless- and con-tinuous jamming of Haw -Haw. This, theythink will choke the pestilential lies thrownso glibly into the air by Goebbels. Theygo on to suggest that even if the Germansretaliated by jamming our radio, we couldstill, through our telephones, listen to ourown broadcasts. This suggestion is tooridiculous to need much comment. Haw -Haw broadcasts in English, and as I haveyet to discover any Englishman who takesthe slightest serious notice of him, exceptwhen we want a little light entertainment,I cannot see why we should run the risk,even if that were possible, of retaliatoryjamming measures. To suggest that weshould listen over our telephones to ourown broadcasts is just too absurd. In thefirst place, less than half of the listenershave telephones, and in the event of airattacks, we should not be able to listen in,as all telephonic communications will besuspended, as of course, will be the broad-casts. I suggest that newspapers shouldinvestigate the_possibilities of their sugges-tions before they make themselves, as wellas this country, look ridiculous. As most-of them have a radio expert on the Staff,why are they not permitted to advise ?Is it because some of the alleged expertsthemselves propound these amusing theses ?

274 PRACTICAL WIRELESS

Comment, Chat and Criticism

June 15th; 1940

Musical History -4The Music of Bach and Handel, by Our Music Critic,

MAURICE REEVE

AT the close of the seventeenth century;music had " attained its majority " ;it was now about to reach its

maturity. It was to be launched on acourse which it has followed down to ourown day, only diverting from it to its ownperil. Many of the writers already men-tioned had produced works which havesince achieved immortality. Harmony andmelody of ineffable sweetness were atcomposers' command. The forms theyfashioned in were for the most part small,and it was in this sphere that the comingcentury was to mark the greatest advanceand achievement. The BeethovenSymphony had still a hundred years towait, and Don Giovanni and the ChromaticFantasy almost as long.

Range of ColoursThe musician's palate had acquired a

wide range of colours. The moderndiatonic scale had reached its definitiveform though chromatics were sparinglyused. The dominant seventh, withoutpreparation, was common in modulation,

was brought into practicaluse. Monteverdi is credited with beingthe inventor of the perfect cadence, andother writers as well as himself used thediminished triad. The orchestra did notyet include the violin, which may soundrather like talking of strawberries andcream only to find that there is no cream !But it was taking an ever larger part inthings with the growth of opera, and theerection of opera houses and their permanentorchestras. The madrigal declined, andwith its eclipse came that great wave ofchurch music so indissolubly linked withthe two names of Bach and Handel. Butonce the works of Couperin, Monteverdi,Scarlatti, Purcell, etc., had been assimilated,music could look forward with confidenceand assurance to producing the men capableof leading it up to the heights which wereso apparently destined for its occupation.It was not to be disappointed.

Bach and Handel were two such men.By the greatness of their vision and theirgrasp of the problems before them, theycreated work, far in advance of anythingpreviously fashioned, or even conceived.Bach, in particular, was a most daringharmonise, and he has, to this day, remainedthe greatest of contrapuntists, and hisorgan works are still the pride and gloryof all that instrument's repertoire, neverhaving been surpassed in grandeur, nobilityor technical resource.

Handel's MusicWhilst much of Handel's music remains

unknown to all but the connoisseur, the" Messiah " has obtained such a uniqueposition in the hearts and affections of theEnglish-speaking peoples that Sir HenryHadow does not exaggerate when he saysthat it might almost be regarded " as apart of Holy Writ." Whilst Bach was thegreat master of elaborate and complicatedtexture. Handel was one of the greatest

melodists who ever lived. This is notmeant to suggest that the one could notwrite fugues, nor the other beautifulmelodies ; far from it. But, broadlyspeaking, that is where their two styleslay. Both men's work is on the loftiestimaginative plane, Bach's perhaps especiallyowing to his lifelong association with theChurch.

Although so closely associated in theminds of most people, and always thoughtof together and analysed as a pair, neitherman ever met the other. In fact, there isno record of either having ever heard ofthe other or of the other's music !

Speaking idiomatically, these two greatmasters may be said to have conqueredthe musical world for Germany. It wascertainly a conquest that lasted far longerthan most of Germany's conquests in other

-

domains ! Actually, a dynasty of Germangiants followed them right down to Wagnerand Strauss, and only to -day does Germanmusic show an appreciable decline fromthis supreme, standard. The sequence ofgreat names that Germany produced fromBach to Strauss is extraordinary, and noteven the great lines of Italian or Dutchpainters, or of English men of letters,surpasses it. The German musicians ofthe eighteenth century rescued music fromthe disgrace that was rapidly overtakingit at the hands of unworthy men elsewhere,As almost all of them were associated withthe Church, it was but natural that theirwork should lay chiefly in the domain ofchurch music or of music with a" churchy"flavour such as Bach's organ masterpieces.

G sharp is B sharp and not C, which arenot one and the same note as you can proveby Ringing the notes instead of playingthem. Furthermore, if you begin thesequence with B sharp instead of with C,you will get further and further away,fromthe original tonality until any combinationwith it becomes impossible. Therefore, tomodulate from D major to- D flat majoris impossible if both scales are accuratelyin tune, as one requires F sharp and theother G flat, which are not one and thesame note.

Perfect IntonationIn Bach's day there were two ways p

facing this problem. One was to merelaccept " perfect " intonation together witthe many restraints that it imposed onmusical thought and development. Theother was to abandon exact purity in orderto obtain the enormous benefit of beingable to employ every key within the frame-work of the one piece. Very much in thesame way as in summer -time," where NVecall twelve -o'clock eleven, in order to enjoYcertain advantages which are denied usotherwise so "equal temperament " callsG flat and F sharp one and the same thing,for the sake of increasing the range andscope of musical composition. By "flatten-ing " here and " sharpening " there-bytelling a few " fibs," as one might say -4-everything that music has to offer isbrought within the range of those bigenough to use it. The rules governingwhether a note shall be deemed G flat orF sharp are very strictly observed gramma-

Although this led to extremes Of conser-_ tically. But both are now combined invatism at the other end of the scale, it the one string, and the same key on thewas just what music wanted to free itfrom the base and impure elements thathad entered into it. After the pendulumhad swung with equal violence in bothdirections, the even greater masters of thelate eighteenth and early nineteenthcenturies were to restore the equilibriumwith a series of master works each con-taining a perfect amalgam of all that wasbest from the past plus their own geniusand prophetic visions, balanced to an exactnicety, and to remain supreme for alltime.

Of even greater importance to the futureof music than the mere writing of magni-ficent works was Bach's insistence on" equal temperament " of the strings of theharpsichord. We have noted the establish-ment of major and minor keys, and thegreat increase and variety of modulationwhich they rendered possible. But a greatdifficulty still remained to be overcome.If a diatonic scale is exactly in tune, onlythe most closely related keys can beblended with it ; any remoter relationshipwould sound intolerable. If you play onyour piano these notes : 0, E, G sharp,and the octave of C,- you will notice that,on your keyboard, the top C is the exact'octave of the bottom' one, and that theinterval between each two notes is thesame. But the true major third from

piano keyboard.Bach took the side of the reformers,

and proved his wisdom by writing hisimmortal " 48 Preludes and Fugues inAll Keys for the Well -tempered Clavier,"works which display a range and varietyof harmony, modulation and contrastthat would have been impossible of achieve-ment under the old rules, but which setthe standard for all future ages.

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Frequency ResponseA Simple Explanation of Response Curves and Their Meaning, With

the Methods of Drawing Them

RESPONSE, as applied to a receiver oramplifier, is generally understood asthe variation in output for an input

pf given voltage over a range of frequen-Cies. Thus it is customary to give a responsecurve or graph on which the output (or afactor representing variation in outpnt)is plotted against the frequency of theinput signal. It would be possible to showthe output in milliwatts and the input incycles per second, but it is more usual toplot frequency against a decibel scale.The graph would then be of the formshown in Fig. 1, which would indicate thatthe receiver or amplifier was in the

sent the ideal, since measurement of outputmight be made without the use of thespeaker to be used with the set. And ifthe response curve of the` speaker" dipped "

below, say, 150 cycles and above 2,000cycles the audible output would fallbelow and above these points.

On the other hand, if the output fromthe receiver went up below 150 and above

2,000 cycles, thesound output mayremain constantover the wholefrequency band,due to the receivercompensating for

10000 the losses in thespeaker. This

principle can beapplied to almostevery componentused in a receiver,

30 100 1000Frequency (cycles/sec)

Fig. 1.-Response curve (practically "straight line") for a high-fidelityamplifier.

j

100 1,000 /Q000

Frequency (cycles/sec)

Fig. 2.-Curve for a well-known parallel -fed L.F. transformer. BrokenI lines show how the curve can be modified by changing circuit constants as

explained in the text.

ii

r43.

Za

'60 /0

°50 100 500 WO 2000

Frequency (cycles/sec.)

Fig. 4.-This transformer curve is of different form from that shown inFig. 2, since frequency is plotted against amplification ratio.

" quality " class, since the output remainsconstant over a wide range of frequencies.

The input is provided by means of amodulated oscillator, which must be a

i precision instrument if useful results areto be obtained, while the output may be

' read on an accurately calibrated outputmeter or valve voltmeter.

In addition to the complete set, individualcomponents such as transformers, pick-ups

1 and speakers. can be tested for response,and curves drawn. Here again, it is nowcustomary to plot frequency against adecibel scale.

Frequency CompensationFor present purposes it is not essential

that the decibel system be fully under-stood (it was explained in our issue dated

j May 11th), for we are mainly concernedwith the variation in output over the rangeof audible frequencies. Theoretically, a

r straight-line curve "-consisting of astraight horizontal line-would indicate aperfect receiver, amplifier or component.

I In practice, however, that may not repre-1

50

40

0

4,000 8000

for it is obviousthat if losses ino n e componentare evened up bycorrespondinggains (or negativelosses) in anotherthe output maystill be straightline."

There is anotherimportant pointwhich should beconsidered. Thisis the scale towhich the graphis drawn ; if thescales are " open,"variations inresponse are farmore _pronouncedthan if t'hedivisions are

275

" crowded." It is customary to use alogarithmic scale for frequency. Thismeans that the distance from zero to100 c/s is equal to that from 100 to 1,000 c/s,and froth 1,000 to 10,000 c/s. Thus, thescale is more " open " at the lower fre-quency end.

Fig. 2 shows a response curve for a well-known L.F. transformer of the parallel -fed type. It will be seen that it givesuniform amplification to audio frequenciesbetween approximately 110 and 12,000 c/s,but that the amplification falls off at eitherend of this range. That is usual in alltransformers in the lower -price range,but it is possible to obtain transformerswhich give " straight line " output fromas low as 50 cycles to well over 10,000cycles. Although componeip,s of this typeare excellent when used With valves forwhich they were designed they may be nomore satisfactory than others costing farless if used in conjunction with any othervalves or in a circuit other than that forwhich they were primarily intended.

Aural EffectsIn considering frequency_ response it is

also necessary to bear in mind that, forpractical purposes, the ear plays an impor-tant part. Its sensitiveness is comparativelylow at frequencies below about 100 cycles

ance of this is especially pronounced at thelow -frequency end of the audio range.Because of this, frequencies below 100cycles are almost inaudible to manypeople, and particularly when accompaniedby harmonics. For example, the frequencyrange of a piano is from 70 to 6,500 cycles,but few people would recognise any differ-ence if, in reproducing piano music, areceiver dicibnot respond to frequencies out-side a range of about 100 to 5,000 cycles.

There is another important aspect of thequestion, which is concerned with the set-ting of the volume control. If the volumeseemed (to the ear) to be constant over thefrequency band between, say, 50 and 10,000cycles with the volume control turned full

(Continued on next page)

(

loo l000Frequency (cycles per sec.)

Fig. 3.-A scientific method of plotting a response graph for a high-grade loudspeaker. This ismore " detailed" than usual, for in most cases the graph is drawn as a smooth curve.

200 a./00 C,

2C

/Q

(I)

50

2

/030/4000

276 PRACTICAL WIRELESS June 15th, 1940FREQUENCY RESPONSE

(Continued from previous page)

on, this condition would not apply when thecontrol was turned down. In fact, if volumewere reduced so that a 1,000 -cycle note(to which the average ear gives maximumresponse) were just comfortably audible,frequencies below 25 cycles would probablybe completely inaudible, whilst freqnenciesover about 3,000 cycles would scarcely beheard. This is the reason why variousmethods haVe been employed with

quality " amplifiers to combine a tonecontrol with the volume control. It alsohelps to explain- why it is often claimedthat good quality reproduction is impos-sible when the output is less than 5 watts,even for a receiver used in the home.

Sound Output as Air PressureIt is not an easy matter to measure out-_

put in terms of sound, or rather in terms ofair pressure, which is a measure of soundas it strikes the ear drum, but by the use

of special apparatus such measurementscan be made. Thus we may have a graphsimilar to that shown in Fig. 3, which is 'acopy of one prepared -by the NationalPhysical Laboratory for a certain Totfd-speaker. It will be seen that variationin sound -intensity is plotted against a fre-quency. scale, whilst on -the right of thegraph there is a scale referring to the acous-tic pressure. Despite the fact that theresponse curve in this case is a very wavyline the speaker to which it refers is a high-grade instrument. This becomes moreevident if a mean or average line is drawnthrough the wavy one, -or if it is noted that(he variation in acoustic pressure does notfall below 5 or rise above 35 dynes/sq. cm.The latter expression, by the way, is simplya measure of pressure corresponding to themuch larger units of lb./sq. in.

Modifying the ResponseAs another sidelight on frequency response

it is interesting to refer to Fig. 2, wherebroken lines are shown as " offshoots " of

the main curve. These show how the fre-quency response ..of the parallel -fed trans-former represented can -be varied by chang-ing the 'capacity of the feed condenser. Thefull line shows the response when using acondenserhaving the usual:capacity for thisposition of .5 nifd., whereas the broken lines-at the bass end of the scale shows the changebrought 'about when the capacity is reducedto .1 mfdi This means that, in this ease, theuse of the lower capacity gives a " bass lift."The brokun line extending to the right ofthe curve, shows the effect of connecting -a.5-megohm resistor across the secondaryterminals.

Another method of drawing a trans-former response curve is shown in Fig. 4,where it will be seen that the verticalscale represents the amplification ratio, thebase line showing frequency, as before.Fig. 4 actually refers to a poor transformer.The curve rises steeply to about 3,000 c/sand falls almost to zero at 50 °M. A curvefor a good transformer, correctly useddrawn to this scale would be very similar,in shape to that given in Fig.

How Frequency Modulation WorksA Simple Explanation of the New Transmitting System

MANY listeners have heard that asystem of broadcasting has beentried out and adopted in certain

parts of the U.S.A. which is claimed to be agreat improvement on existing systems.This new arrangement is known as frequencymodulation, and in view of the increasinginterest which is being shown in it the follow-ing details will, no doubt, prove of interest.They are taken from a talk by Lee McCanne,of the Stromberg-Carlson lIfg. Co., and arereprinted from the American publication" Radio To -day."

Just what is Frequency Modulation ?And how does it differ from AmplitudeModulation, the kind of radio broadcastingwe have all been hearing up to now ? It issimply a different method of superimposingthe programme on to the carrier wave. Letus see what it looks like on the charts onthese pages.

Now whether we are dealing with Ampli-tude Modulation or Frequency Modulation,we start with the same kind of radio carrierwave, and also the same kind of telephonecurrent " programme wave " as picked upby the microphone. With Amplitude Modu-lation, the programme wave is combinedwith the carrier in such a way as to changethe power of the resultant wave. It adds tothe power part of the time, and subtractsfrom the power another part of the time sothat the wave radiated by the broadcastingstation appears as shown at (c).

Broadcast AdvantageIn Frequency Modulation the same

programme wave does not change the powerof the carrier at all but is made to change itsfrequency, speeding up the carrier -wavepart of the time and slowing it downanother part of the time so that the resultantwave from a Frequency Modulation stationlooks like (d).

The chart shows you immediately theadvantage which Frequency Modulationoffers to the broadcasting station. Theprogramme is superimposed on the carrierwithout changing its power. In other words,the broadcasting station operates at fullpower all the time. That means that it canbe much more efficient, use less valves, less

current and even save man power-becausethe management can let the transmitterrun " wide open " all the time and doesnot have to have an engineer who is also atrained musician constantly watching avolume indicator, ready to turn the volumeup when the programme level gets lost in thebackground noise, and to turn it down whena soprano hugs the microphone or theorchestra plays too loud.

In Amplitude Modulation, the broad -

J

(a)

(b)

If

casting station has to be built to handlefour times the rated power of the station onvolume peaks. The Stromberg-Carlsonstation WHAM, for example, is rated at50,000 watts but it has to be built with alarge enough power supply, valves, tuningsystem, and aerial to handle 200.000 watts,and it would even go above that if theengineer was not there to turn the volumedown when he sees his volume indicatorneedle swinging too high. This compressingof the " dynamic volume range " in itselfaffects the musical quality of the pro-gramme. Suppose that an orchestra hasenough instruments to make a maximumnoise equivalent to " 100 decibels " volumelevel when everyone is playing his hardest ;an Amplitude broadcasting station mighthave to squeeze that volume range down to,say, 50 decibels, in order to keep the quietpassages up above, valve noise and studionoise, and to bring the loud passages down

within the capacity of the transmitter. InFrequency Modulation there are no volumepeaks as regards the power being radiated ;all that volume does is to swing the carrierfrequency. So a 50,000 watt FrequencyModulation transmitter can be built todeliver 50,000 watts all the time, and handlethe full dynamic volume range of the music.

" Limiter " ; " DiscriminatOr "In a Frequency Modulation broadcasting

(a) Is a radio carrierwave of constant ampli-tude ; and (b) is a pro-gramme or audio wave

for modulating (a).

station the microphone amplifier is made toaffect the oscillator valve directly, changingthe frequency of the carrier rather thanbeing indirectly added to it by a mixer.At the receiving set you have the same sixvalve functions and two more ; the detectorcircuit in a Frequency Modulation receiveruses three valves instead of one. One ofthese extra valves is called a " limiter "valve, and its purpose is to fix upon andassign control to the strongest signal comingin at the particular wavelength or fre-quency to which your receiver is tuned.Thus the strongest signal gets control.

The other, extra valve is ,called a " dis-criminator," its function being to wipe outany noise or signal other than the strongestsignal coming in. The discriminator alsowipes out rather effectively. any AmplitudeModulation station that might be broad-casting on the same wavelength to whichyou have tuned your Frequency Modulation

4'

June 15th, 1940

receiver and, since most static is essentiallyan Amplitude Modulation. signal, thediscriminator wipes that out, too. ;Theeffect is to, eliminate natural static, man-made static, in fact, practically any kind ofnoise. A good Frequency Modulationsystem eliminates the carrier. " hiss " andmost of the valve noises from. the .broad-casting station as well as any hum modula-tion and valve noise in your receiver. Youdo not even know a Frequency Modulationradio is turned on until the music plays.

Static -freeThat shows how little residual noise there

is in this system. As one writer put it, thiskind of radio can- "transmit silence." (Ofcourse,-noise in the studio may still bepicked up by the microphone. Also whenFrequency Modulation stations are broad-casting gramophone records there may besome needlenoise.) But in eliminating mostof the noises and distortion that mightoccur, Frequency Modulation brings radio-from a tone qualify standpoint-down to asimple static -free long-distance telephonesystem.

To show you how quiet and clear andstatic -free this new system is let me saythat movie producers are interested inFrequency Modulation to give better soundrecordings on their films. Under this plan,rather than taking portable sound -recordingapparatus out " on location," which maybe miles away from Hollywood, they willuse the, best sound recording equipment atthe studio -and will transmit the - pro,gramme to that point by FrequencyModulation radio.

Before leaving this subject, let me pointout why a Frequency Modulation receivermust be a littIe more expensive than anordinary radio. Remember there are twomore valves in the detector circuit, and atuning eye is also desirable, so that a goodFrequency Modulation radio should haveeight or nine valves minimum. To takefull advantages of the better FM tone, theaudio amplifier stage must be a good ClassA distortionless amplifier. The loud-speaker must be designed to respond tohigh tones and overtones beyond the rangeof many of the foudspeakersabeing pro-duced to -day, and it must have a suitablebaffle.

In Frequency Modulation, if the broad-casting station is an Armstrong wide -swing transmitter sending out high fidelity,then the tuning system of your receiving setmust be designed to admit or accept the fullfrequency swing of that transmitter. Up tothe detector valve, that receiver has to begood. Otherwise the Whole reception isdistorted. The receiver ought to be selective,too, against interference from the nextFM channel, but it. must accept the wideband of frequencies on each channel. Thatmeans, if you want a cheap FM receiver,the maker can economise on the cabinet,.'on the loudspeaker, and on the audioamplifier, but the tuning system must begood. And, since the tuning system is oneof the most expensive parts of the wholereceiving set, you might as well make therest of it good enough for eight -octavereception.

No Station ClashesMoreover, an eight -octave FM receiver

is worth the extra cost because you will beable to get the full high-fidelity programmewithout . static and without interference.That is the odd thing about FM. FrequencyModulation transmitters broadcasting onthe same channel just do not interfere witheach other unless possibly you are onthe fringe area of both stations, right onthe ragged edge of their service range.

PRACTICAL WIRELESS 277

Otherwise, the strongest signal takes controland the other one is wiped out. To provethis, three FM transmitters all broadcast-ing different programmes on the samechannel were set up. One was .at Albany,another at Troy, the third at Schenectady-all 12 to 17 miles apart. Engineers wentout with a receiver on a truck, trying tofind ,a place where they could get inter-ference-bring in two of these stations atonce. They could not find it. In fact,that got down to a space of a few inches,that outside this space they would get onlyTroy or Albany or Schenectady-depend-ing on which direction they moved the truck,while inside this small area the programme

(d)

aerial is important, as well as the height ofthe broadcasting aerial. This distancelimitation is the one limitation to goodFrequency Modulation broadcasting ; itwill be necessary to erect transmitters inevery large population centre before wecan all be assured of this new and betterkind of reception.

On the other hand, within its servicerange, a Frequency Modulation stationgives reliable reception, day or night, eventhrough thunderstorms. Listeners locatedat a distance from the city but withinrange of its FM transmitter will probablyget better and quieter reception than theywould from an AM transmitter in that

ulEwould flop back and forth from one stationto another.

Two to Four HorizonsIn the ultra -short waves, Frequency

Modulation radio acts like light, andacts much like television as regards the rangeof the transmitters. Given enough powerand a high aerial, Frequency Modulationstations seem to have a fairly reliable rangeup to two horizons and a possible servicerange (to listeners not located behindbuildings or motmtains or in valleys) upto four horizons. Height of the receiving

(c) is the carrier andaudio wave combined inAmplitude Modulation,whilst (d) is the samecombination in Frequency

Same city. Listeners located beyond therange of the FM transmitter will, of course,have to rely on the AM station the sameas heretofore. High -power cleared channelAM stations will probably always beneeded, as will the International short-wave AM stations. For that reason thesystem proposed to the CommunicationsCommission by the FM Broadcasters,Inc., is a combination of AM and FMstations whereby the cleared channels on theregular broadcast band will be used for long-distance and rural coverage, and FM chan-nels will be used for reliable local service.

Coil for. the. Gas -mask Box ReceiverOWING to the fact that the con-

structional details of the coil usedin the Gas -mask Box Receiver (which wasdescribed in our issue of May 11th, 1940)were given in our issue of September 30th,1939, and that a great number of ourreaders appear to be without this par-ticular copy, we reprint below the essentialdetails. It will be noted that we havevaried the original specification to suit thereceiver mentioned above, by increasingthe diameter of the coil former from ltin.diameter to 2in. The length should be21 -in. A piece of ordinary postal cardboardtubing, or, better still, a length of paxolintubing, can be used for the former. Ifcardboard is used, it is absolutely essentialto see that it is perfectly dry ; in fact, it isadvisable to impregnate the tube afterdrying it in a slow oven for a few minutes.Ordinary shellac may be used for theimpregnation.

Winding DetailsThe actual winding is carried out with

22 -gauge enamel wire, winding this with70 turns close wound, that is, each turnlying close up to its neighbour. After 23turns have been put on a tapping loop hasto be made for the aerial and this isaccomplished by doubling a length of thewire and pushing it through a hole in theformer. It must be kept taut whilst the

rest of the coilis wound. After the windinghas been finished a length of paper orEmpire Tape in. wide should he wrappedround the lower ;end of the winding,its position being about. /,in. from thelower end of the coil. On this insulator20 turns of the 34 S.W.G. enamel wire arewound, and these must be in the samedirection as the first winding. One wayof anchoring the ends of this additional

AERIAL

GRID CONDENSERe, ruNIAI0CONDENSER

LT-F/LAt. too

YREACTION

CONDENSER

XREACTION.

CONDENSER

Coil connection details for the Gas -maskBox Receiver.

winding is by means of sealing -wax orChatterton's Compound, whilst anotherway is to pierce holes through the former,between the turns of the first winding.In this case great care must be taken notto scrape off the insulation where the wirescross.

PRACTICAL WIRELESS

HORT-WA ESECTIONS.W. SET IMPROVEMENTS.

Modifications to Existing Apparatus :Using a Mains Unit and OtherDetails for Obtaining Better Short.wave Results. By W. J. DELANEY.

MANY amateurs, when looking roundto find some means of improvingtheir apparatus, start to buy new

parts or change components, generallywithout any idea as to the ultimate resultof such modifications. It is worth whilein such a case to start by thinking out thelines upon which one wishes to work,so that any changes which are made will

Fig. 1.-Using apotentiometer to

vary grid bias.

L.T. Sup*

lead to improved results and not to somedoubtffil working which may lead tofurther difficulties. For instance, a changein valves may be thought to give animprovement, but such a change may resultin the need to modify a coupling or to usesome different H.T. value, and this in turnmay lead to inefficient working of anotherstage. These facts are more importantin the case of short-wave apparatus asthere is not so much latitude in this typeof equipment. Furthermore, as it isdesirous always to obtain the maximumperformance from short-wave apparatusit is not possible to make do with whatmight be termed " makeshifts." Suchideas as fitting band -spread tuning havebeen mentioned many times in these pages,but there are other directions to whichthe keen experimenter may turn in anendeavour to improve, or " hot up " hisapparatus, and the following will give someideas upon which to draw.

New ComponentsIn the way of new parts, modern short-

wave apparatus may always be taken asan improvement upon old parts, especiallythose which have been primarily designedfor normal broadcast use, but which havebeen included in a short-wave set. Tuningcondensers, for instance, will be found inthe modern short-wave sphere to haveimproved insulation, and this is one of themost important factors of short-waveapparatus. Insulation everywhere shouldbe of the highest, and in addition to this,heavy gauge wiring should always beemployed. If, therefore, your set is wiredwith fine wire, especially if this is capableof a fair degree of movement, it should bereplaced by rigid wiring, and the leadsshould be kept as short as possible. Certainby-pass condensers could also be replacedby mica components if you are using thoseof the paper type. This, of course, wouldcome under the heading of improvednsulation. Better quality valveholders

would also be a worth -while change, whilstif terminal connections are employed itmight also be desirable to make use of goodsolid soldered contacts wherever possible.

New ArrangementsThe above details concern mainly modi-

fications in parts, but there may also befound that a change in circuit design may beemployed.. Many amateurs have a circuitwhich they have found tried and tested,and are not on this account keen to makechanges. There are, however, one or twosmall changes which may be made withoutaffecting the general performance, althoughleading to an improvement. For instance,the average type of short-wave detectorwill have the grid leak taken to the L.T.positive leg of the valve. This is a standardarrangement, but it may be found that incertain circumstances it is preferable tovary the bias which is thereby applied tothe valve. For this purpose an old idea,but one which works very well, is to use apotentiometer connected across the fila-ments, and to return the grid leak to the armof that potentiometer. A value of 400ohms is quite satisfactory, and in use itmay be adjusted so that the reaction is assmooth as desired. Other modificationsof this nature, which do not in any wayinvolve drastic changes, are the parallelfeeding of the L.F. transformer, differentialin place of ordinary reaction condenser,or the fitting of extension controls.

Mains UnitsA form of query which is often being

put to us is " How can I use a mains unitwith my short-wave set ? " It is foundthat when a standard type of mains unitis connected to some short-wave apparatus

Fig. 3.-The arrangement of Fig. 2, plus decouplers

HT(or earth)Fig. 4.-How to arrange for the decouplers shownin Fig. 3, so that they may be included between the

June 15th, 040(and in some cases to ordinary broadcastequipment) results are very unsatisfactory.It is even found in some cases that theunit cannot be used, mainly on account ofinstability. As a short-wave receiver ismore susceptible to hum than ordinarybroadcast equipment the problem is moreintense, and it is usually found that theunit is quite satisfactory if another formof feeding the receiver is adopted. Thestandard type of mains unit is providedwith three or four H.T. positive outputs.In some cases one or more of these may beof the variable type, provided either withalternative sockets, nr a variable control.It is in the form of voltage dropper whichis incorporated inside the unit that thetrouble arises. Even if the receiver isprovided with decoupling devices thetrouble may still arise if two or more ofthe unit outputs are fed to the receiver.The most satisfactory way of overcomingthe trouble is to ignore all the positiveoutputs except the maximum, and thusto connect only one positive lead to thereceiver. This means that you will haveto include decoupling circuits in all parts

HT+ 3H.T. #2HTtl

HFpoke L. S.

Fig. 2.-A skeleton standard 3 -valve circuit show-ing normal anode connections.

of the receiver so that the appropriateH.T. feeds are decoupled, and the corrector appropriate voltages are applied to eachstage. If you do not wish to go to thetrouble of modifying the set wiring thereis another way out, and that is to make up asmall unit consisting of the appropriate ,

decouplers and voltage droppers and toconnect this between receiver and mainsunit. The advantage of this arrangementis that at any time the unit may be dis-connected and ordinary battery suppliesused to feed the receiver. For the benefitof those who are not familiar with de -coupling arrangements we show in,Fig. 2a three -valve skeleton circuit with batteryor separate H.T. feeds, and in Fig. 3 thesame arrangement with deoouplers added.In Fig. 4 are shown the separate decouplingcomponents wired up for connectionbetween the unit and receiver. In this case,of course, the maximum H.T. tapping orsocket is used on the unit. To ascertainthe correct values of the resistances it isnecessary to obtain a good milliammeterand voltmeter. The batteries are con-nected to the receiver in the usual way aidthe milliammeter connected in each positivelead in turn. The current flowing iscarefully noted, and then a good voltmeteris connected between the anodes or otherpoints fed by the H.T. line and earth.Next, the voltage supplied by the H.T.unit must be measured, and a slight droptaken into account to compensate for thefall when a load is applied to it. Thenby subtracting the required voltage from-that given by the unit, and dividing this bythe current in milliamps, you will have thevalue of the resistance required in thou-sands of ohms. As an example, supposeyou calculate that the unit will deliver120 volts, and you need 100 volts for anL.F. stage. If the current at that anodeis 5 mA, then you would need a resistance

receiver and mains- unit. of 120-100 divided by 5, or 4,000 ohms.

)

June 15th, 1940 PRACTICAL WIRELESS 279

Practic- 1HhhatsEasily -made Tweezers

SCREWSand other small parts often

drop into awkward corners in aradio chassis that cannot be reached withthe fmgers, or anything else handy. Insuch gases a pair of tweezers, like thoseshown in the sketch, would be foundvery useful. To make them, take a piece

SPRING

FLATTENED ENDGIA handy pair of tweezers made from a

piece of springy wire.

of hard springy wire about 12in. long,and bend the middle part of the wire rotund-a lin. bolt to form a loop to give theteeezers an opening spring. Flatten out theends with a hammer, and file the endssquare.-R. JENKINS (Caterham).

Illuminated Map Tuning

WHEN"tuning on the medium -wave

band on a home-made set, it isvery convenient to know quickly to which-station the set is tuned. This can be donein the following manner, with the help ofthe accompanying sketch. A 6BA round-

Doh' soldered tomoving vanes

Insulated wiresto indicator

Bolts fixed atstation points

An illuminated map tuning dodge.

headed bolt of about 2in. in length issordered to the end of the tuning vanesof a tuning condenser, as shown in sketch.A piece of 3 -ply wood 4iin. wide and theheight of the tuning condenser's highestpoint when open, is temporarily fixedbehind the tuning condenser, so that awhenthe condenser is tuned right through, itdescribes an arc of 180° on the plywood.The stations wanted are then marked,where the head of the bolt is just touching,the plywood. The wood is then removedand drilled where marked. 4BA bolts(flat -headed) are inserted. The plywoodis then permanently fixed by bracketsbehind the tuning condenser, in the exactposition as before, so that when the con-denser is tuned right through 130° thebolt on the condenser makes contact inturn with each bolt in the plywood. Eachbolt has a corresponding bulb, which should

1.00INM0.0.=.0000.0004M..0411.1M.000001M.0.00.1.104

THAT DODGE OF YOURS 71Every Reader of " PRACTICAL WIRE-

LESS" must have originated some little dodgewhich would interest other readers. Whynot pass it on to us ? We pay 41-10-0 for thebest hint submitted, and for every'other item zpublished on this page we will pay half -a-guinea. Turn that idea of yours to account by zsending it in to us addressed to the Editor,"PRACTICALWIRELESS," George Newnes, zLtd., Tower House, Southampton Street, IStrand, W.C.2. Put your name and address 1on every itetn. Please note that every notionsent in must be original. Mark envelope§" Practical Hints." DO NOT encloseQueries with your hints.

,...otmte

F,110000004001.0.11M.0=0000001001M1.00100.1110.00771.SPECIAL NOTICEI All hints must be accompanied by the! coupon cut from page 288.

LiN001104.01100004,1=Ir041141011.00101.()40000.1010J

preferably be coloured to give an evenluminous effect. The bulbs are then in-serted in a named cardboard map (eitherhome or ready made) in their right places ;connecting, wires are joined, one each fromthese bulbs to their corresponding boltsin the plywood. All the + sides of thebulbs are joined. A switch may beinserted in the + lead if desired. Only onelead will connect to the accumulator as thenegative is already joined via the movingvanes of the tuning condenser.-D. ZEID(Bournemouth).

Fitting Sub -chassis ValveholdersHAVING no facilities for drilling line

holes for the valveholders in analuminium chassis, I devised the followingmethod of overcoming the difficulty.gine valveholders were bolted in positionwith a small washer on each bolt between

A novel method of fitting sub -chassis valveholders

the valveholder and chassis, then theholes were marked out on the undersideof the chassis. These were then drilled

diameter. Although the valve legsdid not touch the chassis, I took ,thefurther precaution of slipping a lin. lengthof insulating sleeving over each valve leg.-N. DAWTRY (Sheffield).

L.F. InstabilityAFTER an H.T. battery has been in

use for several months a set may:.develop a high-pitched whistle and re-ception becomes distorted. The reason

for this is, in the majority of cases, that theH.T. voltage from the battery has dropped,due to a breakdown of one or more of thecells with a consequent increase in theinternal resistance of the battery. Thesedefects are sufficient to produce instabilityin the circuit, particularly in the L.F.portion. Many constructors endeavour toovercome the dropping voltage by connect-ing a new battery in series with the old one,but, unfOrtunately, this does not overcomethe trouble as the high resistance of the oldbattery still remains the same. The onlysatisfactory remedy is to use a new battery.

H.F. Choke LossesA POPULAR method of coupling

between a screen -grid valve and adetector is a tuned circuit, connected to thedetector via the usual leaky -grid arrange-ment and to the anode of the S.G. valvethrough another small fixed condenser, theanode of the valve receiving its H.T.through a suitable H.F. choke.

An important point to bear in mind isthat there must be few losses in this chokeif the full amplification of the S.G. stage isto be obtained. In quite a number of theearly H.F. chokes, which were often con-structe4 down to a low price, insufficientwire was employed to provide the requiredvalue of inductance, while in many instancesthe insulation of the wire was often toopoor to give adequate insulation. If theutmost efficiency is required, it is abso-lutely essential to make sure that an H.F.choke of reliable make is employed.

An Efficient EarthAN efficient substitute for an ordinary

earth tube can be made as follows.:Obtain a 3ft. length of galvanised ironstove -pipe, about 4ins. in diameter, andround it drill a number of tin. holes. Solderon a strip of brass or copper, to the end of

&MTH WIRE

GALVA/MID IRONPIPE'

- t

0

0

o

r-

0

OLDERED

r.C..s--

. J.

5

A piece of galvanised iron pipe makes aneigcient earth tube,

which the earth wire is finally soldered.Dig a hole in the ground, and bury the pipeto within about 2in. of the top, and thenram the earth well down all round the pipe.In dry weather empty a pail of water down.the pipe occasionally to keep the groundin a wet condition.-G. DALLAS (Watford),

280 PRACTICAL WIRELESS June 15th, 1940

From Crystal to ValveHow to Use a Valve as an H.F. Amplifier for aa replacement for the Crystal Detector.

THE addition of a valve or valves foramplification of the signals- afterrectification by a crystal detector

has already been explained, thereforealternative arrangements are discussed inthis article so that a, very efficient crystal/valve combination can be constructed.

After a reasonable period of experiment-ing with the original circuit, plus the L.F.amplifier, the constructor will, no doubt,wish tO make further progress towardsincreasing." the effective range of theapparakusr., .This can be achieved in twoways;,;,to, valve can be used as an H.F.(high-freqnency) amplifier, between theaerial andAe crystal set, or an alternative

mearrangent ;.would be to replace thecrystal with :a triode valve acting as asimple reagtirAg detector. The first methodwould increase range and selectivity whilestill retaining the pure reproduction quali-ties of the crystal, and, for these reasonsalone, many music lovers will be temptedto model a circuit along the followinglines. A stage of H.F. amplification, usinga modern variable -mu H.F. pentode, acrystal detector, and a good L.F. amplifieremploying one, two or three valves accord-ing to individual requirements. The secondmethod; i.e., replacing the crystal with avalve detector, will appeal to those whowish to reach out to the more distantstations, and experiment with aomethingmore alive and more active than a crystal.The advantages to be gained are these :greater'sensitivity, chiefly due to the wiseuse of reaction, improved selectivity, whichcan also be put down to the reaction circuit,and considerable increase in output. Againstthese items we have the possibility ofintroducing distortion, slight extra drainon the batteries, that is assuming thesehave already been secured for the L.F.

-circuits previously described, and the costof an extra valve. The question of distortionis, in the majority of cases, closely connectedwith unwise use of reaction, unsuitabledetector valve and incorrect operatingconditions. It will be seen, therefore, thatas all these items are within the controlof the operator, the whole problem of

faithful reproduction becomes far lessserious if a little knowledge is applied.

H.F. AmplifierA suitable circuit for this stage is shown

in Fig. 1 where it will be seen that a variable -mu H.F. valve is recommended. In prac-tice, it can be of the ordinary S.G. orpentode type, the latter being more satis-factory if powerful signals from a local

A

station are to be received. Many readersmight have a " straight " or non -variable.mu valve on hand, and wish to make useof it. That is quite permissible, althoughthe very fine volume control provided byone of the specified type will, of course,be lost. The only modifications to thecircuit shown, to enable a " straight " S.G.or pentode H.F. valve to be used, are thefollowing. Ignore the potentiometer, fixedcondenser C.3 and G.B. battery, and their

associated connections, and connect thebottom end of the aerial coil direct to thecommon negative earth line.

Fig. 2.-The H.F. transformer system of inter -valve coupling isshown in this circuit.

Crystal Set, or asBy L. 0. SPARKS

Aerial CoilThis can be wound in the same manner

as that described for the original crystalcircuit but, as there will be no need tomake provision for a reaction winding, thetotal number of turns can be reduced to,Say, 60 and the number of tapping pointsconsequently reduced to five. The methodof connection will be clear from the diagram.

An 'additional .0005 mfd. tuning con -

60v,

75 A" onrystat SetTo E on Set

Fig. 1.-An efficientH.F. stage having awide range of applica-tion. An ordinaryS.G. valve can be used

if so desired.

denser will be required, together with afour- or seven -pin valve holder, accordingto the type of valve used, a .1 mfd. and a.0002 mfd. fixed condenser, and a reliablemake of H.F. choke. For those who wisha valve to be specified, I would suggest aCossor 210VPT.

The circuit shown is the simplest arrange-ment ; it is known as a " tuned -grid "H.F. coupling and, apart from being easyto construct, it is capable of giving verysatisfactory results. The output from thisH.F. stage is fed into the crystal circuit

(Continued on next page)

Fig. 3.-Hoto the original coil can be used to form a veryefficient one -valuer.

June 15th, 1940 PRACTICAL WIRELESS 281

FROM CRYSTAL TO VALVE(Continued from previous page). '

via the original aerial terminal and theoutput condenser C.4. The actual aerialnow being connected to the new aerialterminal as shown.

H.F. TransformerThis form of H.F. coupling requires two

windings on the inter -valve coil, namely, aprimary and secondary. The theoreticalcircuit is shown in Fig. 2, and it will benoticed that the H.F. choke and couplingcondenser of the former system are nolonger required. The additional winding,the primary, is connected between theanode of the H.F. valve and its positiveH.T. supply, the fixed condenser C.5 beingused for-decoupling purposes. The windingcan consist of, say, 25 to 35 turns of 32S.W.G. insulated wire wound over. theoriginal coil, preferably at the upper end.

Between the two windings, two layers ofthin, dry paper or other insulating materialshould be placed, and to this the ends ofthe primary winding can be anchored withsmall blobs of sealing wax or Chattertonscompound.

An alternative method of making theH.F. transformer is to make a coil former,from two or three layers of stiff, dry paper,having a diameter slightly less than theinside of the existing former. The coil isthen wound on it, the same number ofturns as mentioned above, and the com-pleted coil slipped inside the larger one.This, perhaps, is the neater and strongerarrangement.

Valve as a DetectorFor those who wish to dispense with the

crystal detector, the tapped aerial coildescribed can be used to good advantagein the circuit shown in Fig. 3.

The majority of connections, and themethod of adjusting them,' are practicallyidentical with those used for the crystalset, but in place of the crystal a gridcondenser is employed to feed the signalsto the grid of the valve. The additionalconnection to the coil is to provide reactionin conjunction with the reaction condenserC.6 which is connected between the anodeof the valve and the end of the winding.It should be noted that while this condenser,which is of the variable type, provides therequired control of the reaction, the amountof reaction effect obtainable with thecondenser specified will be governed by thenumber of turns of the coil used in thispart of the circuit, therefore, by means ofthe crocodile clips, it is possible to adjustmatters until the most satisfactory resultsare obtained.

A suitable valve would be the Cossor210H10.

ritish sta nce1M -criers' Club 4.

ReceiversTHE analysis of members' receivers

makes very interesting reading,especially in view of the minor, controversywhich is still going on between the com-munication receiver enthusiast and the0-v-1 men. On actual score there is nodoubt that the latter are well in themajority, and judging by some of thesplendid logs they send -in, there can belittle doubt that the simple receiver in thehands of an experiended and capableoperator can pull in the DX signals.

So many amateurs are inclined to confusevolume with signal intelligibility, and over-look such things as signal-to-noise ratio.For long-distance work it is absolutelynecessary to consider the readability of thesignal rather than the amount of noisewhich can be reproduced via the headphonea-or loudspeaker. Bearing this in mind, itis not usually advisable to add L.F. stagesin a haphazard manner, otherwise un-wanted noise will be amplified beyond allproportion to the signal. It is in thisdirection that the simple 0-v-1 scores, andif a member using a circuit of this type istempted to add another stage of L.F.amplification, he would do well to pauseand consider whether the actual gain asregards signal intelligibility will be all thatone might expect. Although the untunedstage of H.F. amplification is often treatedwith contempt by those who have failed toappreciate its true worth, there is much tobe said in its favour, especially when one isconsidering any additions to an 0-v-1circuit. It is usually far better to employan additional valve as an untuned H.F.,amplifier than as an L.F. stage, as theformer will improve the over-all efficiencyof the detector by reducing aerial damping,eliminating aerial resonant peaks and like-wise allowing the maximum efficiency tobe obtained from the reaction circuit.

These remarks are not intended to startthe old controversy off all over againthey are only given with the hope -thatthose who have despised the small sets willat least put in a certain amount of activework with them before making a sweepingcondemnation.

Correspondence

N°' that -the practice of sendingreports to transmitting stations

overseas is no longer permitted, one hasnaturally to turn to other items of interestassociated with our work, and Member6732, of Hull, shows us in a very definitemanner what great interest he is obtaining.His own words can best explain hisactivities. " Sincejhe war started I havebeen keeping an accurate record of theweather each day-i.e., temperature,whether sunny or dull, etc.-and I alsomaintain a daily log of all the short-wavestations received, noting such things assignal strength, time, frequency and at-mospherics. The weather records, and thestation, log can be compared with mostinteresting and illuminating results. Forexample, I have found that certain fre-quencies and areas are more affected byweather conditions than others. LastEaster, during the aurora borealis, VLQwas received exceptionally well."

Another very interesting lettei has beenreceived from Member 6443, of Lisburn,North. Ireland, who states, amongst otherthings : " About a month ago I built anR signal -strength meter. It is composedof a transformer, valve, and an 0-7 moving -coil milliammeter. After spending sometime calibrating it I have now got it goingvery well, and it proves of great value whenwriting up my log."

We certainly agree that an instrument ofthis type is most useful, and should certainlyform part of the equipment of -everymember. Why not send us more details,6443, so that we can pass on the benefits ofyour experiments to other members ?

Member 5767, of Bromley, Kent, alsogives details of -some of his practical workrelating to his aerial experiments. " Theaerial here is a half -wave Windom for20 metres, running N.N.E. by S.S.W.The north end is 37ft. high and the south end30ft. The Windom is quite a good arrange-ment for short-wave working once thecorrect tapping point has been found.When bare wire is used for the aerial, it isa comparatively simple matter to connecta crocodile clip to the feeder and determine

the correct tapping point lay adjusting thaclip until the desired results are obtained.When, however, insulated wire is used, thematter is rather more difficult, but byadopting the following idea the trouble wasovercome. I soldered the fe ader to a safety -pin., and was thus able to pieree the in-sulation and make contact with the con-ductor, and then when I had determinedthe correct point I was able to make anefficient weather-proof connection."

Member 6232, of Colwyn Bay, writes :" The sudden lack of interest shown bymembers towards the proposed DXcompetition stirs my own interest to a widerdegree. As yet I have laeen a ' silent 'member, but now I feel that I must havemy say on the subject. It is very disap-

inting that members who have so manytimes asked for such a competition backout at the crucial moment. Why thisstate of affairs should exist in such aflourishing club as our own is beyond me.I quite realise that the new drive to increaseour industrial output will probablynecessitate many of our members hav-ing to work overtime ; but if they werereally keen this would not damp theirardour.

In the issue of P.W.' dated June 1stit is suggested that members could keepin touch with each other by formingdifferent groups in their own area. Per-sonally I would be only too glad to act as a' go-between' for this district. I shallbe here until about the end of July, andduring that time I will place myself at theclub's service to, whip up enthusiasm.

" Wishing you and the club all the verybest."

Change of AddressWILL all members please note that

Member 6172 has now changedhis address to 93, Kinfauns Road, Good-naayes, Essex.. His previous QR-A was64, Nicholas Road, Dagenham, Essex.

StationeryWE are getting so many requests from

new members for details of theB.L.D.L.C. stationery available for theiruse, that we make no apologies for gi'vingthe complete list below.

Pad of 50 verification forms, Is. 6d.Pad of 50 log -book sheets, is. 6d.Packet of 50 sheets headed notepaper,

ls. 3d.White manilla folder for log -book sheets,

7id.Membership badge, is.

282 PRACTICAL WIRELESS June 15th, 1940

A Remote Control SystemConstructional Details of a Novel Apparatus

By Cecil Andrew, A.M.I.R.E.WHEN amateur transmitting was

closed down, there remained a" void " for many readers as

far as experimenting was concerned. Thisled the writer to turn his attention to thedevelopment of apparatus he had beenusing for the remote control of his trans-mitter.

It was found that this method of controlcan be applied to many uses, from trans-mitting gear to receivers, telephones, firealarms, bell circuits, television, A.R.P.requirements, and practically every branchof electrical science.

The basis of the arrangement can be seenin Fig. 1, andis constructed from second-handparts obtained for a few shillings from themany vendors advertising such com-ponents. In the first place a number ofautomatic telephone dials were obtainedfor about 2s. 6d. each second-hand, as well aswhat is technically termed a " uniselectorswitch " sold second-hand for about 7s. 6d.This switch was sd adapted to have 50separate double line circuits, " step bystep," any one of which can be selected bychoosing the number on the " dial.'Upon releasing the last number, a relayautomatically switches out the selectingdevice and places the line calling through towhatever relay or apparatus is con-nected to that number, thus allowing theoperator to control any instrument or, ifpreferred, place himself in telephoniccommunication with that point.

Constructional DetailsAt the bottom of Fig. 1, on the left,

can be seen the dialling box, placed thereonly for the convenience of photographing.Attached to this is a flexible lead con-nected to a valve base with 3 or 4 pins.This can be taken from point to point, andplugged into a valve -holder, so that the" exchange " can be controlled. Nearlyall the relays are home -constructed fromold bell coils, etc., with the exceptionof the bank of relays, seen in the topleft-hand of the illustration, which were

0

Line

Line C

Changeover relay

Fig. 2.-Diagram of connectionsfor the remote control system

described in the text.

purchased for about 2s. each. These con-tain multiple contacts which allow anycombination of circuits. The writer usedto control his transmitter from any numberof points connected to the " exchange " bya pair of line wires and earth connection.By dialling a number the mains were firstswitched on, another number would lightthe filaments, another number would bringin the H.T. to any stage, and,, finally, any" mike " required could be brought in bydialling the appropriate number. It wasfound that home-made polarised and" latchet " type relays were the mostsuitable for this purpose, as one numberwould hold the circuit closed, and anothernumber dialled would then release it.Fig. 1 also shows two " mercury switches "for heavy mains work.

Several of the home -constructed polarisedrelays were made from old type loud-speaker magnets, between the poles ofwhich moving from side to side soft ironarms are pivoted. These were wound with acoil connected by flexible leads to terminals.When the current was sent in one directionthis arm moved across to one side contacts,and when in the other across to the othercontacts.

This relay rack is at prese,nt used toremote control one or two communicationreceivers, and by dialling a prearrangednumber a small induction type electricmotor is operated' through very slow wormgearing to move the tuning dial of thereceiver. Then another number will con-nect a, morse recorder to the output, andso on.

No doubt there are many other readerswho prefer to keep all their apparatusand experimental receivers in their own" den " away from the risk of domesticdamage, and it is in this case that remotecontrol is so convenient, enabling one tooperate from any room or fireside withoutloss of time. Further, a, simple telephonein workshop and other rooms was found tobe very useful, the interconnection beingself-controlled.

Ratchet rev, Fblarized4e4ya

I ' t Magnet ffizolm'

r I9 Coil

Line A

A -Linecontacts00

e WiperA

Selectormagnet

Zero settingcontacts

Contact breakeron selector magnet

I

Fig. 1.-The general arrangement of the completeremote control apparatus.

The dials are purchased with contactsnormally closed, but these were altered sothat they remain open when the dial is atrest, and close as it is rotated. This avoids.the need of extra relays, and is much moresimple for the purpose. The bottom tworows 'of contacts on the selector switch areall connected together \with the exceptionof the last one, and this arrangement isused as a self -restoring device. By pressingthe white button on the control box, thisswitch returns to " zero " ready for thenext selection. Inside the box is a 4-,1 voltbattery connected to a D.P.D.T. switch sothat the polarity can be reversed on the"line " to operate the polarised relays ineither direction.

OperationIn the normal position when all relays

are at rest, the " A " and " B " lines(Fig. 2) can be traced right through fromthe D.P.D.T. switch and the dine plugsockets to the " A " and " B ", contactsupon which the two wipers are resting, sothat any current sent through the line orapparatus connected to it will have athrough connection to any relay on thosepair of contacts on selector switch. Imme-diately the dial is operated and leavesits normal position, the contacts marked" C " will earth the " C " line through the6 -volt battery and operate the change -overswitch, thus pulling the two contacts offthe " A " and " B " wipers over to theselector switch circuit, and every time thedial passes a contact number the selectorswitch is operated once, and moves thewiper on to the next pair of contacts. Thiscycle of movements takes place for thenumber dialled, and therefore the selectorwipers always move to correspond to thenumber required: If we want 18 we dial 10and 8, and we shall find the line throughto the No. 18 pair of contacts, which will

li

li

1,

June 15th, 1940

allow us to operate the relay connectedto this number by turning the D.P.D.T.switch which sends a current through theline, or we can plug into the line anyapparatus such as a telephone, or radio,etc.

When the use of this number has beencompleted the zero set button is pressed,which " earths " the " B " line, and byholding down this button for a few seconds,the bottom zero set contacts, which areall connected together except the last one,pass a current through the selector magnetwhich now has the contact breaker on thisselector in circuit. This, keeps the rotarywipers moving around until they rest onthe last unconnected contact when the

PRACTICAL WIRELESS 283

circuit is broken, and the selector comes torest ready for the next dial operation.

There will be found on the " uniselectorswitch " usually five rows of 25 contacts.The top two rows are used for the linecontacts and the bottom row used andconnected for zero set. If more than 25 linesare required, it will- be necessary to employthe top two rows for, " A " line and thethird and fourth rows for " B " lines,cutting off- the wipers on_ one side only.This will give 50 lines. All this seems verycomplicated but is really quite simple inpractice with the apparatus described. Aratchet relay is shown connected to numbers1 and 2. When -No. 1 is dialled, and acurrent passed through the line, the arma

ture is drawn down under the latch whichholds this down against the bottom con.toots. By dialling the next number thislatch is pulled back to release the armatureagainst the top contact. The polarisedrelay moves from side to side on one numberonly, according to the direction of thecurrent. The writer has so arranged hisrelays that only two lines and earth arerequired for full scale operation, but threelines are shown in Fig. 2 in order to simplifythe diagram, but the reader will soon dis-cover many variations for himself.

It might be mentioned that the selectorswitch can be operated from an old H.T.battery, but the writer uses old wet high-tension accumulators for economy.

LATEST PATENT NEWSGroup Abridgments can be obtained from the Patent Office, 25, Southampton Buildings, London,W.C.2, either sheet by sheet as issued on payment of a subscription of 5s. per Group Volume or in

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Specifications Published.521278.-Cole, Ltd., E. K., and

These particulars of New Patents of interest Shackell, A.-Tuning of radio -to readers have been selected from the Official receivers.Journal of Patents and, are published bypermission of the Controller of H.M. 521209.--Kolster-Brandes, Ltd., andStationery Office. The Official Journal of Beatty, W. A.-Radio-receivers.Patents can be obtained from the Patent 521350.-Cole, Ltd., E. K., andOffice, 25, Southampton Buildings, lindon, Robertson, N. C.-Method ofW.C.2, price Is. weekly (annual subscription

/2 les.). making electric connections inLatest Patent Applications. radio -receivers or the like.

8972.-Amalgamated Wireless (Ans. 521367.-Jones, W., and Pye, Ltd.--tralia), Ltd.-Biasing circuits fer Television and like systems.electron -discharge tube circuit (Cognate Application 143539.)arrangements. May 20th. 521330. - Philips Lamps, Ltd. -

8762. --Philo() Radio and Television - Superheterodyne wireless receiv-Corporation.-Electron-beam de- ing-sets.fleeting circuits. May 16th. 521183.-Philips Lamps, Ltd.-Radio

9077.-Philco Radio and Television receiving -sets.Corporation.-Automatic volume Printed copies of the full Publishedcontrol system. May 22nd. Specifications may be obtainedf rom

8836.-Philips Lamps, Ltd.-Control the Patent Office, 25, Southamptonmechanism of wireless receivers. Buildings, London, W.C.2,-at the IMay 17th. 'uniform price of ls. each. i

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NEW PATENTS

New Cossor TransportableTHEvalves has led to some interesting

recent introduction of " all -dry"

transportable and portable receiverdesigns, the latest of which, from the Cossorfactory, is illustrated on the right. The newvalve dispenses with the need for anaccumulator for the L.T. supply, and as aresult a more compact and lighter type ofreceiver is possible. The L.T. supply isobtained either by a small separate drycell, or by a section of a special type ofH.T. battery designed for the purpose,This is the arrangement used in the Cossorreceiver, the battery being of the 90 -volttype. A self-contained frame aerial is fitted,and there are four valves in the receiver.The output stage is a pentode, and thecircuit is of the superhet type with per-meability -tuned I.F. transformers. Auto-matic grid bias and A.V.C. are included inthe circuit refinements, whilst the outputis fed to an 8in. P.M. moving -coil speaker.There are three controls, the centre being aslow-motion tuning with concentric tonecontrol, the others being wave -change andvolume controls. Normally no earth isrequired with the receiver, but sockets areprovided so that when desired an externalaerial and earth may be employed. Thereare also sockets for the inclusion of anexternal loudspeaker, these sockets beingprovided with switching.

The wavebands covered are from 190 to560 and from 830 to .2,000 metres, and thecabinet measures 20in. by 16in. by 10in.deep. The price of the receiver is£9 17s. 6d.

The new Cossor transportable.

1PROGRAMATEI-

w....NOTES14=14141M.0.0=114M.N100411111.41.11.0411if.I.0 NIM141111111-Ma

Wedding CroupPHILIP WADE, well-kno ckm. member of1 the B.B.C. Repertofy Company,is also familiar- to listeners for his workas an author and an adapter of works forbroadcasting. It will be recalled thata recent adaptation of his was of scenes from" Pickwick Papers." One of his plays,which is to be 'produced in the HomeService programme on June 14th, byHoward Rose, is " Wedding Group." This isa revival of a work which was originallyproduced five years ago. The story beginswith a modern setting on the eve of thewedding of the young couple. The bride-to-be's mother shows the young couple anold family photograph album in which thereis a family wedding group which dates backbefore the Crimean War. In a delightfulway Philip Wade brings this weddinggroup to life and tells the story of theromance behind it.

Cabin No. 3THE cruises of luxury liners have not

provided such a spate of romanceand crime stories as might have beenexpected, but Florence A. Kilpatrick hasused the subject as the peg on which tohang an exciting play which is to beproduced by Peter Creswell on June 13th.This is a story of a trip from Rio de Janeiroto Tilbury, and the main characters are awealthy woman globe-trotter, her prettyyoung companion, and a stowaway whoappears to be mixed up with a gang- ofemerald thieves who are after the globe-trotter's jewels. There is both crime andromance in the play, for the pretty youngcompanion manages to save the designingstowaway from his worst self and the storyhas a happy ending.

BOOKS RECEIVED I._LOAMW041...0410041.10.04111.0.=.041.0.MWOM.01M1.04111M0.W

DEFINITIONS ANii FORMULAE INRADIO AND TELEVISION ENGINEERING.By A. T. Starr, A.M.I.E.E. Published bySir Isaac Pitman and Sons,- Ltd. 50 pages.Price 6d.

THIS useful pocket handbook, intendedfor students of radio engineering,

contains a large number of definitions,formulae and circuits, covering many phasesof radio, and based on standard practice.The tAok contains sufficient information forrefreshing the memory as occasion requires.

284 PRACTICAL WIRELESS June 15th, 1940

Open to. ViicalitanThe Editor does not necessarily agree with the opinions expressed by hiscorrespondents. All letters must be accompanied by the name and address

of the sender (not necessarily for publication).

A "Quality" EquipmentSIR, -I enclose a photograph of my

equipment which I have built withthe aid of the knowledge obtained fromyour excellent paper.

The top of the rack contains an 8in.Epoch speaker (P.M.), which I use forDX work, though this was only put thereto take. ,the place of a low -power C.W.transmitter which I was going to put inthe rack if transmitting had continued.

The middle rack contains the H.F.portion of the rig, and this consists of aVP215 R.F. amplifier which is 'tuned onall bands, which are fed into a triodedetector. This part takes care of theDX, and it has a S.W. range of 12-60 metres,while the usual broadcast bands are alsoincluded.

A turntable is next, and this is fitted witha crystal pick-up for quality work withrecords. The bottom rack holds the mostinteresting part of the gear, which is theL.F. amplifier.

When on the radio the valve line-up isLP2 (transformer coupled to the detector),which is in turn transformer coupled toa 19 (Class B) valve, which has an outputof over 2 watts. When over to the gramo.the line-up is HL2 pre -amplifier resistancecoupled to an LP2, which is transformer -coupled to the output valve. The qualitywith this line-up is exceptionally good, and

-the output of the Class B valve is fed into a.Celestion (P.M.) 10in. speaker. Switchingfor the L.F. amplifier is done by a multi -cut -out switch, while the two D.P.D.T.switches are for cutting out the Class Bvalve when working on 'phones, or theDX speaker. The speaker case may interestsonic quality " fans," as it consists of astrong box 2ft. 6in. square by lft. deep.The inside is lined with felt kin. thick andcorrugated paper on the outside. Thefront is 5 -ply wood, and perforated zincwas used over the speaker aperture. Thisgreatly improves the quality, which wasgood before, but this extends the frequencyrange greatly in both directions. Thewhole rig is built primarily for qualitywork, though it is also good on DX. Theaerial used is a 20 m. doublet, and the rigis entirely run from batteries with 180 H.T.Here's wishing your magazine all the bestin the future.-C. HEYNE (Britton Ferry,Glam).

A Simple S.W. One-valverSIR, -I enclose a short.. log of stations

received with a simple single -valveshort-wave set _I have built, which may beof interest.

Aerial tuning condenser is .00001 mfd.Coil: Grid winding 6 turns No. 16

standard gauge bare copper wire on 3in.ribbed former. Reaction winding 5 turnsNo. 20 D.C.C. wire. Grid condenser is.002 mfd. ; grid leak, 2 mQ. Ordinarybroadcast valveholder. Tuning condenserand reaction condensers, .00015 mfd.These I made by removing half the platesand double spacing ordinary air -spaced.0003 condensers. An ordinary short-waveH.F. choke is used.

With this set and an aerial consisting of10in. of No. 16 copper wire connectedvertically to the grid terminal of the coil,I have logged the following stations :EA7BA, W2ECR, EK1EF, W1CRW,W1DLM, LY1J, ES1E, ES4G, I1JKV,K4END, W4XDK. With an aerial of

Mr. C. Heyne's equipment for quality workwith records, and for DX work.

approx. 15ft. No, 20 enamel -covered wirehung round the room, the followingcommercial stations came in : W2XAD,2R06, DJD, GSQ, WIXAL.

erai s

PROBLEM No. 404.KENT had a three -valve battery receiver

which gave splendid results on 'phones,but when he connected a loudspeaker he wasdisappointed with the results. He was assuredthat the speaker was in good condition andcapable of good results, so he decided thatperhaps results would be improved if he didnot use a direct output feed. He accordinglyderided to parallel feed the speaker, but ashe had not got a suitable L.F. choke availablehe used a 50,000 -ohm fixed resistance (bearingin mind that the effect is similar to resistance -capacity coupling). He fed the speaker fromthe anode through a .1-mfd. condenser, butthe results were worse than ever. What waswrong ? Three books will be awarded for thefirst three correct solutions opened. Entriesmust be addressed to The Editor, PRACTICALWIRELESS, George Newnes, Ltd., Tower House,Southampton Street, Strand, London, W.C.2.Envelopes must be marked Problem No. 404in the top left-hand corner and must beposted to reach this office not later than thelirst post on Monday, June 17th, 1940.

Solution to Problem No. 403.When Alatt hews added his screen potentiometer he

overlooked the fact that this was joined between B.T.positive and 11.T. negative, and accordingly a switchshould have been included to isolate this when theset was switched off. Failure to do this resultedin the battery discharging continuously with theresults which he experienced.

The following three readers successfully solvedProblem No. 402 and books have accordingly beenforwarded to them :M. Buffington, Yorktown Road, Sandhurst, Berks.W. Brown, 40, Blantyre Street, S.W.10.D. McLean, 49, Newark Drive, Glasgow, S.1.

I am a regular reader of your journal,and have always found it a great help tome.-W. G. MORRIS (London, S.E.).

"Radio Training Manual"

SIR,May I take this opportunity of

expressing my complete satisfactionwith the " Radio Training Manual "

I have studied this unique book fromcover to cover, and have found that itfully explains a number of points whichare extremely valuable, and yet are passedover lightly in most text -books.

In particular, I was highly interesteain the chapters on the design of the detectorstage and the principles of receiver design.

Many radio constructors, with a flairfor the technical side of things as well asthe constructive, would, I am sure,appreciate a series of articles in PRACTICALWIRELESS incorporating such interestingsubjects as high -frequency transformers,stability in high -frequency circuits, sep-arating the three electrical currentcomponents and oscillatory circuits.

To be brief, a series embracing the wholeof the modern radio receiver, and dealingwith it in detail, as you have done withthe detector stage in the " Radio TrainingManual." -Most enthusiasts wish to knowwhy this, that, and the other must be ofsuch a value, instead of being told thatit should be so.

PRACTICAL WIRELESS is certainly doingits bit 100 per cent. in keeping the wireless -minded public informed of all the latestdevelopments in this ever-expanding field.

May it long continue to do so !-J. T.JACKSON (New Barnet).

Station CR7BESIR, In a recent issue of PRACTICAC,

WIRELESS, R. Scotten, of Leyton,states that he picked up a Portugese EastAfrican station broadcasting on 30.8 metres.This station would be CR7BE LourencoMarques, Mozambique, on 30.8 metres(9.71 me/s). This station can be heardbroadcasting in English from 20.00 to21.00 B.S.T. every night.-C. MERRETT(Evesham).

Correspondent Wanted

I J. TOBIAS, 56, Reginald Road, Crosby,1 1 Scunthorpe, Lines, would like to getin touch with any reader who has built the" Midget S.W. Two."

RADIO CLUBS& SOCIETIES

EDGWARE SHORT-WAVE SOCIETYHeadquarters : Constitutional Club, Manor Park

Gardens, Edgware, Middx.Chairman : P. A. Thorogood, 35, Gibbs Green, Edg-

ware.ON Saturday, June 29th, from 4 to 10.30 p.m.

there will be an exhibition of apparatus bywell-known manufacturers; a demonstration of theirnew pick-up and loudspeakers by Messrs. VoigtPatents, Ltd.; an unusual junk sale; Morse speedcompetition ; discussion by the secretary of R.S.G.B.;competitions, and in the evening a social and dance.Admission sixpence.

ROMFORD AND DISTRICT AMATEUR RADIOSOCIETY

Hon. Sec. : H. G. Holt, 5, Butts Green Road, Horn -church.DURING April, and the early part of May, many

members demonstrated apparatus they had madeand queries were duly settled.

On May 28th we had a very enjoyable evening withMr. Voigt, and his loudspeaker. Using his new e?c-perimental pick-up he gave a demonstration whichfar exceeded our expectations. Reproduetion was thebest we had ever heard, and everybody enjoyed it.'

June 15th, 1940 PRACTICAL WIRELESS 285

In replyeov

Itryour lettera

Tone Improvement" I have a four -valve battery set which,

whilst good so far as all normal require-ments are concerned, could, I think, dowith some improvement in the quality of itsreproduction. This is particularly ar liv-able to the gramophone side, and I appenda circuit and would be glad if you wouldsuggest a good tone -control device whichwould be guaranteed to give good results.I have tried several schemes withoutappreciable improvement."-K. M. (Wok-ingham).

YOUR circuit modifications may havebeen effective, but your speaker, the

cabinet, or other acoustic details may haveprevented the effects from being noticeable.Have you tried a " constant " tone -controldevice ? This might give the desiredimprovement and it should be includedacross your volume control used with thepick-up. It consists of a 50 millihenrychoke and a 15minfd. condenser in series.The condenser is joined to the earth endof the pick-up volume control and the endof the choke is joined to the arm of avariable resistance having a value between

,5,000 and 10,000 ohms. The resistance isconnected to a tapping on the pick-upvolume control, the best point being foundby experiment. About one -fifth of thewinding is generally satisfactory. Thetapping is, of course, on the earth end.If you cannot tap the volume control youcan obtain a similar effect by connectinga fixed resistance in series, but it is prefer-able to tap the control as this then enablesthe resonant circuit to be cut out at lowsettings and this gives the desired compen-sation.Economy Amplifier

" I wish to build an economy amplifier,capable of giving a really good output fordomestic purposes. I do not want dancingvolume, but something a little above theordinary for good music, and with goodquality. What arrangement and circuitwould you recommend ? "-D. F. (Col-chester).

THE 21 -watt battery amplifier whichwe described some time ago would,

we think, be the most suitable for yourpurpose. This consisted of a single L.F.stage with parallel -fed transformer feedingtwo pentodes in push-pull. The currentconsumption is fairly high, but by using areally good H.T. battery this should notbe a difficulty. The amplifier is economicalto operate and not expensive to build.

, Full details were reprinted in our issuedated December 23rd last.S.W. H.F. Stage

" I have been using a detector-L.F.I short-wave combination, but a friend hastold me that I could improve my results byfitting an untuned H.F. stage in front of thedetector. I do not see how an untunedstage can give any appreciable advantage

land should like your advice on this beforemaking any change."-J. E. T. (Cam-

' bridge).A LTHOUGH an untuned stage may not

11-X giveas muc'h improvement as a tunedstage there are advantages to be. gained by

its inclusion. Firstly, the inclusion of thevalve before the detector will result in aslight increase in amplification. Secondly,the inclusion of the stage between aerialand detector tuned -grid circuit will over-come erratic effects due to the dampingof the aerial, and probably result in smootherreaction and consequent increase in thestrength you -may obtain on some distantstations. Another point is that on thelower wavelengths the gain obtained byusing a tuned H.F. stage is not appreciableand thus there is no object in using thetuned circuit. The buffer stage, as theuntuned arrangement is called, is, however,always worth while.

RULESPIIMµ049

We wish to draw the reader's attention to the Ifact that the Queries Service is intended only Ifor the solution of problems or difficulties Iarising from the construction of receivers idescribed in our pages, from articles appearingin our pages, or on general wireless matters.We regret that we cannot, for obvious reasons- - I

(1) Supply circuit diagrams of complete Imulti -valve receivers.

(2) Suggest alterations or modifications of Ireceivers described in our contem-poraries. -

(3) Suggest alterations or modifications tocommercial receivers.

- (4) Answer queries over the telephone. i(5) Grant interviews to querists.A stamped addressed envelope must be

enclosed for the reply. All sketches and idrawings which are sent to us should bearthe name and address of the sender.

- Requests for Blueprints must not be enclosedwith queries as they are dealt with by a ii separate department.

! Send your queries to the Editor, PRACTICAL WIRELESS,LGeorge Newees, Ltd., Tower Rouse. Southampton Street,Strand, London, W.C.E. The Coupon must be enclosed

with every query.1111=1141.1.111MINNEMIII.4.111100.11104

Noisy Controls" I am finding it difficult to adjust my

volume control without a noisy background.I am afraid, however, that the control itselfis not responsible for the noise, as I haveexamined it very carefully and the arm ismaking good contact all over the element.Is it possible to get noise with good contact ?Failing this, is there any way in which Ican -find out the cause of my presenttrouble ? "-A. E. (Isleworth).IT is possible to get noise from a controlI which apparently has good contact.But this would only occur if H.F. waspresent, and then a poor H.F. resistancewould be responsible. In some cases thiscould be overcome by shunting the controlby a fixed condenser. If the control ison the L.F. side of the receiver there shouldbe no H.F. present and faulty H.F.-filteringwould therefore be the thing to overcome.You may find, however, that the controlitself is not responsible, but that whenvolume is turned up the increased vibrationcauses a poor contact to be jarred in someother part of the set and that this givesrise to the noise.

L.F. Transformer Design" I have been looking into the question

of improving my set and have come acrossa small problem which I should like you tosolve. I have two transformers, at leastfive years old. They are substantiallyconstructed, andrI have always understood

that this is one of the most importantfeatures of an L.F. transformer. It nowseems that modern transformers are ex-tremely small and I am not certain that bypurchasing one of these I shall obtain anyimprovement from the quality point of vie*.I wonder if you could confirm this ? "-H. E. R. (Colindale).

YOU have apparently overlooked thefact that modern transformers do not

always employ the old form of core. Inthe early types of transformer ordinaryStalloy stampings were employed, but manymodern transformers make use of specialalloys, such as nickel -iron. This permits ahigh inductance to be obtained in a muchsmaller space, and therefore, it may bepossible in certain circumstances to obtainbetter quality with a modern small trans-former than with an old pattern largetransformer of poor design. It is, of course,still possible to obtain high -quality trans-formers of substantial design, such as theFerranti.S.W. Oscillator Coil

" In reading about superhets I see thatthe oscillator section has to tune to 465 kc/s.From tables I find that this, 13 equivalent toabout 600 metres. - Now I have a short-waveset tuning down to 9 metres, and on lookingat the oscillator coil I find it only has a fewturns of wire and is tuned by a very smallvariable, so I fail to see how it can go upto 600 metres. Perhaps you could explainthis point to me."-S. L. (S.E.14).

THIS is a point where thinking in metres,rather than in kilocycles, has helped

to confuse you. You have also overlookedthe principle of the oscillator stage. It istrue that the I.F. stages are tuned to 465ko/s and that this frequency has to bedeveloped in the frequency -changer stage.However, if you think in kilocycles you willsee that the required frequency is obtain-able, even down to 1 metre. Take, forexample, a station working on a wave-length of 15 metres. This is a frequencyof 20,000 kc/s. Now to obtain a beat of465 kcis you must tune the oscillatorcircuit to either 20,465 kc/s, or 20,000minus 465 kc/s, or 19,535 kc/s. A frequencyof 20,465 kc/s corresponds to a wavelengthof about 14.65 metres and 19,535 ice'scorresponds to about 15.5 metres, and thus,you see that there is very little differencebetween the actual tuning circuit and theoscillator circuit, and you do not need alarge coil to cover 600 metres. We trustthe above explanation clears up yourdifficulty.Midget Coil Connections

" I built up your small receiver sometime ago, with a Bulgin midget coil in it.I dismantled the set and now wish to makeup a similar one, bat I have lost the codefor the coloured leads of the coil. Is itpossible to give me the wiring for thiscomponent? "-R. T. (Edinburgh).

THEcoil in question had a fixing bracketon it and you should not have re-

moved this in dismantling the set, as it isin contact with the lower end of the gridwinding and must be earthed. The tuningcondenser should be joined to the red leadand the on -off wavechange switch shouldbe joined between earth and the blue lead.The grid condenser may be joined eitherto the green lead or to the red one whichis already connected to the tuning con-denser (the other side of which is, of course,earthed). The primary winding is betweengrey and yellow, the latter being the earthyend.r,.....14WIHNIN.014=011.111.11=1.11.111.11111.11.11111.141.1.11.r6

The coupon on page 288 must bg

1.0attached to every query

286 PRACTICAL WIRELESS June 15th, 1940

Reducing Adjacent Channel InterferenceDetails of a New Type of I.F. Transformer

THE sensitivity of modern super-heterodyne receivers is such that,when they are tuned to the frequency

of any given broadcast channel, theyrespond to some extent to signals in anadjacent channel, and it is desirable toprovide some means for reducing theadjacent channel interference. Such re-duction is best effected in the intermediatefrequency circuits because in these circuitsthe currents to be transmitted are of thesame frequency for all signal channels and,as the broadcasting wavelengths are spacedapart by 10 kilocycles, the currents corres-ponding to adjacent channels to beattenuated are also of fixed frequency of10 kilocycles above and 10 kilocycles belowthe intermediate frequency respectively.

The coupling arrangement about to bedescribed provides an inexpensive meansfor the reduction of adjacent channelinterference, and has the advantage that itrequires a minimum of adjustment.

+

To superhet converter

Fig. 1.-The early stages of a

Fig. 1 shows three cascade -connectedvalves, 1, 2 and 3, the first of which is afrequency changer, while the others areintermediate frequency amplifiers. Theanode circuit of the valve 1 includes theprimary winding 4 of a coupling trans-former which has secondary windings 5 and6, connected in series in the input circuit ofvalve 2. Similarly, the anode circuit of valve2 includes the primary winding 7 of acoupling transformer which has secondarywindings 8 and 9 connected in series inthe input circuit of the valve 3.

The primary windings 4 and 7 and thesecondary windings 5 and 8 are tuned tothe intermediate frequency by condensers10, 12, 11 and 13 respectively, thesecondensers being adjustable, in the mannerof the usual trimmer condenser, for thepurpose of initial adjustment. When sotuned, the voltage in the secondary ofeach transformer is displaced in phasewith respect to the voltage of the primarywinding by 90 degrees when the inter-mediate frequency is correct. When thisfrequency shifts, however, from the correctvalue, this phase relation changes and thesecondary voltage becomes more nearly inphase with the primary voltage, or morenearly opposed to it, depending upon thedirection of the shift in frequency and, ofcourse, the magnitude of the phase displace-ment is dependent upon the magnitude ofthe frequency shift. The secondary winding6 of the first transformer and the secondarywinding 9 of the second are untuned, but

these windings are coupled to theirrespective primary windings to haveinduced in them a voltage which is at afixed phase relation with respect to theprimary winding, and which thereforeconstitutes in the secondary circuit areplica of the primary voltage. Accord-ingly, this voltage may be made exactly toequal and oppose the voltage in thesecondary winding 5, or 8, when thefrequency has shifted from the intermediatefrequency by the amount of 10 kilocycles.The voltage of the secondary winding 5may be adjusted to be equal and oppositeto the voltage of the winding 6 at afrequency of 10 kilocycles above theintermediate frequency, and similarly thevoltage of the winding 8 may be madeequal and opposite to the voltage of winding9 at a frequency of 10 kilocycles belowthe intermediate frequency. Thus, atthese two adjacent channel frequencies,infinite attenuation in the system is

produced, the adja-cent channel abovethe intermediate fre-quency being infinitelyattenuated in thecoupling device 4, 5, 6,and the opposite ad-jacent channel beinginfinitely attenuatedin the coupling device7, 8, 9.

Voltage A d jus t-mentThe voltage adjust-

ment producing infinite- attenuation can best

be obtained by theuse of resistors as

standard superhet. shown at 14 and 15,one of these resistors

being in circuit with winding 6, and theother in circuit with winding 9. Theseresistors may be either in series with therespective windings or in shunt, and thediagram illustrates both methods of con-nection. Both resistors may; of course beconnected in the same way, and theirvalues are selected to assist the securing ofthe desired phase relations between thevoltages in the secondary circuit.

Since the voltage on winding 5 is displaced

in phase from the voltage on winding 6 by90 degrees at the intermediate frequency,it will be observed that windings 4 and 5may comprise the normal transformer

16

11111 NMIIa MIR

111N1111111111

1111111111111111111111

4

Fig. 2.-Method ofwinding the type ofI.F. transformer des-cribed in this article.

coupling coils, since at the intermediatefrequency the coupling is not appreciablyaffected by the winding 6. For this reasonmanufacturing variations in the coils 4and 5 do not seriously affect the frequencyat which infinite attenuation occurs, withthe result that adjustment to take care ofsuch manufacturing variations to securethe infinite attenuation of the adjacentchannels is not necessary. In fact, it hasbeen found that the adjustment of thecoupling between coils 4 and 5, and similarly7 and 8, to secure infinite attenuation atthe adjacent channels is not more criticalthan the adjustment for critical couplingin the conventional intermediate frequencytransformer.

Fig. 2 shows the structure of the trans-former comprising the windings 4, 5, 6,in Fig. 1. While the arrangement of thewindings and the structure of the trans-former may vary widely, in the constructionshown the secondary winding 5 comprisestwo coils mounted at one end of a coilsupport 16, and the primary winding 4comprises two coils arranged near theopposite end. The winding 6 is arrangedon the opposite side of winding 4 from thesecondary winding 5 and is more closelycoupled to winding 4 than is winding 5.

This system was developed in thelaboratories of the G.E. Company ofAmerica.

NEW. RECORDSDance Music

AMBROSE and his Orchestra have madetwo new records this month. They

are " You Made Me Care" with a viral byVera Lynn and " In an Old Dutch Garden 'on Decca F 7451, and " Indian Summer "and " My Capri Serenade " on DeccaF 7452. Jack Payne and his Band alsosupply dance music with " Walkin' Thru'Mockin' Bird Lane " and " Me and the Mokeand Liza " on Decca F 7454 and " Let theCurtain Come Down " and " RainbowValley " on Decca F 7453. Alex Moorepresents Mantovani and his Music forDancing on Decca F 7455-6. On the firstrecord is a slow fox-trot " No Souvenirs "and a quick -step " Dancing is AnotherName for Love and on the other aquick -step " There's a Boy Coming Homeon Leave " and " You Made Me Care,"which is a waltz.

Rex

VOCALrecordings are supplied this

month by that popular radio starElsie Carlisle, with her version of " Walkin'Thru' Mockin' Bird Lane " and " A LittleRain Must Fall " on Rex 9775 ; Joe Peterson,who sings " I'm Praying To -night forthe Old Folks at Home " and " YouMade Me Care " on Rex 9771, andGeorge Eldrick with " Arm in Arm " and" Light up Your Face with a Smile " onRex 9773.

Dance music is supplied by Billy Cottonand his Band with " The Navy's Here "and " Let the Curtain Come Down " onRex 9770, and " In an Old Dutch Garden "and " Rainbow Valley " on Rex '9766.Another popular dance band-Jay Wilburand his Band-have also recorded thismonth. They play " Indian Summer "and " Dancing is Another Name for Love"on Rex 9767.

June 15th; 1940

-.4 --.1:77

p yourSet a

1111E SI I

D .lzeh AVOMINORC.

Regd. Trade MarkELECTRICAL MEASURING INSTRUMENT

-Use the D.C. AvoMinorperiodically to check upvalve performance, bat-teries and power output.Thus you'll always keepyour set in. good trim. Ar3 -range precision - builtinstrument, it tells you allyou need to know, and en-ables you to locate speedily

the seat of any trouble.Direct readings. No

calculations. Com-plete in case, with.instruction booklet,leads, interchange-able test prods andcrocodile clips.

Voltage Current Resistance0- 6 volts 0-240 volts 0- 6"m/amps. 0-10,000 ohms0- 12 volts 0-300 volts 6,- 30 al/amps. 0-60,000 ohms0-120 volts 0-600 volts 0-120 m/amps. 0-1,200,000 ohms

0-3 megohms/MAD MADE

Write for fully descriptive literature andcurrent prices of all 'Avo' Instruments.

Sole Proprietors & Manufacturers :-Automatic Coil Winder & Electrical Equipment Co., Ltd,Winder House, Douglas St., London, S.W.1. 'Phone : Victoria 3404/7.

tOR THERADIO SERVICE MAN,DEALER AND OWNERThe man who enrolls for an I. C. S. RadioCourse learns radio thoroughly, com-pletely, practically. When he earns hisdiploma, he will KNOW radio. We arenot content merely to teach 'She prin-ciples of tadio, we want to show ourstudents how to apply that training inpractical, every -day, radio service work.We train them to be successful!

INTERNATIONAL,e0RiltirliNDENdE SCHOOLS:' . .

Dept. 94A, International Buildings,Kingsway, London, 'N.C.2.

Please explain fully about your Instruction inthe subject marked X.

Complete Radio EngineeringRadio Service Engineers

Elementary RadioTelevision

If you wish to pass a Radio examination,indicate it below.

Inst. of Wireless TechnologyP.M.G. Certificate for Wireless Operators

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(Use penny stamp on unsealed envelope.)

PRACTICAL WIRELESS

Metal PanelsTHE present shortage of -metal has

resulted in some difficulty inobtaining aluminium or copper for panels,chassis, etc. It may therefore be worthwhile remembering that a solid sheet ofmetal is not essential for the . averagereceiver where screening is required. Avery, good substitute for a metal panel is anebonite or wooden sheet, backed by ordinaryperforated zinc, This is still Callable,in fair quantities and as a screen is quitee ective. Another alternative is to usefine meth wire -netting. This is obtainablewith Bin. ,mesh (used for bird aviaries)and on most apparatus, when earthed,offers a satisfactory H.F. screen. Thismaterial is, in fact, often employed infactories and test -rooms for screening offa section of the area for test purposes. Achassis could be made from perforatedzinc, which soldeil nicely and offers onlythe difficulty that it is not rigid. Strength--ening pieces" may, however, be placed atcorners or along edges, and ordinary ply-wood or angle strip from the popularconstructional toy may be used for thispurpose.

Test ApparatusA DIFFICULTY which often besets

the designer of an all-purpose typeof test instrument is the selection of variousvoltage or other tappings. There is usuallya range of outputs available at each typeof test and it is often found that the mostsatisfactory plan is to use one or twoflexible leads in conjunction with a numberof sockets. Whilst this is a good planthere is the difficulty that should the plugbe in a wrong socket the instrument may bedamaged by connecting the test leads to,say, a high voltage for test purposes. Sucha ciculy may be overcome by fitting a fuse,but this generally affects the range of theinstrument or its accuracy. A better planis to make use of a multi -contact selectorswitch, or a bank of such switches, soarranged that all that has to be done is.to turn the indicator to the range desiredand the internal switching is automaticallycarried out. This, of course, still leavesthe human element, namely, that one mustwatch that the instrument is correctlyindicated before making the test, but itsimplifies this procedure as there will onlybe one indicator to attend to.

A New Book

NEWNES SHORT-WAVE MANUALA Complete Treatise on the Design, Construc-tion and Operation of all Short-wave Equipment

Price 5/- or 5/4 by postfrom George Newnes, Ltd., Tower House.

Southampton Street, Strand. W.C.2.

ELECTRAD IX-A Plain Scale

MAGNETIC I':"COMPASS

Indicates Currentflow, polarity, aerialbearings, visualtuning, etc. Bevelglass, brass d.body. A Britishitem for only

3;9 AI IL LI AM METERS. -New. Fer-ranti. Something simple withoutcalibration for tuning or gaivo fortesting. Back of panel type. as illus.,8 m.a. full scale. Great Bargain at 3/1).MILLIAIVILMETERS.-MovIng coil, 510, 25, 50, 500 MA., in various sizes, from21in, to Sin. dia. Switchboard Meters,3 to Sin. dial, all ranges. 3/9SOLENOIDS. -6 -volt for relay or distance switch,core travel din. pull 1 oz., 3/6. A.C. Magnets. 230 volts.30 m/a., 14 ozs. 11 t, 2/6. A.G. All voltages and sizes.

PHONES. For all purposes. Tableor wall, house or office 'phones, 10 -Headphones. Pocket teyoPe W.leather Headbands and 819.Radio 4,000 ohm 81W lightweightHeadphones, 5/- pr. 13.TH., Brownslightweight B.. 7/6.MORSE KEYS. Service Keys, seeList. A good small key on mouldedbase is the TX Mvot arm, excellentor learners. 3'd. Full size, wellnished key, all brass, solid pivot

bar, adjustable tinsion, etc.. 3.2, 7(6.Superior Type FF., full adjustable,nickel finish. 911.

High Grade Type IV, plated fittings,polished wood -base, a fine key, 1016.Special Key on 3 -switch box forbuzzer and 2 lamps, C.A.V., 612.BUZZERS, small type, withcover, lie. Power Buzzers,with screw contact and adjustable spring armature,

in Bakelite case. 376. MagnetoExploders, 25/-.

BuzzerCABINET AND COM-PONENTS FOR PAY,

80/- THREETudor Oak Cabinet 130th.by 71n. by 7in. Fitted.0005 Slow Motion Con-denser dial window, ver-nier microdenser 3 -wayswitch, chassis valve-holders wiring. clips,threeAxed condensersand 10 terminal panelstrip. 12:6.

Complete as detailed ONLY 101- to callers ; or 14/- Postand Packing Free.

CONDENSERS. Fixed electrolytic 12+4 mfd. =16 mid.275 volts, 1/6 6+6=12 mfd. 250 volts, 1/6 - 8+8-16 tad.350 volts, 116 : 24+8=32 mfd. 400 volts,. 2/6; 8 mfd-500 volts, 116 ; Iklansbridge Paper and Mica, 4 mfd.. 3;6 ;8 mfd.. 516 ; 10 mid., 7:- : Dubilier .01 for 1,000 volts, lib ;.005 Mica Trans. 5,000 volts, 2(6 ;10,000 volts, 5/- ; 25 mid.mica, 4,000 volts. 8/-. Oil Condensers, 1 mfd. 3.000 volts,5/6. Special Condenser Bargain to clear surplus.Western Electric 1 mfd. and 2 mid.. 3d. each. 2'- doz. ;12/ -gross. carriage forward. Ideal for makers of smooth-ing and filter Circuits. Offer open till cleared.VARIABLE CONDENSERS. .005 mfd. Tekade, 1/3 ;.00075 Polar Compax, 1:- , S/W Formo. 2/- ; J.B. Midget,.0001, 1/6 ; 2 -gang variable, .0005.1/6i 3 -gang, 2/-,BUTTON MICROPHONES for sound detection.Usually sold at 216. Our price has always been 11-. 1/..We have supplied thousands.

VIBRATOR BATTERYSUPERSEDER, with metalrectifier. for H.T. No need forH.T. Battery ; 3 output volttappings. Reduced from23/15/- to sale price, 35;-.CRYSTAL SETS. plug-lncoils, 2 tuning condensers,semi -perm. detector. 7i6.Boudoir enclosed type, simpleto use, 6/6. One for yourA.R.P. shelter.

RELAY CIRCUIT BREAKERS. Trip overload.Magnetic Blowout, enclosed. S.P. to 4 amps., 7/6. Withthermal delay. 10;-. S,P.6 amps., 10! -. Or with thermaldelay, '12/6. S.F. 10 amps., 141-. B.P. 15 amps., 18/-.S.P. 20 amps.. 18/8.YOU MUST KEEP YOUR BATTERY PREPA ICEDCharging on A.C. Mains. The NITNDAY will keepyour battery fit without attention.Model N6A21, charge 2 volts. I amp., 12/6.Model N/A61, Trickle Charger, charge 6 v., I amp..17/8.Model N/B6/1., Car Charger, charge 6 volts 1 amp., 241,Model N/B8/1i Car Charger, charge 6 volts 1i amps., 27/8.Model IN/C6/2 Car Charger, charge 6 volts 2 amps., 37/..Model N/D12/1, H.M. Car Charger. To charge 12 volts1 amp., 38/-.

MOTORS FOR A.C. MAINS. Repulsion,self-starting. 1/60 H.P. with pulley. Type 50.1,500 revs., 18/6. Ditto, Type 36, 1,25 H.P.G.E.C., 3,500 revs., 2716. Endo. Induction1110 H.P. Motors, 2,500 revs., 35/-. self-starting, I H.P., 1,425 revs., 49/-. etc.. etc.D.C. Mains Motors, 1/40 110 or220-v. series. Type KM., 1,750 revs., 15"-.Ditto, G.E.C., 230-v., 2.1191 revs., 18/, Croy-rdeovns 1/1200 /-11. .P ., 110 and 220-v. shunt, 1,700

5/- EMERGENCY PARCELS of usefulexperimental electrical and radio repairmaterial and apparatus. 7 lbs. for 5/-.Post free.

1 TPE atiV,:'

SAN -VERYi11111t111111111(01MT-

SUPERSEDE

Stamped addressed envelope must be enclosed for oarNew 1940 Bargain List " N," or reply to all enquiries.

ELECTRADIX RADIOS218 Upper Thames Street, London, E.C.4

Telephone , Central 4611

288 PRACTICAL WIRELESS June 15th, 1940

Classified AdvertisementsADVERTISEMENTS are accepted for these columnsat the rate of 2d. per word (minimum charge 2/-each paragraph). Series discounts of 5 per cent. for 13,10 per cent. for 26 and 15 per cent. for 52 insertionsare allowed, All advertisementa must be prepaid.EACH paragraph will commence with the first wordprinted in bold face capitals. Additional words inbold face capitals are charged at 4d. per word.ALL communications should be addressed to theAdvertisement Manager, " Practical Wireless," TowerHouse, Southampton Street. London, W.C.2.

CABINETSA CABINET for every radio purpose. Surplus cabinets(undrilled) from noted makers. We have hundreds instock (no catalogues). Send measurements of chassis,etc., and say what kind of cabinet required. Stampfor reply. Inspection invited.H. L. SMITH AND CO., LTD., 289, Edgware Road,W.2. 'Eel: Pad. 5891.

LITERATURE

NEW Edition. American Amateur Relay LeagueHandbook. 500 pages of up-to-the-minute technicalinformation, 7/- post free. 1940 Jones Handbook ;approximately 700 pages dealing with every aspect ofShort-wave Radio, 8/6, post free.-Webb's Radio, 11,Soho St., London, W.1. 'Plioue : Gerrard 2089.

LOUDSPEAKER REPAIRS

LOUDSPEAKER repairs, British, American, anymake. 24 -hour service, moderate prices. -SinclairSpeakers, Pulteney Terrace, Copenhagen Street,London, N.1.

REPAIRS to moving coil speakers. Cones/coilsfitted or rewound. Fields altered or wound. Pricesquoted, including eliminators. Pick-ups and speakertransformers rewound, 4/6. Trade invited. Guaranteedsatisfaction. Prompt service,L.S. Repair Service, 5, Balham Grove, London, S.W.12.

- Battersea 1321.

MORSE EQUIPMENTFULL range of Transmitting Keys, Practice Sets,Oscillators, Recorders and other Radio TelegraphApparatus, designed and manufactured by T. B.McElroy, World's Champion Telegraphist. Sole dis-tributors Webb's Radio, 14, Soho Street, London,\V.1. 'Phone: Gerrard 2089.

MORSE TRAININGWIRELESS Code Courses. "Book of Facts" Free. -Candler System Co. (L.0.), 121, Kingsway, London,W,C.2.

NEW CHASSISARMSTRONG Co. recommending the followingeconomically priced Radio Chassis for good qualityreproduction.ARMSTRONG Model AW38.-8-valve All -waveRadio -gram chassis, incorporating the latest circuit,including 6 watts push-pull output. Price £8/8/05% war increase.Armstrong Co. have many other models of equalinterest, please write for catalogue, 6d. Post -Free.Armstrong Manufacturing Co., Warkehs Rd., Holloway,London,

NEW -LOUDSPEAKERS

3,000 Speakers, P.M. and energised 4in. to 141n. In-cluding several Epoch 18in.-Sinclair Speakers,Pulteney Terrace, Copenhagen Street, London, N.1.

e5. WE SUPPLY LATEST MODELS OF

ARMSTRONGQUALITY CHASSIS ONEASY TERMS

...L. WRITE FOR FULL DESCRIPTIVE LIST &TERMS. PROMPT DELIVERY, CARR. PAID.

LONDON RADIO SUPPLY CO.," WINDER," ARDINGLY RD.,BALCOMBE,

SUSSEX (Evacuation Address).

tt 1.2.MillM1.114=NONIM011,1111111.41.1.1)4111eili(11...11111.1111M4

I

I

FREE ADVICE BUREAU

COUPONThis coupon is available until June 22nd, 1940,and must accompany all Queries and Hints.

PRACTICAL WIRELESS, 15/0/1940..1111141.1/1.11110-04111101111.04111111.1.111=0.141.11.141M41.1.0i0.114)

PUBLIC APPOINTMENTS

MINISTRY OF AIRCRAFT PRODUCTIONAERONAUTICAL INSPECTION DIRECTORATEVacancies exist for une.stabli,hed appointments asExaminers in the General Engineering, W /T. andInstrument Branches.QUALIFICATIONS.All candidates must have good general education, beable to read drawings, understand specifications, usemicrometers and other measuring instruments.(a) Applicants for the General Engineering Branchmust have had practical experience in an engineeringworks. An elementary knowledge of materials testingis desirable.(b) Applicants for the Instrument Branch must haveknowledge of physics and training in light engineeringor instrument making. Candidates with knowledgeof optical instruments are also required.(c) Applicants for the WIT Branch must have practicalknowledge of W/T and electrical equipment withtechnical training in radio communication equal toCity and Guilds final examination standard.ACCEPTED candidates will undergo a period of trainingin inspection as applied to the above subjects, notexceeding three calendar months, and will be paid£3 15s. weekly during training. Subsistence allowanceof £1 Is. weekly during training is payable to marriedmen normally residing outside the training area. Onsuccessful completion of training, candidates will beappointed as Examiners at a salary of ti270(payable monthly in arrear subject to a deduction of£12 p.a. for each year of age below that of 24. Exami-ners must be prepared to serve in any part of theUnited Kingdom,NORMAL age limits 23 to 60. -

CANDIDATES should indicate on their applications.for which vacancy they wish to be considered -a, bOr e.APPLICATIONS must be made on form 786, copies ofwhich can be obtained on application, by postcard only,to : The Inspector -in -Charge, A.I.D. Training School,(I.C.S. Bee. 53), Brandon Steep, Bristol, 1.

RADIO MAP AND GLOBE

WEBB'S Radio Map of the World enables you tolocate any station heard. Size 40' by 30' 2 colour heavyArt Paper, 4/6. Limited supply on Linen, 10/6.WEBB'S Radio Globe -superb 12 full -colour model.Radio prefixes, zones, etc. Heavy oxydised mount.Post Paid, 27/6.-Webb's Radio, 14, Soho Street,London, W.1. 'Phone,: Gerrard 2089,

READERS' BARGAINS

RADIOGRAM, good tone and working order, also all -wave table receiver. Must be good and reasonableprice. -Roberts, Rural Structures, Amersham.

BTY. B.T.S. Trophy S.W. Three, £2 15s. Bty. straightS.W. Three, 26s.-8, Retford St., London, N.1.

FERRANTI in/c Meter, 500 in.a., 211," dial, 15/-;Weston ditto, 200 m.a., 12/6 ; Eddystone lv. batterypre -selector -converter, 251- ; S.A. " Mercra " Mercuryvapour lamp, 150 -watt, 10/- ; 100 copies " PopularWireless," August, 1935 to December, 1937, 10/- ;150 copies " PRACTICAL WIRELESS," August, 1935 toDecember, 1938, 15/- ; 1lb. mercury, 10/- ; parcelcomponents (used), £1. All post free. -130, Holly -croft, Hinckley.

RECEIVERS AND COMPONENTS

LONDON CENTRAL RADIO. WE HAVE A FEWSPARTON 5 -VALVE PRESS -BUTTON RECEIVERSLEFT, 24151- EACH. FOR FULLER DETAILS SEE"PRACTICAL WIRELESS," JUNE 1st ISSUE.

OLD H AMS.-Ex-Poliee, slightly used, unspillable2v. 7 amp. Accumulators, suitable for Lamps, Port-ables, etc., 2/- each. Heavy Black Twin Flex, 20yards, 2/6. -London Central Radio, 23, Lisle Street,W.C.2. Gerrard 2969.

VAUXHALL. -All goods previously advertised arestill available ; send now for latest price list, free. -Vauxhall Utilities, 163A, Strand, W.C.2.

COULPHONE Radio, Ormskirk. Collaro A.C. Motors,12in. turntable, 27/6. Radiogram Units, 45/-. Highquality crystal pickups, 25/-. Rola G.12 speakers,energised, 52/6. G12 P.M., 65/-. Guaranteed Americanvalves, 4/6 ea. Octal, 5/6. ld. stamp for lists.

RECEIVERS AND COMPONENTS

RADIO CLEARANCE, LTD., 63, High Holborn, London,W.C.1. TELEPHONE : HOLborn 4631.

LISSEN 2v. screen -grid valves, 8.0.215 and S.G.2V,3/6 each.LISSEN 2v. battery pentode, 4 -pin side terminal P.T.2Aand P.T.225, 3/11 each.LISSEN 4v. mains screen grid A.C./S.G., 4/11 each.LISSEN 4v. mains output pentode A.C./P.T., 4/11 each.LISSEN rect ifier valve V650, 2/11 each.ALL -WAVE super -het. chassis, 5 valve A.C. LatestMullard valves T.I1.1.B., V.P.4.B., T.D.D.4., Pen.A.4., L.W., 4/350v. Ranges : short wave 16-48 metres.Medium wave 200-560 metres. Long wave 800-2,E00metres. Size of chassis, 141in. long, 71in. deep, heightoverall 8110. Controls, tuning at side. Volume on/oilat side, wave -change, provision for pick-up. Com-plete with valves and knobs, W1716. SpecialSpeaker, 1,500 ohms field, 10/8 each.ULTRA SHORT and short-wave choke. Lissen HI -Q.Inductance, 100 microhenries. Boxed, list, 2/-. Ourpricc 1/- each.ULTRA SHORT and short wave double wound low -resistance choke. Lissen Hi -Q. resistance less than.05 ohms. Boxed, list, 2/6. Our price, 1/3 each.LOW -LOSS Ceramic valve -holders. Lissen Hi -Q.Baseboard and chassis, 5- and 7 -pin, 10d. and 1/- each.MANSBRIDGE type condensers. Lissen Hi -Q. 250D.C. working. Moulded case with feet .1 mfd. and 1DIM., 6d. each.ROTARY COIL UNIT. Lissen Four -band from4.8-91 metres, can be selected by a turn of the knob.With circuit. Boxed. List, 15/6. Our price, 6/11 each.LOW -LOSS short-wave variable condensers. Ceramicinsulation. Rrass vanes. Lissen Minimumcapacity 5 micro -micro farads. Two types. Boxed.With knobs. 160 mmfd. List, 7/6. Our price, *each.20 mmfd. List, 5/6. Our price, 2/6 each.CONE speaker unit. Lissen. Complete with reed andchuck. Boxed, 1/- each.PUSH-PULL switches. Lissen. 2 -point, 4d. each.3 -point, 6d. each.ROLA P.M. speakers. Latest type, 71" cone withpower and pentode transformer. Boxed. 15/- each.CLOCK -FACED dials. 5" x 31". With printed 3 -wavescale. Ox -copper escutcheon and glass, 3/6 each.Ditto, less escutcheon, 2/6 each.FILAMENT transformers, input 200-250v., output4v.4 amps., 4v.6 amps., 4111 each.MAINS transformers. Plessey. 350-0-350v., 90 m.a.,4v. 2.5 amps., 4v. 6 amps., 8/6 each.MAINS transformer. G.E.C. American windings,350-0-350v., 65 m,a., 5v. 2 amps., 6.3v. 2.5 amps,Suitable for replacements in G.E.C. models, 3/6 each.MAINS transformers. Wearite. Type 11.0.1,250-0-250v., 80 4v. 2.5 amps., 4v, 4 amps., 9/11each. Type R.C.2, 350-0-350v. 120 m.a., 4v. amps.,4v. 4 12/6 each. Type R.C.3, 350-0-350v.150 m.a., 4v. 2.5 amps., 4v. 2 amps., 4v. 5 amps.,16/- each. Type 31.0.4, 500-0-500v.150 ma., 4v. 2 amps.,4v. 2 amps., 4v. 2.5 amps., 4v. 5.6 amps., 21/- each.All above are centre -tapped windings. Type R.C.5,100 -watt auto transformer, 100-110v., 200-250v.,reversible, 12/6 each. All transformers 200-250v.tapped primaries.CHASSIS mounting valve -holders. American 4 -5 -6-7 -pin, 4d. each. Octals, 6d. each. Loctals, 10d. each.7 -pin English type, -3d. each.1 WATT resistances, Polar N.S.F., 4d. each ; 3/9 dozen.All sizes up to 2 meg.VOLUME CONTROLS. American C.T.S., finest made,divided spindles. Length 21'. With switch 2,000,5,000, 10,000, 25,000, 100,006, 250,000, 500,000, and1 meg., 2/6 each. Wire wound, 5 watt (less switch),2,000, 5,000, 10,000, 20,000, and 25,000 oluns., 2/ -each.24 MFD. can type electrolytics, 450v. working; 1/6 each.MAINS TRANSFORMER, type R.C.D. Drop through.Capped, 250/350v. 100 ma., C/T. 4v. 6 amps 4v. 2.5amp, Wire -ends, 10/6 each.SPEAKER transformers, pentode output, 3-4 ohm.matching, 3/- each.B.I. wire -end type, bias electrolytics, 50 mfd 12v.,1/6 each. Ditto, 50 mfd., 50v., 2/- each.TUBULARS wire -end, non -inductive paper condensers.All sizes up to 0.1, 5d. each ; 4/9 dozen.0.1.8 ic.8 mfd.. 550v. working. Cardboard electrolytics,3/6 each. Ditto, 16 x 8 ma, 4/6 each. Ditto, 8 mfd.tubulars, 2/- each. Ditto, 4 mfd. tubulars, 1/9 each.Ditto 2 mfd. tubulars, 300v. working, 1/3 each.All orders must include sufficient postage to cover.Hours of busluess a.m.-6 p.m. weekdays; 9 a.m.-

1 p.m. Saturdays.PLEASE WRITE YOUR ADDRESS IN BLOCK

LETTERS.WE CANNOT UNDERTAKE TO ANSWER EN-QUIRIES UNLESS ACCOMPANIED. BY FULL

DENCO.-Ultra low loss S.W. components, receivers, POSTAGE (20.).polystyrene insulation. Send 2d. for Catalogue.- RADIO CLEARANCE, LTD., 63, High Holborn, London,Wsrwick Road, Clacton, Essex, W.C.1. TELEPHONE : HOLborn 4631.

June 15th, 1940 PRACTICAL WIRELESS 111

RECEIVERS AND COMPONENTS

SOUTHERN Radio's Bargains.ALL Guaranteed, Postage Extra.5/- Parcel of useful Components, comprising Con-densers, Resistances, Volume Controls, Wire, Circuits,etc. Value 25/-. 5/- per parcel.15/- Service Man's Component Kit. ElectrolyticCondensers, Volume Controls, Resistances, Tubular,Mira, Paper Condensers, Valve Holders, etc. 120articles contained in strong carrying case, 9" x 7",15/- the Kit.211- Small Trader's Parcel of Components. 150Articles comprising all types Condensers, ValveHolders, Resistances, Chokes, Coils, Wire, etc. Value85/-, 21/- the parcel.5/- 100 Wire -end Resistances, assorted capacities.

and 1 watt, 6/- per 100.ORMOND Loud -speaker Units, 2/6; Crystal Sets,5/6; Westectors Type W2, 2/6; Crystal Detectors,2)-; Crystals, 6d.; Marconi V21 Valves, Dd. "P.O."Microphones on Stand, for use with any receiver, 51.;Telsen W349 Iron -core Midget D.R. Coils, 5/6; MorseTapping Keys, 3/-; Buzzers, 1/6.2/- Tool or Instrument Carrying Cases, ex Govern-ment Stock . Wood 0' x r xr, 2/-.SOUTHERN Radio, 46, Lisle Street, London, W.C.Oerrard

SCIENTIFIC SOCIETIES

RADIO Society of Great Britain. Reduced subscrip-tions during war. Details and "T and it Bulletin"h/-.-16, Ashridge Gardens, London, N.13.

TUITION

PRACTICAL Postal Courses, radio television, testequipment design, trade -test coaching for R.A.F.poste, 1.P.R.E. and I.W.T. exams. ; booklet free.-Secretary, 1.P.R.E., 3, Shirley Road, London, W,4.

YOUNG MEN (15-20 years) urgently needed to trainas Radio Officers for the Merchant Navy ; militaryexemption obtainable; short training period; lowfees; boarders accepted. Write or call for full par-ticulars, Wireless College, Calmore, Southampton,or Wireless College, Colwyn Bay,

Turn Your SurplusComponents Into Cash

Those components for v,hichyou have no further use can beturned into cash through theclassified columns of "PracticalWireless." Readers' adver-tisements are inserted at thespecial rate of Id. per word

(minimum charge 1/-).

Send your advertisement with P.O. to :-Advertisement Manager, " PracticalWireless," Tower House, Southampton

Street, London, W.C.2.

This unique Handbookshows the easy way to

secure A.M.1.C.E.,A.M.I.E.E.,

A. M. I. A. E., A.M. I.W.T.,A.M.1.R.E., and -similar qualifica-

tions.. WE GUARANTEE-" NO PASS-NO FEE."Details are given of over 150 Diploma Courses in allblanches of Civil, Mech., Elec., Motor. Aero, Radioand Production Engineering, Television, Building,Di: smanship, Tracing. Inspection, Govern-ment Fintdov men t, etc. Write for this enlighteningHandbook to -day FREE and post free.dien with radio knaultdge run hidain ant -acne.: posts hi the Cervices.Britis`l Institute of Engineering Technology,

909, Shakesnraro HCI1SP,- H. 15. 19, Stratford Pt., W

PREMIER RADIONEW PREMIER S.W. A.C.

RECEIVER KITIn response to many requests, we'have now pro-duced an A.C. version of the pOpular PremierShort Wave 803 Kit. Complete Kit of Partswith drilled chassis, all components, Plug-inCoils covering 13-170 metres, 4 valves and fullinstructions and circuits, 84/10/-. Completelywired and tested, 85/10/-. Send for full details.PREMIER SHORT-WAVE BATTERY -OPERATED

KITS for OVERSEAS NEWSIncorporating the Premier 3 -Band S.W. Coil. 11-86Metres without coil' hanging. Each Kit is completewith all components, diagrams, and 2 -volt valves.3 -Band S.W. 1 -Valve Kit, 14/9. 3 -Band S.W. 2 -Valve Kit, 22/6.

DE LUXE S.W. KITSComplete to the last detail, including all Valvesand coils, wiring diagrams and lucid instructionsfor building and working. Each Kit is suppliedwith a steel Chassis and Panel and uses plug-inCoils to tune from 13 to 170 metres,1 Valve Short -Wave Receiver or Adaptor Kit 20/-1 Valve Short -Wave Superhet Converter Kit- 23/-1 Valve Short -Wave A.C. Superhet Converter

Kit .. . . 26/32 Valve Short -Wave Receiver Kit .. 29/-3 Valve Short -Wave Screen Grid and Pentode

SPECIAL- OFFER. ROTHERMEL PIEZOBRUSH CRYSTAL PICK-UPS. New JuniorP.U. with arm, 19/6. Standard S.8 Model witharm, 29/6. P.U. head only, De Luxe Model, 19/6.ROLA MOVING COIL SPEAKERS, complete withTransformers, Nin, P.M. 12/6, Sin. P.M. 16/6,10in. P.M. 22/6, G12 P.M. 66/-.ENERGISED SPEAKERS, Plessey Sin. 175 ohmField, 7/6 ; 7,000 ohm Field, 7/6 ; with transformerRola G.12 Energised 1,250 or 2,100 ohm, 59/6.MAINS TRANSFORMERS. Manufacturers' surplus.All brand new and Guaranteed.Input 110 v. and 220 v. A.C. Output 325-325 v.,

-120 m.a. 6.3 V., 2-3 amps., 5 v. 2 amps, C.T.,7/6 each. Input 230 v. A.C. Output 325-325 v.,75 m.a., 5 v. 2 amps., 6.3 v. 2-3 amps. C.T., 6/6each. Input 100-250- v., 300-300 v. GO m.a. 4 v.5 a. C.T., 4 v. 1 a., 6/11. Output 325-325 v. 180m.a., 5 v. 2-3 stops., 6.3 v. 2-4 slops., 12/6.PREMIER BATTERY CHARGERS for A.C.Mains. Westinghouse Rectification complete andready for use. To charge 2 volts at i amp., 11-19 ;6 volts at amp., 19/- ; 6 volts at 1 snip., 22/6 ;12 volts at 1 amp., 24/6; 6 volts at 2 amps., 37/6.

REPLACEMENT VALVES FOR ALL SETSEUROPA MAINS VALVES. 4 v., A.C. Types,A.C./H.L., A.C./L.,A.C./H.P., A.C./V.H.P. (5 -pin), all 6/3 earl,.A.C./H.P., A.C./V.H.P. (7 -pin), 7/6. A.C./Pens-I.H., 716 ; 7/3 ; Oct. Freq. Changers,8/6 ; Double Diode Triodes, 7/6 ; 350 v. F.W.Rect., 5/6 ; 500 v. F.W. Rect., 6/6 ; 13 v..2 amps.Gen. Purpose Triodes, 5/6 ; H.F. Pens and Var.-Mu H.F. Pen., Double Diode Triodes, Oct. Freq.Changers, 8/6 each. Full and Halt -Wave Rectifiers,616each.TRIAD 1110K GRADE U.S.A. VALVES, all typesin stock. Standard tubes 5/6 each. Octal Basetubes, 616 each.PREMIER Short -Wave Condensers all -brassconstruction, with Trolitul insulation. 15 minf.,1/9; 25 mmf., 1/10 ; .40 mmf., 2/-; 100 mmf.,2/3 , 160 minf., 2/7 ; 210 mmf., 2/11.PREMIER SHORT-WAVE COILS, 4- and 6 -pintypes, 13-26, 22-47, 41-49, 78-170 metres, 2/- each,with circuit. Special set of S.W. Coils, 14-150metres, 4/9 set, with circuit. Premier 3 -tenetS.W. coil, 11-25, 19-43, 38-86 metres. Suitableany type circuit. 2/11.NEW PREMIER 2 -Gang S.W. Condenser 2 a.00015 mf. with integral slow-motion, 5/9.CARDBOARD ELECTROLYTICS. 100v. working,4 inf. or 8 nif., 1/9 8-F8 mf., 3/6; 50 mf. 12 v.,1/2 ; 25 mf, 25 v., 1/2 ; 50 inf. 50 v., 21-.Coil Formers, 4- and 6 -pin, plain or threaded,1/2 each.UTILITY Micro Cursor Dials; Direct and 100 : 1

Ratios, 4/3.LBSSEIV Dual Range SCreen0 Coils, medium andlong waves, 2/9 each.Orders 5/- and over sent Post Free. Under .5/ -please add 6d. postage. All enquiries must beqecempanied by 21,1. stamp.

YOU MUST HAVE APREMIER 1940 CATALOGUE

111 PAGES . . PRICE 6d. plus postage, 2A.GET YOUR COPY TO -DAY !

ALL POST ORDERS TO : Jubilee Works, 167Lower Clanton Road, London, E.5. Amherst 472,CALLERS TO : Jubilee Works, or our NEW

PREMISES, 169, FLEET STREET, E.C.4.Central 2833

or 50, High Street, Clapham, I' .W.4. Macaulay 2381

1141 F

WANTED

WANTED, copy " Popular Wireless " dated November14th, 1036, or Wiring Diagram of A.C./ST.800. Also,Mains Valves, Coils, Components: Write : Johnson,3, Eric Road, Willesden, N.W.I0.

WANTED. Communications receiver. Pay £2 deposit,10s. weekly. Or small A.C. short -Wave receiver. -88, Wentworth Read, Barnet, Herts..

WANTED. S.W. Communication Receiver. Reason-ably. priced. -71, Sudbourne Road, Brixton, London,S.W.2.

WANTED. 'Westinghouse Metal Rectifier, type -L.T.4. House Service Meter, 230v. 2.1a.. 1 -phase, 2 -wire.Approval against cash.-C. W. Castleton, 47; BernardStreet, Russell Square, London, W.C;1.

WANTED, one Colvern's 3 -gang coil unit, EGO,KGO, KOR.; state price.-Beal, 3, Greenhead Lane,Dalton, Huddersfield.

BOOKS

YOU need this war atlas when you read the newspapersor listen to the wireless bulletins. " NewnesCompre-hensive War Atlas" will make the priigfess of theconflict understandable in all its aspects. 8 pages ofwar maps, including the ScandinaViiin countries,Western Front, Balkans, and al, countries whosefrontiers adjoin or are near those if the belligerents.With war strengths of the Armies, Navies, and AirForces of the Nations.-Of all newsagents and book-sellers, ls. 6d. net, or by post Is. 9d. from the BookPublisher, George Newnes, Ltd., Tower House,Southampton Street, Strand, London, W.C.2.

KNOW what to do before the doctor comes,-Learnfrom the new book,- "First Aid For The Householder,"and your knowledge may be the means of saving a life.Deals in simple language with all kinds of injuriesand their treatment, war -gases, shock, and the minorailments of everyday life. Clearly illustrated.-Of all booksellers, Is. 6d. net, or by post ls. 9d. fromthe Book Publisher, George Newnes, Ltd., TowerHouse, Southampton Street, Strand, London, W.0.2.

The " Fluxite Quin. s" at work.

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See that FLUXITE is always by you-In thehouse-garage-workshop-wherever Speedy sol-dering is needed. Used for 30 years in govern-ment works and by leading engineers and manu-facturers. Of Ironmongers-In tins, 411., 9d.,114 and 2:8.Ask to see the FLUXITE SMALL -SPACE SOL-DERING SET-compact but substantial-complete with full instructions, 7/6.

TO CYCLISTS I Y0111' wheels will NOT keepround and true, unless the spokes are lied withfine wire at the crossings AND SOLDERED.This mains a much stronger wheel. It's Simple-with FLUXITE-but IMPORTANT.

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"MO RENGINEERING"FOR LESS THAN HALF PRICESKILLED motor engineers arein greater demand than everto -day. The war effort calls forthousands of experts-men whoare specialists in repair andservicing ; and these skilledengineers are wanted on thehome front and in variousbranches of the Services.

That is why, to -day, we areoffering " MOTOR ENGIN-EERING " for less than halfprice-for only .E2 2s. insteadof 10s.-in order that it shallbe.within the scope of all whowould like to benefit from itsvast store of knowledge... .

What is even more interesting,it will be scut to yon for a firstpayment of only 2s., the rest bysmall monthly instalments.

These Wonderfully IllustratedVolumes comprise : Vol. I :ENGINES-Overhauling, Re-pair and Testing. Vol. II:ELECTRICAL - EquipMentand Accessories. Vol. :CHASSIS-Bodywork, Brakesand Braking Systems. Vol. IV :TRANSMISSION - Clutches,Gearboxes, Universal Joints,Differentials, Rear Axle andSteering Reran5.

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"MOTOR ENGINEERING" contains 2,160pages of invaluable knowledge and wellover 2,000 illustrations, including Diagrams,Plans, and specially taken " action " photo -

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1 agree to remit a further 8 monthly subscriptions of5/- to complete the purchase price of 52/2/-. Cashprice with order £2.

POSTTHIS

ORDER FORM

TO -DAY

All you do is fill theOrder Form on rightand send with PostalOrder for 2/- to theaddress given.

Name

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Occupation Arm. P.-NV.15640. I11111111111111111111111111111111..... NO BM PIM OM NM 1.111 IMO MN Meli Ms Me An ME Ms BM MO ISMGeorge Nesties, Ltd.

RADIO 1TRAINING -MANUAL P.W.

ARN RADIFROM A to Z

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IRadio technicians are wanted

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T-H.-E----OUTLI;;;;."1:;;;L--ES";""--By RALPH STRANGER

This book covers the theory and practice of Wireless Recetion from A to Z, and makes everything plain even to tmost " non -technical " reader. Step by step, it teachyou everything you want to know about wireless. OvBoo pages, fully illustrated. los. 6d. net. (By post it

THE PRACTICAL WIRELESS ENCYCLOPZEDIBy F. J. CAMM

Wireless Terms and Definitions stated and explainedconcise, clear language. 392 pages. Over 500 illustration

7S. 6d. net. (By post 89

EVERYMAN'S WIRELESS ROOK.By F. J. CAMM

A Radio Consultant for the Listener, Expert and AmateConstructor, with Special Chapters on the Principles oRadio Telephony, Installation and Systematic Faulfinding. With 200 'Illustrations and Diagrams.

5s. net. (By post 5s. 6d.)

PRACTICAL WIRELESS SERVICE MANU'.By F. J. CAMM

A complete work on the testing and servicing of all typof wireless receivers. Packed with information. Wiover 220 illustrations. 228 pages.

6s. net. (By post 6s. 6

NEWNES SHORT-WAVE MANUALBy F. J. CAMM

Deals thoroughly with the problems underlying the desof short-wave apparatus, the special circuits which yiethe best results, and designs for receivers based on thocircuits. 5s. net. (By post 5s. 6

WIRELESS TRANSMISSION FOR AMATEUBy F. J. CAMM

Covers fully and clearly the whole field of wireless tramission. With 120 illustrations.

3s. 6d. net. (By post 4.

WIRELESS COILS, CHOKES ANDTRANSFORMERS

By F. J. CAMMFor every home constructor.x26 illustrations.

3s. 6d. net. (By post 4Of all Booksellers, or by post from the Publishers, GearNewnes, Ltd. (Book Dept.), Tower House, Southampt

Street, Strand, London, W.C.2.

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Published every Wednesday by GEORGE NEWNES, LIMITED, Tower House. Southampton Street, Strand, London, W.C.2, and Printed in Engl.by THE. NEWNES & PEARSON PRINTING CO., LTD., Exmoor Street. London, W.10. Sole Agents for Australia and New Zealand GORDOGOTCH, -LTD. South Africa :CENTRAL NEWS AGENCY, LTD. Subscription rates including postage 23s.10d. per annum, 11.s. 11d. formonths',; Abroad 21s. 8d. per annum, 10s. 10d, for six months. Registered at the General Post Office for the Canadian Magazine Post,CONDITIONS OF SALE AND SUPPLY.-This periodical is sold subject to the following conditions, namely, that it shall not, 'without the wriconsent of the publishers first given. be lent, resold, hired out or otherwise disposed of by way of Trade except at the full retail price of : Octit shall not be lent, resold, hired out or otherwise disposed of in a mutilated condition or in any unauthorised cover by way of Trade ; or affixed to or as part ofpublication or advertising, literary or pictorial matter whatsoever.