INCORPORATING ELECTRONICS, TELEVISION …americanradiohistory.com/Archive-Television-UK/Electronic...n 11 -British product-tire result of excensive ... The coils are contained in enclosed

INCORPORATING ELECTRONICS, TELEVISION AND SHORT WAVE WORLD

PRINCIPAL

CONTENTS

I I \

Electronic Colour Television

Magnetic Materials

Electronic Organ Design

Cathode Ray Tube Traces -- 4

A New Electronic Beam Switch411111011111111111111111111111111111111111111111111111101210011

PDF compression, OCR, web optimization using a watermarked evaluation copy of CVISION PDFCompressor

ii Electronic Engineering September, 1944

n 11 -British product-tire result of excensive

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The fact that goods made of raw materials in short supply owing to war conditions are advertised Inthis magazine should not be taken as an indication that they are necessarily available for export.


September, 1944 Electronic Engineering 133

WIDE -RANGE TRANSFORMERSTYPES D-106 & D-139

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for Modern Communications Equipment

FREQUENCY CHARACTERISTICSPractically linear from the lower audio frequenciesto over 100 Kc/s. (See representative curves.)

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134 Electronic Engineering September, 1944

RESISTNCEWIRES

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Please quote priority and Contract Nos.

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Units are made with a number of variations inwindings and coupling coefficients for certain circuitconditions, but five Standard types are recommendedfor general use.

Wartime restrictions prevent ouraccepting orders other than thosecovered by priority numbers

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ElectronicEngineering

SEPTEMBER, 1944

Volume XVII. No. 199.

CONTENTSPAGE

Editorial-Colour ? 139

Electronic Colour Television 140

Magnetic Materials-Part I ... 142

Electrophotography 145

A Vector Calculating Device 146

Suppression of Radio Interference fromMains Voltage Fluorescent Lamps 148

Problems in Electronic Organ Design ... 149

Cathode Ray Tube Traces 153

Mechanism of Leaky Grid Detection-- Part 2 158

A New Electronic Beam Switch .. 162

The Multiplier Photo -Cell ... 164

A Valve Voltmeter for Higher Voltages ... 168

Correspondence ... 170

September Meetings ... 170

Abstracts of Electronic Literature 172

Book Reviews 174

CONDITIONS OF SALE-This periodical is sold subject to thefollowing conditions, namely, that it shall not without thewritten consent of the publishers first given, be lent, re -sold,hired out or otherwise disposed of by way of Trade except at thefull retail price of 2/- and that it shall not be lent, re -sold, hiredout or otherwise disposed of in a mutilated condition or in anyunauthorised cover by way of Trade, or affixed to or as part ofany publication or advertising, literary or pictorial matter

whatsoever.



LOOKING FOR TROUBLE . . .Hidden from the human eye, deep in an intri-

cate metal casting or a delicate machine -part,

a tiny flaw may lurk, perhaps to cause, if

undetected, the failure of a vital war -machine, a

gun, a plane, an engine . . . perhaps to cause an

accident, a loss of life.

Here the new wonder -science of electronics

makes yet another contribution to industrial

progress. An electronic device - the Electronic

Flaw Detector-looks for trouble of this kind,

searches for the flaw . . . . and infallibly finds it.

Flaws, that once would go unnoticed, now can-

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PROPRIETORS :

HULTON PRESS, LTD.ectronicEngineering

EDITOR:

G. PARR.

EDITORIAL, ADVERTISING AND PUBLISHING OFFICES, 43-44, SHOE LANE, LONDON, E.C.4

TELEPHONE:

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Monthly (published last day of preced-ing month) 2/- net. Subscription Rates :Post Paid to any part of the World -3 months, 6/6 ; 6 months, 13/- ; 12

months, 26/-. Registered for Trans-mission by Canadian Magazine Post.

TELEGRAMS

HULTONPRES, LUDLONDON.

THE brief report of J. L Baird'sfurther development in colour

- television, given overleaf, re-minds us that since the appointmentof a Television Advisory Committeesome time ago, very little has beensaid in this country about plans forthe future.

Papers have been read givingsuggestions for an improved tele-vision service-mainly from thetechnical point of view-and recom-mendations have certainly been madeto the Committee by outside bodies.but it is doubtful whether the view-ing public are aware of the discus-sions, and certainly so far there hasbeen no attempt to arouse theirinterest in post-war television.

To those who think that such anattempt would be premature, we canpoint to the American manufac-turers, who are already showingevery sign of stimulating publicinterest in what they will have tooffer after the war.

Admittedly, the conditions overthere are different from those in thiscountry, and the scope for develop-ment is wider, but locally theproblems are not so widely diverse.(The recent N.B.C. questionnaire, forexample, showed that there were

Colouronly 5,000 receivers in the New Yorkarea). The manufacturers are givingpublicity to their schemes for ex-tending the service to cover themaximum number of viewersthroughout the country-advertisersare being told how television willhelp to sell products-televisionorganisations of both broadcastersand producers are being formed, andthere is the general preliminarystirring of enthusiasm for the future.

There are several interesting newsitems about this growth of televisioninterest. One is the formation of theR.K.O. Television Corporation,which has for its object " to makeavailable to the producers of

FREQUENCY MODULATIONis the title of the first ElectronicEngineering Monograph, written by

Dr. K. R. Sturley,which has been reprinted several timesin response to the demand.

The publishers have pleasure inannouncing that a

2nd Edition,revised by the author, is now availableand can be obtained from technicalbooksellers or direct from the Circula-tion Dept., 43 Shoe Lane, E.C.4

Price 2/6, or 2/8 post free.

television entertainment a completeprogramme -building service." Thismeans that expert news -cameramenwill be available to film eventsof outstanding interest for thesole purpose of television broad-casting later. Their service is avail-able to all commercially interested intelevision, and they have already putout a slogan, "Televise to Advertise."Surely this is the first instance of acommercial firm devoting a specialbranch of its organisation to tele-vision service.

Another matter of interest is thepublication of a brochure by theColumbia Broadcasting System " tostate and describe the tremendousopportunity which the war has givento television and to state the pro-blems which lie in its path." Natu-rally the report is coloured (no punintended) by the outlook of theC.B.S. on the future of colourtelevision, but it is nevertheless aserious attempt to present the fun-damental problem of post-war tele-vision in all its aspects :-shall westay where we are for the time beingand develop an improved systemlater, or shall we hold things upuntil we can have the best ? Whichwould you do ?



ElectronicColour

Television

J. L. Baird's

New

"Telechrome

J. L. Baird with his new two-colourCathode Ray Tube.

ON Wednesday, August 16, J. L. Bairddemonstrated to members of the Press

his new system of colour television using aspecial cathode-ray tube capable of givingdirect colour rendering without the aid ofintermediate filters.

Hitherto the reproduction of televisionpictures in colour has necessitated the useof a rotating disk synchronised with thescanning and having colour filters throughwhich the image was seen.

Baird has held the view that the intro-duction of mechanical moving parts into anotherwise electronic system is a retrogradestep which would detract from the meritsof a full colour television system.

An earlier experimental method avoidedthe filter disk by focusing separate pictureson to a screen through stationary lensesand colour filters (see this Journal forJanuary, 1943), but had several disadvan-tages.

The present method, which is trulyelectronic, has overcome these disadvan-tages and is an important step forward inthe development of colour and stereoscopictelevision.

The experimental receiver and Tubeassembled.

PP


ANL


Fig. 2, Diagram of Tube for television repro-duction in three colours, the mica screen

being stepped on one side as shown.

THE `°Telechrome " televisionsystem is entirely electronic, thecoloured image appearing 'di-

rectly upon the fluorescent screen.Two cathode-ray beams are requiredfor a two-colour system and three fora three -colour system. These cathode-ray beams are modulated by theincoming signals corresponding to theprimary colour picture and impingeupon superimposed screens coated withfluorescent powders of the appro-priate colours. For example, in atwo-colour system the two cathode-raybeams scan the opposite sides of athin plate of transparent mica oneside of which has been coated withorange -red fluorescent powder andthe other with blue-green fluorescentpowder. Thus the screen has formedupon its front face an image contain-ing the orange -red colour componentsand on its back face an image con-taining the blue-green components,these images being superimposed andthus giving a picture in naturalcolour (Fig. 1).

Where three colours are to be usedthe back screen is ridged and a thirdcathode-ray beini added; the frontface of the screen then gives the redcomponent, one side of the backridges gives the green components,and the other side of the ridges theblue component (Fig. 2).

A two-sided tube has been de-veloped to receive a picture from a

600 -line triple -interlaced moving spottransmitter using a cathode-ray tubein combination with a revolving diskwith orange -red and blue- greenfilters. The receiving cathode-raytube is shown in the diagram (Fig. I)and in the photograph. The screenis a to in. diameter disk of thin micacoated on one side with blue-greenfluorescent powder and on the otherwith orange -red fluorescent powder.The colour may alternatively be pro-vided for the back screen by using awhite powder and colouring the micaitself.

The tube shown in Fig. I may beviewed from both back and front,but if used in this way one set ofviewers sees a mirror image. Also,coloured mica must not be used, anda filter has to be inserted between theback viewers and the tube to keepthe colour values correct and com-pensate for the light lost in the micaand fluorescent powder when thedirection of viewing is reversed.

The tube shown in the photo-graph of the apparatus can only beviewed from the front, but having onecathode-ray beam perpendicular tothe screen simplifies the set up of theapparatus. The tubes give a verybright picture due to the absence ofcolour filters and the fact that specialpowders are used giving only the de-sired colours, which are seenadditively.

The tubes give excellent stereosco-pic television images when used witha stereoscopic transmitter, the blue-green and orange -red images forminga stereoscopic pair and being viewedthrough colour glasses.

New Form of ScanningIn the present form of scanning all

the lines in successive frames are ofthe same colour, the colour changingwith each successive frame.

In a new form of scanning nowbeing developed, successive lines areof different colour and the number oflines is made a *non -multiple of thenumber of colours, so that every lineof the complete colour picture has

B

THIN MICA SWEET

Fig, I, Diagram of double screen Cathode RayTube for viewing television images in

two colours,

successively shown each of the pri-mary colours.

The object of this is to reducecolour flicker. Where frame -by -frame colour alteration is used flickerbecomes prominent in any large areaof a single colour, for example, ifthe picture is showing a large bluearea, this blue appears in the blue*frame only. While the red and greenframes are appearing, it is not shown;so that the frequency of the repetitionis reduced and flicker accentuated.With line -by-line colour alteration,each colour appears in every frame.

This form of scanning does notlend itselfto the revolving filter disksystem.

BIBLIOGRAPHY

Nature, August 18, 1928, pp. 232-4.Television, Vol. 1, No.' 7, pp. 20-21.Television, August, 1928, pp. 9-10.Wireless World, August 17, 1939, pp. 145-146.Electronics & Short Wave World, March, 1938,pp. 541-542.British Patent No. 473323., 1935.Electrician, December 27, 1940.Wireless World, February. 1941.Electronics & Television & Short Wave World,February, 1941.Electrician, January 1, 1935.Wireless World, February, 1943.Electronic Engineering, January, 1943.British Patent No. 562168, 1942.British Patent No. 562334, 1942.British Patent No. 652582, 1941.Electrician, December 20, 1941, pp. 359.Wireless World, February, 1942, pp. 31-32.Electronic Engineering, February, 1942, p. 620.


142 Electronic Engineeril September, 1944

Magnetic MaterialsI. The Domain Theory of Ferro -magnetism

By F. BRAILSFORD, Wh.Sch., Ph.D., A.M,I.E.E. *

Fig. I. Illustrating the arrangement of the atoms in a crystal (a) Iron, (b) Nickel.

1.1 Introduction

DURING the past twenty yearsremarkable progress has beenmade in the commercial de-

velopment of high permeability andother special alloys for light currentelectrical engineering, and in theproduction of new permanent magnetmaterials. Developments are alsooccurring in the sheet steels used formagnetic purposes in heavy electricalplant. In parallel with these achieve-ments, laboratory experiments haveled to noteworthy advances in ourknowledge of the physical basis un-derlying ferromagnetic phenomena,and from this has been built up auseful working theory of the subject.

It is proposed in this series of ar-ticles to discuss some aspects of themodern theory of ferromagnetism andsome relevant laboratory investiga-tions and commercial developments.1.2 The Nature of Magnetic Materials

The ferromagnetic elements iron,nickel, cobalt, and their alloys, incommon with other metals, are crys-talline in character. Examination ofa sample under the microscope, andMetropolitan Vickers Electrical Co. Ltd.

even in some cases with the naked eye,shows the metal to consist of a largenumber of grains with well-definedgrain boundaries. Each grain is asingle crystal consisting of atomswhich are arranged on a regularpattern or lattice. The type of latticeand the spacing of the atoms areknown from X-ray examination. Inthe case of iron the atoms form aregular cubic pattern known as" body -centred," as shown in Fig.t(a), in which,, if we consider anelemental cube of the crystal or unitcell, there will be an atom at eachcorner of the cube and another at thecentre. Nickel also crystallises on acubic lattice, but in this case, the dis-position of the atoms is " face -centred " with an atom at each cornerof the unit cell and one at the centreof each cube face, as shown inFig. 1(b). The lattice points, how-ever, determine only the mean posi-tions of the atoms which will, depend-ing on the temperature, be in a stateof more or less violent thermalvibration.

In the case of ferromagnetic alloys,atoms of more than one element willbe present. In those having high

permeability and low hysteresis loss,.the " soft " magnetic materials, themetal is always single phase, that isthe grains composing it are all of thesame composition and type of crystalstructure and the atoms of the alloy-ing element substitute themselves forthe pa rent atoms at a number of lat-tice points. The permanent magnet,or magnetically hard alloys, however,require, by contrast, the largest pos-sible area of hysteresis loop. Theymay contain five or six constituentelements and are more complex.Their exceptional magnetic propertiesare the result of their ability toseparate out into more than one phase,these phases having differing compo-sition and lattice type or lattice *men-sions. In these articles, however, weshall be mainly concerned with thesingle-phase alloys of cubic crystalstructure.1.3 The Elementary Magnet

Many engineers and. physicists stillthink of magnetic materials in termsof Ewing's theory. Ewing supposedthe material to be made up of elemen-tary atomic bar magnets capable ofrotation about their centres. Thesemagnets were influenced not only by



the applied field but also by theirmutual interaction. Models made upof large arrays of small pivoted per-manent magnets exhibited similarforms of magnetisation curve andhysteresis loop to those observed inpractice. In spite of this qualitativesuccess,, however, the theory failsquantitatively when the appropriate values, not known when the theorywas introduced, are inserted for themagnetic moments and spacing of theatomic magnets. Ewing's theory,consequently, is now only of historicalinterest and has given way to moreprecise modern ideas.

Now the atom of any element ismade up, as is well known, of a cen-tral positive nucleus surrounded by anumber of electrons which may bethought of as describing orbital mo-tions around the centre in more orless well-defined shells and sub -shells.It is also found necessary to ascribeto the electrons a spin, as though theywere minute gyroscopes spinning ontheir own axes. We should thereforeexpect to find associated with eachelectron a magnetic moment both onaccount of its spin and its orbitalmotion, just as in the case of a currentflowing in a circular path. In somecases, however, the magnetic mo-ments, due to all the electrons in the

atom, balance out and the whole atomhas no resultant moment, but withother elements this is not the case.The atoms of the latter have a mag-netic moment which is due, accordingto the experimental evidence, to theunbalanced spins of a few of theelectrons in the atomic structure. Theexistence of this magnetic momentmay be experimentally demonstratedby deflecting a beam of the atoms bymeans of a non -uniform magneticfield.

Thus the elementary magnetic par-ticle is the spinning electron whichimparts to the atom a magneticmoment.

1.4 Paramagnetism

If now we consider .a substancemade up of atomic magnets, the ap-plication of a steady magnetic fieldwill tend to rotate their magnetic axesinto line with the field and thus tomagnetise the material. This processwill, however, be violently interferedwith by thermal agitation. Langevin,neglecting any mutual aligningforces between neighbouring atoms,calculated the relation between, theapplied field and the resulting inten-sity of magnetisation, in terms of theatomic magnetic moments and thetemperature. The equation of the

magnetisation curve, modified morerecently by an assumption of quan-tum theory, may be written as

-= tan hkT

where I is the intensity of magnetisa-tion produced by the field H, J. isthat for parallel alignment of all theatomic magnetic axes, µ is the mag-netic moment of the atom, k isBoltzmann's constant and T the abso-lute temperature.

From this expression we may de:termine numerical values for thesusceptibility or permeability of agiven material, but these are found tobe of the right order of magnitudeonly for those feebly magnetic ma-terials known as paramagnetics,whose permeability is only veryslightly greater than unity.

If we substitute in the equation theappropriate values for iron, then remembering that flux densityB=H+4./r/, we obtain for iron at200 C.

o.5tHB-H=21,900 tan h

106

where B-H and H are in gauss.This magnetisation curve is plotted

in Fig. 2 at A and it indicates that a

(I)

Fig. 2. Calculated Magnetisation curves of Ironaccording to theory of Paramagnetism.

Fig. 3. Saturation Value of Iron in relation totemperature observed and calculated.



field strength of many millions ofgauss would be required to saturatea sample of iron. This is, of course,not in accord with the results of ob-servation since iron, in common withother ferromagnetic materials, maybe brought near to saturation by afield strength of only a few hundredgauss.1.5 The Domain Theory

To overcome this difficulty, Weissmade the hypothesis that in the ferro-magnetic materials there was spon-taneously produced a " molecularfield " acting in the direction of themagnetisation and proportional inmagnitude to it. Thus if H was theexternally applied field, and 1 theintensity of magnetisation in thematerial, the total field acting toalign the atomic magnets was H+NIwhere N was a constant. We mayrepresent this molecular field by thestraight line F in Fig. 2 intersectingthe magnetisation curve calculatedfrom the paramagnetic theory at (a).A little consideration shows that ifsuch a molecular field exists then,even in the absence of any externallyapplied field, the material would beunstable in the uninagnetised condi-tion, represented by the point 0, andwould spontaneously magnetise itselfat zo° C. to the intensity correspond-ing to the point of intersection at (a).In Fig. z the straight line F, repre-senting the relation between the mole-cular field and (B -H), has been drawnat the correct slope as determined bycertain considerations mentioned be.low. We thus see that for iron atroom temperature the molecular fieldacting has a value of about sevenmillion gauss, or about twenty tithesgreater than the highest field strengthattained, even momentarily, in thelaboratory, and the material is spon-taneously magnetised to a saturationvalue.

Now it is experimentally observedthat the saturation value of any ferro-magnetic, as determined in thelaboratory, falls with rising tempera-ture finally becoming substantiallyzero at a temperature known as themagnetic change, or Curie point. Athigher temperatures than this thematerial is no longer ferromagnetic.In the case of iron the Curie tempera-ture is 770° C. and the observed rela-tion between saturation value andtemperature is shown in Fig. 3 Cal-culated paramagnetic magnetisationcurves for iron are shown in rig. 2by A, B and C, drawn by way ofexample for temperatures of zo, 400and 77o° C. respectively. The

theoretical values of saturation mag-netisation for these temperatures aregiven by the intersection of the curveswith the molecular field line F, theposition of which is fixed by makingit tangential to the calculated curve Cwhich corresponds to the Curie tem-perature. The theoretical relation be-tween saturation value and tempera-ture may also be expressed by theequation

IT iTTet anh

lo 1.Twhere /T is the saturation value ofthe intensity of magnetisation at tem-perature T and T is the Curie tem-perature in absolute units. This re-lation is also plotted in Fig. 3, forcomparison with the observed curve,and shows the agreement betweentheory and experiment. Similarcurves may be drawn for nickel andcobalt and these give even betteragreement.

The Weiss molecular field hypothe-sis thus leads to the conclusion that,in the ferromagnetics, the materialeven in the absence of an appliedfield,, is spontaneously magnetised tosaturation. Since, however, we knowthat a piece of soft iron, for example,with no field applied shows no ex-ternal evidence of being magnetised,Weiss made the further assumptionthat the material was made up of" domains," or small regions of thematerial, each of which was saturatedbut in which the neighbouring mag-netisation vectors were randomlydirected. With no applied field therewas therefore no external magneticeffect.

Sample Amplifier LoudSpeakr

Fig. 4. Illustrating the Barkhausen effect.

The assumption of a molecularfield, and the sub -division of a ferro-magnetic into spontaneously satu-rated domains, form the foundationof the modern domain theory of ferro-magnetism, a theory now well sup-ported by experimental evidence.

The origin of the Weiss field wasat first a mystery since it could not beaccounted for in any way known toclassical mechanics. However, it nowappears from the mathematics of thewave or quantum theory, that if cer-tain dimensional relations connectedwith the atom and the crystal latticeare fulfilled, powerful mutual forcesmay exist between the spinning elec-trons in the atoms, which are theelemental magnetic carriers, makingtheir directions of spin the same andtheir axes parallel. Those elementswhich exhibit ferromagnetism con-form to the necessary conditions.They will thus be spontaneouslysaturated by these internal mutualforces of " exchange " betweenspinning electrons. The source ofWeiss's postulated molecular fieldthus becomes apparent.1.6 The Barkh Effect

On the application of an externallyapplied field to a piece of iron orother ferromagnetic material, mag-netisation occurs by sudden reversalsor changes of direction of the mag-netisation in whOle domains as thefield increases. These re -alignmentsof the magnetisation vectors in thedomains can be produced by muchlower field strengths than are re-quired to orientate the atomic mag-netic axes in a paramagnetic, as willbe described in more detail later. Thehigh permeability of the ferromag-netics is thus explained. Moreover, itfollows that the magnetisation willincrease, in part at least, by a Seriesof discrete jumps. Thus the mag-netisation curve, greatly magnified,should show_a series of steps as illus-trated in Fig. 4. A simple but strik-ing demonstration of the existence ofthese "Barkhausen jumps " can begiven by inserting a small piece ofiron, or any other ferromagnetic, intoa search coil of a few thousand turnsconnected to suitable telephones. Onmagnetising the sample, for example,by approaching it to the poles of apermanent magnet,, a rustling noiseis heard under quiet conditions in thetelephones. By the use of a suitablevalve amplifier this noise may be con-verted into a roar in a loud -speaker.The noise is, of course, due'to a rapidsuccession of minute E.M.F.s inducedby the jumps in magnetisation.


0

10

-/

September, 1944 Electronic Engineering

a0....

-0..

li

To

x

ce.1

Averagee

DomainVolume

.S.

o

-5

5

0 5 /0H in gauss

Fig. 5. Hysteresis loop and average volumeof Domains for annealed Iron.

Oscillographic records of thesediscontinuities were obtained byBozorth and Dillinger, from whichthe approximate domain sizes werededuced. Fig. 5 shows the averagevolume of the domains so obtainedfor one-half of the hysteresis loop ofa sample of iron. It will be seen thatthe largest discontinuities, cor-responding to domains with a lineardimension of the order of o.00i cm.occur near the steepest part of thehysteresis loop.1.7 Structure of a Ferromagnetic

We have thus seen that a ferro-magnetic element or alloy owes itsspecial characteristics to the existenceof elemental magnets or spinningelectrons which are spontanecnislypulled into parallel alignment toform the saturated domains. In generala large number of such domains goto make .up each grain in a poly-crystalline material. For example, ifthe latter contained 000 grains percubic mm.-, and the largest domainshave a volume of io c.c., therewould be at least i,000 domains ineach grain.

The observed magnetic propertiesof a material are,, of course, the re-sultant of the magnetic processes oc-curring in all its constituent crystals.It is thus important to understand' theprocess of magnetisation in singlecrystals. This subject will be con-sidered in the next section.

(To be continued.)

Electro-photographyIn the July issue of RADIO NEWS

magazine, Nicholas Langer des-cribes a new system of producingphotographic prints by electronic ac-tion instead of by the conventionalchemical processes.

The method, which is due to C. F.Carlson of New York,* uses a photo-sensitive plate which is coated witha uniform layer of " photo -conduc-tive insulating material." This ma-terial has a high insulation resist-ance in the dark but becomes conduc-tive under the action of light, return-ing instantaneously to its originalresistance when the light is removed.

Sulphur or anthracene are quotedas suitable materials, being appliedto the metal plate in a layer about.00i in. thick.

The surface of the material isgiven a uniform charge by rubbinglightly with a dry cloth or brushafter loading into the camera in thedark. When the plate is exposed, theparts of the coating illuminatedallow the charge to leak awaythrough.the photo -conductive layer tothe backing plate, and the image isthus stored on the plate as an elec-tric charge pattern-an electrostaticlatent image.Development

The development of the image iscarried out by utilising the attractionof the charge for fine particles. Thesemay be in the form of lycopodiumpowder, powdered ink, resin, orcarbon. The photo -conductive pow-ders are near the negative end of theelectrostatic series, and for best ad-hesion it is therefore preferable to usea powder which becomes positivelycharged on agitation.

Resin powders, which acquirenegative charges, will be repelledfrom the charged areas of the plateinstead of attracted, and can thus beused to produce a negative imageinstead of a positive one.

After dusting the powder over thesurface of the plate, the excess isgently blown off, leaving the blackportions of the picture with a fullcoating of powder and the rest inproportion to the tone of the image.

*U.S: Patents: 2,221,7762,227,0132,297,691.

Printing off

The image as viewed on the coatedplate will, of course, be a mirrorreserve of the original scene. To ob-tain a final ropy it is therefore neces-sary to roll or press a moist sheetof paper against the coated sur-face. The damp paper causes thepowder to adhere mechanically, re-moving it from the coated plate. 'Themethod of fixing the powder on thepaper varies with the type of powderused. If a fusible resin, the papercan be warmed sufficiently to melt thepowder on to it. If it is soluble ina solvent such as alcohol, the paperis moistened to carry the dye into thesurface. Other powders can be fixedby a thin coating of adhesive. In allcases the- result is a positive perma-nent print. Best results are obtainedwith powders of .00i" grain size.

The size appears to contribute tothe ability of the particles to flow orroll readily over the surface of theplate and distribute themselvesuniformly in the electostatic field.

The development of the plate bydusting is fairly rapid, requiring onlya second or so, as it depends on themechanical means for applying thepowder.

Exposure

The exposure required is longerthan that for ordinary plates -3o sec.with a f.4.5 is quoted for an anthra-cene plate for two photoflood lamps.The maximum response of anthra-cene is at 5.400 A., and that for sul-phur 4,70o A., above which it fallsrapidly.Line Blocks

One suggested application of theproCess is in the making of lineblocks on metal, the coating usedbeing anthracene.

Anthracene has a melting point of217°C. and a boiling point of 354°C.,but in thin layers the transition frommelting. to vapour takes place almostinstantaneously. After exposure,the coating is dusted with asphalt andis then heated slowly to i50-zoo° C.,causing the anthracene to volatiliseand allow the asphalt to settle on themetal surface. This forms an acidresist for the conventional etchingprocess. It is stated that the methodworks equally well for lithographicplates.

REFERENCESMonkman, Proc. Roy. Soc., 46, /43' I889.Kurrelmeyer, Phys. Rao. (2) 3o, 893, 194.Pauli, Ann d. Phys. 4o. 686, 1913.


146 Electronic Engineering. September, 1944

A Vector Calculating DeviceBy C. SNOWDON, B.Sc.*

The conversion of vectors of the complex form (a jb) to the polar formgraphical method

IntroductionELECTRONIC engineers and

power engineers too are oftencalled upon to use vectors in

their calculations. These vector cal-culations, though usually simple, in-volve tedious work in the conversionfrom the complex form (a + jb) to thepolar form r I 0. Engineers are con-versant with the usual method em -employed, i.e., r = V a' + b' and

b0 = tan -1,

aIt will thus be appreciated that the

conversion not only causes delay incalculations, but is also a fruitfulsource of errors. Special types ofslide rule have been designed whichenable such conversions to be madewith the same ease as ordinary arith-metic calculations, but the number ofengineers possessing these instru-ments is insignificant compared withthe number who use vectors.

The purpose of this article is todescribe a means of reducing thistedious work to a minimum, consistingessentially of a baseboard chart anda transparent cursor,, both of whichcan easily be constructed.

In order to understand the construc-tion of the chart, a considerationshould first be made of the theory ofthe Argand Diagram (Fig. i).

OX and OY are rectangular axes," a " being measured from 0 alongthe X axis and " b " measured fromO along the Y axis.

The point P has coordinates of(a.b) which produce the vector OP,of length r measured from 0 andangle 0 to the OX axis.

If the radius vector r is keptconstant, but the angle 8 varied,then the locus of the point P is acircle about centre 0. Thus variousvalues of r give rise to a family ofconcentric circles of appropriateradii.A calculator embodying these facts

suffers from the practical difficultythat all the loci have different curva-tures. If then a calculator could beconstructed on which all the loci haveexactly the same curvature, such a

Messrs' Reyrolle & Co. Ltd.

Fig. I. The Argand.Diagram.

locus could be drawn on a transparentcursor and placed in the requiredposition on the main diagram.

Such a result has been obtained byplotting the Argand Diagram ondouble logarithmic paper.

Construction of Calculator(i) Baseboard Chart

The chart consists of a sheet ofstandard double logarithmic graphpaper, the so -inch square type beingvery suitable, and such paper canreadily be constructed using anordinary so -inch slide rule. Eachscale is marked from o.s to so, valuesof " a" being plotted along thehorizontal axis, and those of " b "plotted along the vertical axis. Adiagonal is drawn from the bottomleft to the top right corner (Fig. 2).

On this chart is drawn the radiusvector r of value so units, by plotting" a " = so cos and " b " = sosin8along the horizontal and vertical axisrespectively, values of 0 being from00 to go°. This curve will be foundto be asymptotic to the lines a= loand b= so, due to the fact that theorigin of both a and b axes is atinfinity. Such a curve using thisparticular size of paper can only beplotted between the angular limitsgiven by : o.i

0= tan -1-- = tan - 'o. o =.75°IO

and so0,,=tan--1----=tan-' T00=89.250

o.Ibut this must not be regardedas a serious drawback since

rl 6 and vice versa by a

such extreme cases are rarely metwith in practice, and in mostcases the smaller term would beignored. If greater accuracy isrequired, however, a larger sheetof graph paper (i.e., one containingmore than two octaves) must be used,but for all practical purposes thechart will be found to be sufficientlyaccurate if the curve is connected tothe point o. i, so.

If the angle is kept constant thenthe locus is a line parallel to thediagonal, again due to the fact thatthe origin of the paper is at infinity.The curve may be calibrated withrespect to these angles, there being nonecessity to draw the whole line.

For the proof of these facts refer-ence should be made to the Appendix.

(ii) The:CureorThe curve drawn on the chart for

r = so units, the diagonal and alsothe asymptotes should be transferredto a transparent cursor, preferablymade of celluloid though tracingcloth could be used.

The cursor should then becalibrated with angles indicated bythe expression

bl a "=. tan

suitable values of a and 0 beingselected and b calculated. A con-densed table of such values is givenin the Appendix.

Consider the vector (a + jb) inFig. 2, and hence determine the pointP. The cursor is now moved over thechart, with the two diagonals super-imposed, until the curve on the cursorpasses through the point P. The inter-section of the asymptote on either ofthe axes then giving the value r, theangle of the vector being read offfrom the scale on the cursor.

Examples in Using the ChartN few simple examples are now

enumerated in order to illustrate themanipulation of the calculator.I. To convert the (I +12) to the form

rl 0

Determine the point (i -1-j2), movingthe cursor along the diagonal until itpasses through the point. Theasymptotes intersect both horizontal



BaseboardChart 71

6 6

10

968

The Chart with Cursor Superimposed

7ft"-----i-----/-,:_.lanimmill.1111=3EVE0110 P -EtE-.....-..- ..' .7-,53==... ziaz=mar.IIIIIIII.L"m4r.M=wilEE°"'"'"'"AMI. MOMMI

211-EIMITI111114111111:-.%1,== -; -1-1..-M.1--111=i- ra"Illti

1121111=3 MINIIIMINOFrAmmissonsms me mummusn assmunen n:

..149111 I Iiili" I I Iling 8MFON -/Esisilaitno eimeiniesim mu_

as 121111111 NIIIIIIIIIIIS%Man 11110e11 MI:i ZIMIWIlla II 11:

esemeensemsEsssieserseseeseses. um_

um Nem a iiimor.mmesssessummarssimmuo miemsmourisimo mom IIMEMINIIMINISINIIIIIIIIMIMIIIIIIIIIIIIIIIIIMMIIIIIIIIIIC

MII 111111111111111=NIIMMIIIIIIII1111111 IIIIIIIIIII WAIMIIIIIIIIOISMIIHIIIMMVINIMIIIIII 11111111_

1111M1111111111111111111111111M MIME =1,41111111111M11111111114:07.111111118811111111111111111111111111_

M111111111111.111111111111111111111ME11111111111111 MN 1111//110111111 1111 111111111111111 11111111111111_

1. , IIIIIMILINIIIIIIIIIIIIIIIIII11111111111111111111111111111 IV IMI all 111111111!! 11______........... -.9 9 ....2 jjj;i . -Iiiit: im . .. . .=aomaXiMaili :iliae bTialmilha

=5.....-===.1.Z.s..7 litalemasa ril--=ac-1.11.,6 iniumiimuilimanipmeio i

.3

2

04

==.7.

...

MNMN

a., SINEWS'

Masser=

Imilumm=so2111011111111111

.... .... e = 11. MU a...........

..W.===== ..... = ..... ............i :::::::=2.11=nr.,=======::::::....= ,=., : :==r1.1...""°==ii.::'

12

=r",,T=2:========.2,17:4=1=1.11:11:=1=1:11=BIII.1111111111111111111111111111111111111111111111111111111111111111161 SIM

.1111111111111111MMINIIMIIIINIMMIIIMINNIMM111111111111111111111111MINIIIIIIIIIIIIMICIMII MINIIIIIIIN 1111111MINIM NIHON

lal: la =I illal=h111114

nirinna=1:: Lar...11 I1 :I i ii1101111111111M11111111111111111111111111111

11111

111111111111Y4FAMIIM=MISIMIIIM11011111111111111IIIM1111

OE01111181111111111111=111111111 111111111111111111111

1111

EMU. IIIIIIIIIIIIIWIIIIN111111111111111111111111MMIll 111112111111111111111111 MIMI IIWM= EomppinssmmmsnsmomIIIIIIIIIIII!IIIIIIIIIIIIIIIMIIIIIIIIIII MIMIMUM IIIMMINIIIIIMIMIIIIIMENIIIIIIIIIIIMMEMIIIIIIIIIMFAMEMMENI=11111111111111111.11111111.11111111111111111 11111111111111111111MM11111111

.4111111111111111111111 111111111111111111111111111111111111 1111111111111111111111111111111111111

01 -4 5 .6 1 II .9 1.0 2 3 4 5 6 7 8 9 /01.0011, NO 1,

VALUES 'a1.1

TransparentCursor

Fig. 2.

and vertical axes at the value of= 2.24, and from the position of the

point on the curve 0 `=- 62.5°(i.e.) (1 1-j2) -= 2.24 62.50

2. To convert the vector (30 + 175) to theform r, 9 .

This examplels solved by determin-ing r and 0 as described in (1). but for(3 + j 7.5) whiCh-will be found to givevalues of r=8.i and 0=680. How-ever, the decimal point must now bereadjusted giving r=8i and .0=68°.

(i.e.) (3o+j75) = 8i 68°

3. To convert 3122° to the form (a + jb)

Move the cursor along the diagonaluntil the asymptote intersects bothhorizontal and vertical axes at 3 units,then tracing 'along the curve to the

angle 22° the values of a and h maybe read off a = 2.8 b= 1.1.

(i.e.). 3 1 220 (2.8 + j 1. 1)

Concluding NotesThe following are a few of the

peculiar advantages obtained by theuse of double logarithmic graphpaper :-

(i) An extremely wide range ofvalues is obtainable.

(ii) The percentage accuracy ofreading is (as on a slide rule) con-sistent, no matter what point is chosenon the scales.

(iii) All loci are of same form andtherefore only one such locus is re-quired on the cursor, thus obviatingthe necessity of the estimation ofvalues by eye between closely drawnloci on the chart.

APPENDIXProof of the Cursor construction

Equation to the circular locus des-cribed by the point P in Fig. i is

a2-1-b2=r2Introducing the parameters.r and 0

a=r cos 0 and b=r sin 0Plotting on logarithmic scales, as in

Fig. 3.x=m log a

and y=m log hwhere m is the scale modulus.

Consider. the equationa=r cos 0

...log a=log r+log cps 0.

Now from above log a=-m

x.. -=log r+log cos B.

m...x =m log r+m log cos 0.



r

93

ConstantAngle Loci

, ConstantRadiusVector Loci

in log a

Fig. 3.

Similarly y=m log (r +,m)log sine.Consider 0 as constant and

x, y and r variable.dx dy

Then andd (log r) d (log r)

Thus the slope of the curve is inde-pendent of the value of log r, i.e., of r.

The loci are therefore of the samecurvature and same shape and maybe represented by one such curve.

x=m log r+m log cos 0i.e., m log r=x--m log cog' 0

Also y=m log r+ m log sin 0i.e., m log r=y-m log sin 0

.. x -m log cos a=y -m log sin 0y=x-m (log cos 0 -log sin 0)

=x -m log cot 0.

Thus if 0 is kept constant the equa-tion is that of a straight line at aslope of 45°, and for various valuesof 0 is a series of parallel straightlines.

This enables the curve to be cali-brated for various values of 0.

Table of values for constructing the cursor.Plot out on to double log paper and trace on to cursor.

a 10 9.9 I 9.8 I 9.6 9.4 9.2 9.0 8.50 8.0 7.5 7.0 6.5

jb 0.1 1.39 I 1.98 I 2.81 3.4 3.92 4.35 5.27 6.0 6.61 7.14 7.6

' a 0.1 1.39 1 698 1 2.813.92 I 4.35 5.27 6.0 6.61 7.14

610 9.9 I 9.8 I 9.6 9.4 I ?.2_ 9.0 8.5 8.0 7.5 7.0

For marking of the angles on the curve, plot the following points anddraw a line parallel to the diagonal until it intersects the curve, mark thispoint with the appropriate angle. b/a = tan 0

i.e., b = a tan 0.Plot for a = o. to.0 45 50 55 60 65 70 75 80 i 85

jb 0.10 0.119 0.143 0.173 0.214 0.275 0.373 0.567 I 1.14

Plot for jb = 0.10.0 45 40 35 30 25 20 15 10 5

a 0.10 0.119 0.143 0.173 0.214 0.275 1 0.373 0.567 1.14

Suppression of Radio Interference from Mains Voltage Fluorescent LampsBy J. N. Aldington, B.Sc., F.Inst.P. *

In common with other cases whereinterference is caused by electricalapparatus adjacent to a radio re-ceiver, it is considered that each ofthree main factors may contribute tothe audible manifestation of inter-ference currents generated by afluorescent lamp. These are :-

Direct radiation from the lampitself with which is closely con-nected.

(2) Radiation from the wiring con-necting the lighting point.Feed -back into the lighting mainsof interference currents, whichmay therefore, after rectificationin the receiver, reach the loudspeaker via the power unit.

(t) Direct Radiation from the Lamp.There is small likelihood of direct

radiation from a fluorescent lampproducing a disturbing effect from aradio receiver in the same roomunless the lamp is mounted veryclosely to the radio, and even thenthe normally fitted suppressor con-denser is generally found adequate.

(I)

(3)

* Siemens Electric Lamp and Supplies, Ltd.

(2) Radiation from the WiringConnecting the Lighting Point.Experiments have shown that the

field from the conductors becomesvery attenuated a few feet from thelamp. In general if the cables con-necting the lamp are in properlyearthed and bonded steel tubing orlead -covered cable, and, the controlgear is mounted in an earthed metalcontainer, it is unlikely that inter-ference due to radiation from thewiring will be encountered.

In order that radiation should bereduced to a minimum, the length ofthe leads from the lamp to the sup-pressor condenser should be kept asshort as possible, even if this entailsmounting the ;suppressor condenserseparately to the choke coil.(3) Feed -back into the Supply Mains.

A simple arrangement of con-densers will normally be found effec-tive as a filter. One, of o.o5 mfd.,is connected across the lamp, whiletwo others, of the same value, areconnected one from each lampterminal to earth. The three

condensers may be mounted in aconvenient and ocmpact unit whichshould be located as near to thelamp as possible, so that be-sides reducing feed -back into themains supplying the radio, the lengthof wiring which might directlyradiate is reduced to a minimum.

In conclusion, the magnitude ofthe interference currents produced byfluorescent lamps is generally of asufficiently low order to be sup-pressed adequately by the condenserfitted for this and other purposes.The development of interference'from an established installatfonmay denote the necessity for re-lamping a lighting point. It hasbeen found that the same factorswhich make this necessary from thepoint of view of lamp flicker at theend of life may increase the parasiticradiation from the lamp; in whichcase lamp renewal will effectivelyrestore the original condition of atrouble -free installation.

(Extracted from Electrical Times,June 22, 1944.)



Problems in Electronic Organ Design

OUT of all the electronic musicalinstruments which have beendevised, the electronic organ has

unquestionably the widest appeal. Itsproduction on a commercial basis hadbeen well established in the yearsimmediately preceding the war andthere- is no doubt that when peacereturns its technical development willbe resumed.

The fact that electronic organs havebeen placed on the market is sufficientproof that the technical problems ofdesign and construction can be solved.In commercial instruments, however,it is possible to resort to productionmethods which are beyond the scope ofthe private experimenter. Before he can attempt the construction of suchan instrument with a reasonablechance of ultimate success he mustassure himself that he has reduced themechanical problems to the mostrigorous simplicity. Otherwise, theinescapable complexity of the organwill magnify the task until it assumesprohibitive proportions.

It is not the purpose of this articleto discuss the problems of the com-mercial types of electronic organ. Forinformation on such instruments thereader is referred to a paper by Winchand Midgley' and to the relevantpatent specifications.2 The presentaccount is a review of the author'sexperimental approach to the design ofan electronic organ of modest characterand of such a nature as to be suitablefor private construction.

Fundamental PrinciplesIn the first place, a choice can be

made between two alternative prin-ciples of tone generation : (i) thedirect production of complex, charac-teristic musical tones for subsequentselection by a keyboard system ofcontrol, or (z) the production of pure

By S. K. LEWER, B.Sc.

sine waves and their synthesis intocharacteristic tones by harmonic addi-tion, followed by a system of keyboardcontrol. Further, there exists a largerange of methods by which acoustictones may be generated in electrical.circuits, such as electrostatic or electro- magnetic alternators, photoelectricgenerators, valve reaction and relaxa-tion oscillators. Amongst thosemethods which rely on electro-mechanical effects, there is a choicebetween rotary and vibratory systems.

The earliest experiments carried outby the author utilised the first of thealternative principles mentioned above-the generation of " ready-made "electrical wave -forms corresponding tothe desired musical tones-and theelectrostatic method of generation wasselected _as being the most suitablefrom the ,standpoint of experimentalconstruction. The chief reasons forpreferring the electrostatic methodwere (a) that it would obviously beeasier to cut and shape metal elec-trodes than to wind large numbers ofelectromagnets or to set up precisionoptical systems, and (b) that thedesired " attack " or " envelope " ofthe wave -form could easily be achievedby the use of time -delay resistance -capacity circuits for the control ofexcitation potentials, or by themechanical time -variation of capacity.All the investigations which weresubsequently carried out have con-firmed the belief that the electrostaticmethod of tone geneilation is the beston the score of effectiveness, simplicityand flexibility.

Electrostatic Tone -GeneratorThe basic circuit of the polarised

electrostatic tone -generator is shown inFig. i. A high resistance R (of severalmegohms) and a small capacity C(possibly of ,f pF) are connected

Fig. I. The basic circuit of the polarisedelectrostatic tone -generator.

in series to a source of potentialdifference E. When the capacity of Cchanges, the charging or dischargingcurrent flowing through R gives rise toa voltage e. If the capacity is changingcyclically at an audio frequency, thevoltage e will have the same frequencyand may be amplified suitably to drivea loudspeaker.

Any number of capacities, eachvarying at its own specific audiofrequency, may be connected inparallel with the condenser C as shown,and the corresponding tones will beheard simultaneously from the loud-speaker. No tone is produced by anyof the separate capacities which areallowed to remain fixed in value.

Provided certain conditions arefulfilled, a sinusoidal tone will begenerated if the capacity of any ofthese condensers is caused to varysinusoidally. The amplitude of thesound is proportional to the value ofthe excitation potential E. Conse-quently, tones of different amplitudescan be produced by connecting thecapacities separately to points ofdifferent potential, as in Fig. 2. If theindividual capacities C1, C2, C3, C, arevaried sinusoidally at frequencieswhich are harmonics of some funda-mental frequency f (i.e. f, zf, 3f, 41. . .),the acoustic tone will be a complex oneof that freqUency and may becharacteristic of a flute, clarinet ortrumpet according to the number andamplitude of the several harmoniccomponents. This is the well-knownprinciple on, which the characteristictones of an organ can be simulated byharmonic synthesis of a number ofsinusoidal component tones.

Alternatively, any individual capa-city can be made to vary cyclicallyin such a form as to give the requiredtone directly. Such a complex time -variation of capacity can be achieved

13001

excitationPotential A,

J. 1.

c4

A.RAmplifie7O-0(

Fig.2. Harmonic synthesis of complex musical tones by theaddition of harmonically -related sine -waves of suitable amplitude



Pickup Element

300vExcitation Potential

-

Fig. 3. The author'sfirst experimentaltone -generator, usinga succession of com-plex waveforms carr-ied on a rotating diskof insulating materialand scanned by anarrow pickup elec-

trode.

in several different ways. In the firstexperimental model made by theauthor, a rotating insulated disk(actually a o -inch gramophone recordcarried on a turntable) was providedwith a number of equally spaced wave-forms cut from tinfoil and pasted on tothe disk A narrow metal strip fixedrigidly at a small distance from therotating disk constituted the otherelectrode of the tone -generating con-denser, the effective area of whoseelectrodes varied as the disk rotated.Fig. 3 shows the elements of thisarrangement.

The need for a high degree ofmechanical precision was immediatelyapparent, for any deviation fromflatness in the disk, or any wobble orrumble in the shaft or its bearings, orany departure from constant speed,became evident as an unwantedmodulation or variation of the musicaltone. The effect of electrostaticcharges on the surface of the insulatingdisk was also quite pronounced. Tonescould still be easily obtained when thetinfoil had been stripped off.

Although the troubles due to surfacecharges were overcome by resorting tometal disks, the problem of mechanicalimperfections remained. The wave-forms consisting of cut-out shapes offlat metal sheet, were made stationaryand were scanned either by radialarms extending from the periphery ofthe disk or by radial slits in the diskitself. In this case, as with the disksof insulating material, the generationof the higher harmonics by the use ofa complex wave -form did not appearto be possible for ordinary speeds(about 1,200 R.P.M., correspondingapproximately to boo cycles persecond) unless the scanning electrodeswere made very narrow (less than rmm.) and were placed very close tothe rotating surfaces (less than 0.5mm.). This was evidently due toelectrostatic edge -effects.

The sensitivity of these systems was

MIL

reasonably high, and a three -stageamplifier having a gain of about 70 dbwas adequate for experimental pur-poses.

Absolute PitchIn addition to the problem already

mentioned, there was the question ofthe accuracy of frequency, or pitch.Whenever a tone is produced by arotating system, there must obviouslyalways be an integral number ofelements contained within the peri-phery, and they must all be uniformlyspaced. If several different tones areto be generated by elements mountedon a common rotating disk or cylinder,the speed of rotation must be chosento suit the frequency requirements ofthe several tones. In practice, one canchoose between two methods. Thefirst is to mount all the A elements onone shaft, all the A# elements onanother shaft, all the B's on a third,and so on. These shafts will then haveto run at successively higher speeds,the ratio between the speeds ofadjacent shafts being 1 sA/2. Thesecond method is to arrange all the12 rings of elements contained withinone octave, A, At B Gt, ona common shaft running at a suitablespeed, and to operate a similar disk (orcylinder) at twice the speed, therebygenerating all the tones in the nextoctave higher, and so on. Theproblems involved here are mainlyarithmetical, but careful considerationmust be given to the permissibledeviations from the recognised scale ofmusical frequencies, for such devia-tions are inevitable where one shaft isused to drive several different tonegenerators. For further discussion ofthese matters, the reader is referred tothe paper by Winch and Midgleys.

Vibratory SystemsPractical experience with rotating

systems and the numerous mechanicaldifficulties associated with themseemed to indicate that an easiersolution might be found in vibratory

Fig. 4. The basic arrangement of electrostaticpickups in combination with stretched wirestuned to the musical scale. The arrangementexactly as shown is directly applicable with

good effect to an ordinary piano.

systems. Therefore, still using theelectrostatic principle, experimentswere conducted to examine the possi-bilities of generating pure and complextones and of providing satisfactorycontrol with variousvibratory elements.The first trials were made with strings,or more accurately, stretched wires.The results were distinctly encouragingand led incidentally to the adaptation

.of the principle to an ordinary piano.Fig. 4 shows the elements of thisarrangement. The electrodes P1, P2,P2, etc. are small metal plates, orstuds, placed close to the piano stringsat some selected point along the lengthof each string. The vibrations of thestrings produce variations of capacitywhich are translated into oscillatoryvoltages across the high -resistance R,exactly as in the system illustrated inFig. 1. Such electrostatic pianos havebeen marketed by Everett and byChappell. The chief feature abouttheir performance is the remarkablyfine " body " of the bass tones and, ofcourse, the wide range of acousticpower outputs.

Maintenance of VibrationFor the generation of sustained

tones, as in the organ, it was necessaryto be able to maintain the wires insteady vibration. It was obviouslypreferable to keep all the stringsvibrating continuously and to excitethem electrically when the corres-ponding tone frequencies were requiredby applying the necessary excitationpotentials, as in the case of the rotarysystem. The continuous vibrationwas achieved by feeding some of theoutput current from the amplifier backinto the string itself, a magnetic fieldbeing arranged transversely to thestring. The alternating current there-by produced an alternating deflectingforce, and since in this arrangementthe alternating current was generatedby the vibration of the string itself inthe capacity -pickup circuit, the stringwas automatically in resonance withthe current. Consequently, a state ofsteady oscillation was readily attained':



All the required strings covering themusical frequency range were con-nected in parallel and fed from thecommon amplifier which itself was fedfrom all the corresponding capacity -pickups, similarly connected together.For selecting and converting intomusical sound the various tonesgenerated by these strings it wasnecessary to provide a further set ofpickups connected through suitablekeying devices to a sound amplifier,separate from the maintaining ampli-fier. Fig. 5 illustrates the basicfeatures of this arrangement.

It is quite unnecessary, of course, todesign the frame which carries thestrings on the lines of a piano frame.No acoustic output is required fromthe strings ; in fact, the quieter theyare, the better. Consequently, thestrings can lr made quite light, of thinwire and of short length, the tensionbeing very much less than thatrequired in any normal stringed in-strument. Brass or phosphor -bronzewire is satisfactory. Even copper wire,of gauges between 20 and 36 s.w.g.,was frequently used in the earlyexperiments. The magnetic field wasproduced by two small bar magnets,about 2 inches long, placed on oppositesides of each string.

Good electrostatic screening is essen-tial, as in all of these polarised -capacity systems, to prevent humpickup.

The keying of the pickups in such asystem as this cannot easily beachieved by controlling the excitationpotentials of the separate stringsbecause all the strings are connectedtogether for the supply of the main-taining current from the commonoutput transformer. It would bepossible to use a separately insulatedwinding for each individual string,perhaps covering the whole range in aseries of eight or ten output trans-formers with multiple secondaries, butthis method must be regarded as toocumbersome to be worthy of seriousconsideration. Alternatively, the key-ing can be performed by mechanicaldisplacement of the pickup electrodesor the mechanical adjustment ofauxiliary condensers of simple con-struction connected in the pickupleads.

An examination by means of acathode-ray oscillograph of the wave-form produced by these string genera-tors showed that, provided certainprecautions were taken in the choiceof the dimensions, the tones could bemade reasonably pure. In this con-nection it is interesting to note that

300v

Mechanical Keyingof tone pickups

300v.

Rigid frame carrying stretch&wires tuned to full range ofmusical frequencies.

Fig.& The basic arrangement of an electronic organ using stretched wires continuouslyvibrating as tone -generators. One amplifier is used to maintain all the strings In

vibration. A second amplifier provides the acoustic output.

the existence or absence of harmonicsin the vibration of the string is verylargely determined by the position ofthe maintaining capacity -electrode andthe magnetic field. The best resultsare obtained when they are placedrespectively at the centre and at onethird of the length of the stringmeasured from either end. By reasonof the feedback principle, there cannotthen be a node at either of thesepoints, so that the second and thirdharmonics are precluded. If required,the system can be elaborated withfurther pickups and magnets, either toassist or oppose harmonic phases.

The major practical difficulty en-countered in all the experimentalmodels based on this principle was themaintenance of absolutely constantamplitude of all the strings. Slightvariations in the sensitivity cff thepickups or in the gain of the maintain-ing amplifier, and also certain un-intentional feedback effects in thecircuit arrangement, gave rise to slow,wandering changes in the amplitude ofvibration. Such variations are fatal tothe satisfactory synthesis of complexwave -forms, and therefore an improvedmethod of driving the strings wassought.Air -jet String Resonators

It was discovered that stretchedwires could be made to vibrate very

Fig. 6. Air at low pressure emerging from aJet placed close to the stretched wire will

maintain it in steady vibration.

easily at their resonant frequencies bymeans of an air jet (see Fig. 6) and thatit was possible to produce quite largeamplitudes which remained stable overlong periods of time without specialattention being given to the airflow.This method appeared to be highlysatisfactory for all strings vibrating atfrequencies lower than about 800 c.p.s.It was extremely difficult, however, toproduce vibrations in strings of smalldiameter or length and where thetension was high (i.e. the conditionsnecessary for the higher frequencies).Although the air -driven string evident-ly had very promising characteristicsfor the lower frequencies, and mightsuccessfully be combined with someother method better suited to theupper frequencies, no further investi-gation of its possibilties was made,and the search for a system capable ofsatisfactory application over the wholeaudio range was continued.Reed Tone -Generators

At this stage, attention was turnedtowards the vibrating reed as avariable -capacity element. With suchelements it is, of course, preferable touse the principle of variable -spacing atconstant area (as in the case of strings),rather than that of variable -area atconstant spacing (as in the rotarysystem). Again, therefore, the problemresolved itself into the question ofgenerating pure sine -waves for har-monic synthesis, the production of arange of " ready-made " complextones by a variable -spacing methodbeing regarded as unattainable.

There ,seems to be a peculiarprejudice in some quarters against theuse of reeds in electronic musicalinstruments, apparently arising fromthe belief that the acoustic reed -tonewill persist in any electronically-



around the pickup ; e.g. the shape andposition of the electrode in relation tothe reed.

Experiments were made first withreeds taken from a harmonica (morecommonly known as a mouth -organ).Apart from the difficulty in handlingthe very small frames when they hadbeen cut out of the instrument, thesereeds proved to be quite serviceable.Subsequently, ordinary American or-gan reeds were used instead, and these,being much larger, facilitated the workof assembly. A model containing 4.0such reeds, spread over a range of

5 -OCTAVE MANUAL

I I "

NS. -"NZAT ' ' ArAYN.I.07 Arfe,

OTTON r COL GG

c=ic=c=i REED FRAME ,==i uctc:= 40 Experimental c= y.,== Reeds 32-2000c1s,==

Stop -selectorswitch system

A.F. AMPLIFIER

Fig. 7. The general arrangement of the experimental organ using vibrating reed tone -generators with provision for harmonic synthesis of complex tones.

produced tone derived from the reeds.This is entirely without technicalfoundation. The characteristic tone ofa reed is due primarily to the suddenchanges in the flow of air through thereed mounting -frame. Thus, air -wavesrich in harmonics may be generatedeven when the reed is swingingsinusoidally, and the electrical outputfrom a capacity pickup in such a casewill be unaffected by the existence ofthe steeply -fronted air -waves (exceptin a negligible degree by the effects ofacoustic reaction on the movement ofthe reed). The electrical output willbe controlled predominantly by theconditions governing the electric field

amn

Fig. 8. A simplified circuit diagram of the electronic organshown in fig. 7. The depression of the key G causes suitablepolarising potentials to be applied to the pickup electrodescorrespDnding to the G tone -generator and the harmonics 2G,30, SG, etc. Clicks from the switching system are prevented

by the filter

frequency from 32 to 2,000 c.p.s., waseventually constructed with a 5 -octavesingle manual in order to test thecapabilities of the system on a reason-ably large scale. A diagram of thegeneral arrangement is given in Fig. 7.The elements of the circuit corres-ponding to each key on the manual areshown in Fig. 8.

There was considerable difficulty insilencing the direct acoustic outputfrom the reeds, but while it is true thatall the reeds are operating all the time-and therefore the sound might beexpected to be very much greater thanin an ordinary American organ-thereeds themselves were mounted with-out any resonators or sound -chambers,and it was only the higher -frequencyreeds that gave serious trouble. Thebass reeds made very little sound. Inthe experimental model, absorbingscreens of cotton -wool were set upround the reeds, but no doubt one ofthe special sound -absorbing materialswould have been more effective.

Key -click filters consisting of a high -resistance and a capacity were placedas close to the pickups as possible toprevent sudden unwanted changes ofpotential of the pickup electrodes.These filters are shown in'Fig. 8. Bychoosing suitable values, they couldconveniently be made to provide thedesired degree of " attack."

The experiments were conducted farenough to show that the system ofcontinuously -operated reed generatorswas capable of reliable performance asan electronic organ, using the methodof harmonic synthesis for the produc-tion of complex tones. There washowever, a rather troublesome fault,which did not appear in the earlier,smaller models but only when the

(Concluded on p. 161)

M

AichR.F.

OSCILLATOR

MasnteriltControl

O

./...SMaft *St

SELECTIVEMIXER

AMPLIFIER

FIXEDR.F

OSCILLATOR

7.45Mc/s .

ela

O

Fo

RECTIFIER

& A.F.AMPLIFIER -c(

LOUD

SPEAKER

Resonance freauencyof selective

amplifier JI

Frequency of oscillator M

Fig. 9. A system of tone -generation for an electronic organusing the principle of frequency -modulation. The frequencyof the acoustic output is determined by the frequency of vari-ation of the tone -capacities and has no relation to the

frequencies of the R.F. oscillators.



100 kc/sOscillator

Cathode -Ray Tube TracesA Series to Illustrate Cathode -Ray Tube Technique

Part I.-Lissajous Figures (concluded)

By HILARY MOSS, Ph.D., A.M.I.E.E.

Short periodTime constant

H.F AmplifierAttenuatorPhase Shifter

H F AmplifierAttenuatorPhase Shifter

NegativePulse

Suppressor

7-r -

Low series resistanceor

Damped Oscill'ey Ciec-t oX,

°)(2

Additional Harmonic Channelsfor Complex Figures

°y2

Fig. 6, Circuit Network for pulse injection into Lissajous figures,

8.-Pulse InjectionIT has been shown in the preced-

ing articles that the full interpre-tation of the Lissajous figure re-

quires that the direction of rotationof the spot round the figure beknown. In addition it was shown inthe last article that the synthesis ofthe complex figure from the compo-nent oscillations, requires that thecommon points on these componenttraces are also defined. These twodistinct requirements may be simul-taneously met by the injection intothe -igure of a sharp pulse formed bydifferentiating a square wave. Bysuitable choice of the circuit timeconstants, the relative steepness of thewave front and wave tail can be ad-justed so as to form a marking" arrow," with the more graduallysloping wave tail pointing in thedirection of rotation.

Fig. 6 shows the fundamental cir-cuit arrangement of Fig. 2 for thegeneration of the Lissajous figures,but modified to provide for pulse in-jection. The method is fairly obvious,and is a perfectly standard technique.The too kc/s. oscillator frequency isused as the source, since we requireonly one pulse per complete cycle.Derivation of the pulse voltage from

D

one of the higher frequency har-monics would, of course, provide thesame number of pulses per cycle asthe order of the harmonic. Theoutput voltage is " squared " by theusual saturated valve method relyingon grid current. The squaring isassisted by preliminary amplificationof the applied voltage so as to pro-duce a sharper cut-off/conductiontransition in the squaring valvt,which should, of course, have asshort a grid base as possible. Theapproximately square wave output isthen differentiated by the short timeconstant resistance/condenser net-work, this producing the familiaralmost arrow -shaped figures of bothnegative and positive polarity. Bymeans of a triode valve biased beyondcut-off the negative pulse is sup-pressed, and the residual positivepulse appearing in its anode circuitis series connected into the output ofthe too K.C. component oscillation.Note that this technique ensures aperfectly synthronised pulse.

Figs. 55, 56 and 57 show someresults obtained with this method.Owing to the high frequency of thedriving oscillators for the figure, thearrow does not indicate the directionof spot rotation too clearly, since it

is difficult to produce a sufficientlysharp wave front. In this connectionit should be remembered that thetotal " X " sweep time is only 5

microseconds, and the wave front ofthe pulse shown is about 1/5 micro-second. When actually using the ap-paratus, however, there is no diffi-culty hatever, since by adjustmentof the pulse amplitude control it isvery easy to distinguish the wave tail.

In view of this lack of sharpness inthe marking arrow, the use of a" ringing " circuit was tried. Thepulse was employed to shock excitea damped resonant circuit of suitablenatural frequency. Photos 58-66 in-clusive show the results obtained,which are perhaps a little clearer, asthere is less difficulty in recognisingthe exponential tail to the oscillationgroup.

Comparison of photos 58-6o and61-63 shows the different resultantfigure arising from identically -shapedcomponents, due to a reversal in thedirection of rotation of the spotaround one of the component figures.Compkrison of photos 61-4 and 64-66illustrates that a reversal Of directionof rotation in both components merelyreverses the direction of scan in theresultant without alteration of itsshape. These questions were dis-cussed fully in the last article.

Conversion of Lissajous Figures toLinear Time Base - Ryan's Con-struction

Where at least one of the compo-nent axes of oscillation contains onlyone harmonic term, Ryan's construc-tion readily permits the wave shapeon the other axis to be displayedagainst the time (or angle 0) inseadof against sin 0 as in the photosshown.

Fig. 7 illustrates the method whichis almost self evident. A circle isdrawn,, of diameter equal to the totalsweep amplitude on the axis of thesingle harmonic term. This is dividedinto a convenient number of sectors(say 2.n) having the same angle at



Pulse Injection into Lissajous Figuresx = A sin 0 y = B sin (20 + a) x = A sin 0

55

x A sin () y B sin (20 + a)

58

x = A sin 0 y = B sin (20 + a)

61

y = B sin (40 + a)

56

x= A sin 0 y B sin (40 7.)

59

x= A sin 0 y= B sin (40 H-

62

Resultant

57

Resultant

60

Resultant

63



Pulse Injection into Lissajous Figuresx = A sin 0

x = A sin 0

y = B sin (20 + a) x = A sin 0 y B sin (40 Hc)-

64 65

Phase -Splitting with impure waveformy - A sin (0 - x - A sin 0 y B sin (30 + ix)

67 68

Resultant

66

Resultant

69

Pulse Injection into Low -Frequency FiguresImperfect circle due to harmonic on 50 c/s mains Rotation anti -clockwise

70 71 72



the centre, so that AB=BC, etc. Theaxis containing the single harmonicterm is then divided into n equalparts by vertical lines LM, NP, etc.Verticals are then projected from A.B and C, etc., cutting the curve atA', B', C'. . . . Horizontal linesare then drawn from B', C' . . . tothe appropriate vertical lines LM,NP, . . . cutting them in b, c, . . .

which are points on the requiredconverted curve.Phase Splitting with Impure WaveForm

A form of complex Lissajous figureof some interest in cathode ray tubework is that arising from the phasesplitting circuit of Fig. 8. If a puresinusoidal voltage is maintained be-tween the points A and B of this net-work, then the voltages across thecondenser and resistances are alsopure sine waves, but are in phasequadrature, i.e., may be expressed :

.e=A.sin 0 (resistance)y = B. sin (0 - /2) (condenser)

which is merely a special case of (3)and gives rise to the figures of thetype shown in traces Nos. I, 2 and 3(article I). Very often, however,, itis required to produce such a circularor elliptical trace from an impuresource such as the so cycle mains.The latter usually contains a pro-ndunced harmonic content, with thethird harmonic dominant, and weshall now investigate the effects pro-duced thereby.

Suppose that the equation to thevoltage wave applied across AB is

e=E,.sin 0+E0.sin (n.0+ a) ... 20)Since the circuit is linear, the re-sultant current will be the sum of thecurrents due to each component of thevoltage wave considered separately.Thus

sin (0 + R) En.sin (n.0+7)(21)

VCR' +1 1

0(2 -IVP'

n2,02C2

where as usual 0=co.t, and (0=2.'74.In practice .E.<Ei, so that in orderto obtain an approximately circulartrace, R is adjusted to satisfy theequation

R = 1163C (22)[deflector plate sensitivities assumedequal.]

Substituting this value of R into(21) gives

E,.(0C.sin (0 + fl) E..n.wC.sin (n0+7)

VZ V (n2+ i)(23)

as the resultant current through thenetwork.

NEM

Fig. 7. Ryan's construction for conversion offigure to linear base.

A

Bo y2

Fig. 8.

Again : e = i.Z (24)where Z is the impedance of the cir-cuit element. Using (22) and (24) in(23) then shows that the voltageacross across the condenser, ec, isgiven by

Ei. sin (0 + /3 /2)ec =

V.2E.sin (n.0+ y.--.7/2)

A/ (n'+ 1)(25)

while the voltage across the resist-ance er is

E,.sin (0+R) E.. n. sin (ne+er

V.2

Comparison of these last twoequations shows that the harmoniccomponent of the voltage across thecondenser is only lin of that acrossthe resistance. To a first approxi-mation, therefore, the voltage appliedto one set of plates of the cathode-ray tube can be considered as simpleharmonic, since in practice E./ E,.Photos 67-69 illustrate the curveshape synthesis where n = 3. Photo67 shows the perfect circle cor-responding to pure harmonic oscilla-tions on both axes in perfect phasequadrature. Photo 68 shows the thirdharmonic component and 69 theresultant distortion of the circle.

Photo 70 shows the actual circleform obtained from the circuit of Fig.8 energised from so cycle mains. Thethird harmonic content is smallerthan for No. 69 so the circle has lessgeneral distortion, but the same effectis clearly visible. The smaller oscil-lations are presumably due to toothripple on the alternators, or perhapsto multi -phase high -power arc recti-fiers. The essential point is that to ob-tain a good circle from the resistance/condenser phase splitting circuit, itis necessary to energise it from asource of low harmonic content, say,less than 2 per cent. total.

Photos 71 and 72 illustrate pulseinjection on to low frequencyLissajous figures. The technique usedwas identical to that for the highfrequency cases treated earlier. Thetechnique is much easier, however, asthe pulse writing speed need only bequite low, without detriment to thesharpness of the wave front, and itis now an easy matter to see whichway round the spot is moving.Literature

These four articles are intended asan introduction to the mathematicaltheory of the Lissajous figure. Theydo not 'pretend to be complete in thatthey do not contain specific referenceto circuit investigation. The litera-ture on the specific application ofLissajous figures to special problemsis very extensive, but also very acces-sible. The reader is referred to thestandard texts* to which these articlesare merely a complement. Oncethe basic theory is understood, theapplication is generally fairlyobvious.

Part 2 in this series will commencein a few months and will deal withthe choice of Time Bases.

V (n'-1- 1) * e.g. "The Cathode Ray Tube and its Applications"(Chapman & Hall), which contains an extensive

(26) bibliography.

4



VIBRATION destroys ACCURACY

Delicate pivots,fine gearing, tiny links and other details of sensitive instruments are damaged.by vibration.

Sometimes the damage is not instantly destructive, but is in the form of accelerated wearwhich sets up extra friction, impairs accuracy, and reduces the value and reliability ofthe readings obtained.The importance of providing adequate vibration -absorbing mountings for aircraft

instruments is well understood-for these are among the very many thousandsfor which we design and supply Metalastik mountings.But in the factory and the power -station, the effects of vibration are not so

fully realised, perhaps because they are not so obvious-but serious harmoften results.We in this company maintain a full research staff, with complete equipment,who can analyse vibrations from any source and of any character, and

can design mountings to reduce or eliminate their harmful effects.The small illustrations show three extremely simple Metalastik instrument

mountings, the low -frequency type, left, the cross -type, centre, andthe stud -type right.An important development, the Statostable system of mounting,will be shown in a later advertisement.

Metalastik Ltd., Leicester.

'-11,71-1

MET LASTIk



The Mechanism of Leaky Grid DetectionBy S. W. AMOS, B.Sc. (Hons.) Grad.I.E.E.

Fig. I (above). Apparatus for determining Ig-Eg characteristics.

Fig. 2 (right). Ig-Eg characteristics for an AC'HL.

Determination of Ig-Eg Characteristics

THE experimental work under-taken to illustrate this analysiswas carried out with a 4 -volt

indirectly -heated triode, actually anAC/HL. The first step was to deter-mine the Ig-Eg curves and the circuitused for this purpose is representedin Fig. i. It was made clear in Part Iof this analysis that it is possible toconstruct one Ig-Eg curve for any par-ticular value of A.C. input suppliedto the valve. This being so the valuesof input chosen were zero volts (i.e.,no input at all), and 1, 2, 3, 4, 5 and 6volts peak value (i.e.,"readings of .71,1.42, 2.123 2.83, 3.53 and 4.24 voltsR.M.S. as indicated on the movingcoil instrument used). For each valueof input, two lg-Eg curves were taken,one with the anode of the valve con-nected to a source of 140 volts posi-tive, the value of the H. T. supplyused throughout the experiments, andthe other with the anode disconnected.

The procedure for taking thecurves is straightforward enough;the curve for zero volts input is takenwith the slider of potentiometer R, atthe bottom of its travel, by varyingpotentiometer Rg in a series of steps*and noting at each setting the read-ings of voltmeter V. and microam-eter µA. In general we are only con-cerned with readings of µA of less thanloo µA. The readings of µA and V, arerepeated with Si closed. R1 is thenadjusted so that voltmeter V. reads

Part II

- - - - with no anode connexion /g with anode 140r.- 4 pouter

60A c PEA< VOLTS I I

I

I

8 5 4 3 2 1

ifi 3 i tI

I1

1608

e I

I

I I

:I

II e

Z tI

.40

1 I1,rn

I 1 I "e 'i

1 : h 6 ,

20 ,

,

witft2, -4 -3 -2

--E, IN VOLTS -ri0

.71 volts R.M.S. (1 volt peak) and theprocedure of reading µA and V. is re-peated as before with Si open andthen closed. Readings were thus takenup to 6 volts peak value (4.24 voltsr.m.s.). The results for the AC/HLvalve are given in Fig. 2, in whichload lines for grid leaks of value100,000, 250,000 and i,000,000 ohmsare also drawn. This diagram is, inreality, a more comprehensive versionof Fig. 4 of Part I and from it onecan determine the value of Eg forany given value of A.C. input. andgrid leak. For example, if the A.C.input is 5 volts and the grid lealr isioo,000 ohms (point A in Fig. 2) thepeak value of grid current is 46 µAand the peak value of direct potentialdeveloped across the grid leak is, fromthe graph or by calculation, obviously4.6 volts. By increasing the value ofR to 1,000,000 ohms (point B) /g fallsto 5 µA and E.' increases to 5 volts.In the absence of an input signal thevalue of /. is 4 µA with a grid leakof ioo,000 ohms (point C), giving anegative bias of .4 volts and .5 µA ifR is 1,000,000 ohms, making the bias-.5 'volts (point D). The above re-marks only apply accurately when theanode is connected to the H.T. supplyof 140

Determination of A.C. and D.C. Characteristics

In order to assess the performanceof the valve as a detector it is neces-sary to determine how the anode cur-rent depends on the grid potential

both for steady values of grid poten-tial and for A.C. inputs when the biasis obtained automatically from a gridcondenser -grid leak combination. Theformer la-Eg curves, usually knownas dynamic characteristics, we willcall, for want of a better name, theD.C. dynamic characteristics and thelatter will be termed the A.C. dy-namic characteristics. Using the samevalve and H.T. supply the la-Egcurves were determined for variousvalues of anode resistance, R . The,circuit used is illustrated in Fig. .3

and by noting corresponding readingsof V and µA for various values of REthe D.C. dynamic characteristics catbe determined. They are given inFig. 4. The circuit drawn in Figs 5

is the one used for determining theA.C. dynamic characteristics. Thealternating input was provided by thealternating current mains and inorder that the grid condenser shouldpresent the correct value of reactanceat this frequency of its value was in-creased to 2 µF. At 5o c/s. the re-actance of a condenser of 2 ILF.capacitance is 1,592 ohms, the samevalue as for a condenser of too µµF.capacitance at t,000 kc/s. The deter-mination is similar to that of D.C.dynamic characteristics the readingsof the milliammeter being noted forvalues of alternating input of o, i. 2,35 4, 5 and 6 volts peak value. Theresults are given in Fig. 6 for variousvalues of grid leak. The anoderesistance used had a value of 25,000



ohms. The curves are seen to differconsiderably from the D.C. dynamiccharacteristics. To enable a con-venient comparison to be made theD.C. dynamic characteristic forRI = 25,000 ohms is reproduced alsoin this diagram. The disagreementbetween A.C. and D.C. curves at lowinput voltages is due to the presenceof the grid leak and grid ccndenserin the A.C. case, which, as explainedearlier, develops a small negativebias even in the absence of an inputsignal and so reduces the anode cur-rent below the D.C. value. The dis-crepancy for inputs greater thanabout 2 volts is due to the fact,pointed out in Part I, that no matterhow great the value of the alternatingpotential applied, grid current alwaysflows in order to produce the neces-sary grid bias, and therefore someanode current is bound to flow on thepositive peaks of the: A.C. supply.This is obvious from Fig. 8 of Part I.Distortion Content of Detector Output

It can immediately be seen that therelationship between alternating volt-age input and D.C. anode currentoutput is far from linear, and a linearcurve is, of course, the ideal require-ment. The straightest part of thecurve, and this is only approximatelylinear, occurs for alternating inputsof between zero and 2 volts. This partof the curve is very similar to theD.C. dynamic characteristic of apentode valve. It shows some cubiccurvature and we can therefore de-duce that there will be some thirdharmonic distortion in the detectoroutput Elsewhere the curve showsmainly quadratic curvature, suggest-ing the presence of second harmonicdistortion. If a carrier of t voltamplitude when unmodulated is ap-

Fig. 3 (above). Determin-ation of D.C. dynamic

Eg-la curves.

Fig. 4 (right). D.C. dyna-mic Eg-la characteristics

for AC/HL. -5

Cc'

-4 -3 -2E IN VOLTS

cP°

4

3

z

plied to the valve, when modulationoccurs the detector output will con-tain some third harmonic distortionbut small amounts of second har-monic. This will be true for anydegree of modulation up to too percent., when the R.F. amplitude isvarying from zero to 2 volts. If thecarrier amplitude is 3 volts then verysevere distortion will result on modu-lation and the amount produced willdepend, from the curve, on the depthof modulation. If a carrier of, say,6 volts amplitude is applied and ifthe modulation is so shallow that thebend at 2 volts input is not involvedin the detection process, then distor-tion, mainly second harmonic, will besmall as the curve tends to straightenat large input signals. The amplifi-

Mains

AC/HL 14.0v

Fig. 5 (above). Determination of A.C. dynamic Es-Iacharacteristics.

Fig. 6. (right). A.C. dynamic characteristics for Ri 25,000 0.

cation for these big inputs is verysmall, however, being measured bythe slope of the curve.

The graph of the peak appliedalternating potential and the effectivegrid voltage, Eg:, can be deducedfrom Figs. 4 and 6. It is given inFig. 7 and from this we can deter-mine how the amount of distortion in-troduced by the AC/I-14 for a givenmodulated signal on the grid, de-pends on the value of the grid leak,the anode load and the depth ofmodulation. Some numerical exam-ples will make the method of calcula-tion clear,Numerical Determination of Second Har-monic DistortionConsider first an R.F. signal of 3



-3

00

R 1000 000 flR 250 000I1,R . 100 000R C00.13,

J

2 3 4

Af.. INPUT PEAK VOLTS

t

6

Fig. 7. Relationship between effective grid voltage Es and A.C. input peakvolts fer AC, I -IL for various values of grid leak.

volts amplitude when unmodulated,and suppose we require to know thepercentage of secon4l harmonic dis-tortion introduced by the valve fortoo per cent. modulation. The alter-nating input clearly varies from zeroto 6 volts as shown in Fig. 7 so thatthe effective grid signal, Ea' changesfrom -2.77 volts to -.52 volts, themean value being -2.32 volts. Thesefigures apply for, a grid leak of250,00o ohms. Reference to Fig. 4shows that such changes in the valueof E g" will produce correspondingfluctuations in the value of 1., whichwill vary from .97 ma. to 3.2 ma.,with a mean value of 1.36 ma. for ananode load of 25,00o ohms. Of course,it is possible to deduce these resultsdirectly from Fig. 6, which relatesanode current directly to the alternat-ing input for a load resistance of25,000 ohm', but the advantage ofusing Figs., 7 and 4 is that thismethod can be used to give theanswers for any value of anode load.Having found that the anode currentvaries unsymmetrically for this par-ticular value of input we can calcu-late the amount of second harmonicdistortion from the usual expression .percentage of second harmonic dis-

i(1max +irnlo) -'meantortion, D = too x

hoax -Imin

Substituting the values obtained,namely, /,..x=3.2 ma., ./,,,,=97 ma.and 1.....W=1.36 ma., we have :-

a= oo x(3. 2 -I- - I.36

3.2 - .97-32.5 per

cent.This calculation has been made for

various values of input signal (tooper cent. modulation was assumedthroughout) and for various values ofanode load and grid leak and theresults obtained show that the per-centage of harmonic distortion intro-duced, whether second or thirdharmonic, is practically independentof the value of the grid leak andanode load resistance, but varies con-siderably with the amplitude of theinput signal. The latter result wasanticipated, of course, from theinspection of Fig. 6, but it is interest -

Fig. 8. Distortion of ACIHLfor 100 per cent modulation(with 50,000n anode load and

140 volts H.T. supply.'

ing to consider the numerical. resultsexhibited in graphical form ill Fig. 8.The minimum at 1 volt input isnoteworthy.Voltage Gain of the Detector

From Figs. 4 and 7 it is a com-paratively simple matter to evaluatethe overall voltage gain of thedetector. We will measure thevoltage gain by the fraction :-average peak value of the swing of anode potential

(i.e., mean value of both peaks)peak value of R.F. carrier input when unmodulated

As an example of this calculationwe will evaluate the voltage gain fora carrier of 1 volt peak value, a gridleak of 250,00o ohms and an anoderesistance of 5o,000 ohms. For tooper cent, modulation the value of thepeak alternating potential swingsfrom zero to 2 volts and, from Fig. 7,the effective grid voltage swings from-.53 volts to - 1'79 volts. Thisbrings about a change in. anode cur-rent from 1.63 ma..to .95 ma. for ananode resistance of 5o,000 ohms, thisbeing obtained from Fig. 4. TheP.D. across the anode load hence

1.63 x 50,000swings from =81.5 volts

1,000

.95 x 50,000- 47.5 volts, so that

t,000the average single peak value is

81.5-47-5- 17 volts. The voltage

2

gain for these condition's is then17

clearly - or 17 times. This calcula-i

tion has been repeated for variousinputs and the resulting curve isgiven in Fig. 8. It is interesting tosee that minimum distortion and

2040

...--,,---1-

15

ME -411111111Mg ..AMENIIII :All

RYAMSM.; ., ,c, ..,

PA lb. cb. -. 1

g A Milibi..-r -Li

§

.I MINNIIIIIb....4N,.

,.4 6 .....' .

5.

6' Hillre'Ha manic Di tort for ICO% Mix ulalion

00-O 2 3 4 S II

PEAK VALUE Of CARRIER IN VOLTS WHEN UNMODULATED



maximum gain occur for the samevalue of input, namely, 1 volt. Aninteresting point to notice is that thedetector has a certain amount ofinherent A.V.C. for inputs greaterthan z volt.

Distortion and voltage gain will beless for modulation depths of lessthan too per cent. To confirm thiswe could evaluate the voltage gainand distortion for, say, 3o per cent.modulation using Fig. 7. For anR.F. peak of x volt peak value thealternating potential will swing from.7 volts to 1.3 volts in value, andusing the same technique as above,we could find the correspondingvalues of E5" and then,40 so obtainingthe percentage distortion. This cal-culation has been done and the resultsare given in Fig. 8. The voltagegain is, as might have been antici-pated, about .3 of that for too percent. modulation. This curve is notincluded in Fig. 8.Numerical Determination of Percentage of

Third Harmonic Distortion

It is frequently stated that the" unpleasantness " of harmonic dis-tortion is dependent on its order,increasing with increase in the order.A particular percentage of third har-monic distortion is thus more un-pleasant than the same percentage ofsecond harmonic distortion. Thisbeing so, it is necessary to estimatethe amount of third harmonic dis-tortion given by the leaky griddetector; the cubic nature of the A.C.dynamic characteristic has alreadyshOwn the presence of this form. Anapproximate expression for the per-centage of third harmonic distortion,D', is* :-

D'-= max/180 - Io - 1.41 (1.131 - 144

( /58. - I, + I.41 /1,5 - /45)in which /180 represents h... and /0,

Ens and /0 are the values ofthe anode current an eighth of acycle earlier than /,00x and laterthan I. respectively.

Consider a carrier 1 volt in ampli-tude when unmodulated. For a gridleak of 250,000 ohms Eg" is -.5 volts(corresponding to /.0.) for zero input,and -1.88 volts (corresponding to/.5.) for 2 volts input, these two casesrepresenting the extreme values fortoo per cent. modulation. 1155 occurswhen the input is .293 volts (i.e.,1-.707) and Eg" for this value of I.is -.68 volts. /45 occurs when the

Radio Designers' Handbook, edited by F. I..Smith, p. 184.

input is 1.707 volts (i.e., 1+.707) andEg" for this is equal to -1.68 volts.These were obtained from Fig. 7 byinterpolation. From Fig. 4 thevalues of /, , /155 . I145 and /0 forR1= 50,000 ohms are respectively1.65 ma., 1.52 ma., 1.07 ma. and .95ma., giving the third harmonic dis-tortion content as

I 00 X1.65-.95-1.41 (5.52-1.07)

1.65-.95+1.41 (1.52-1-07).70 - .63

= 100 X.70 + .63

.07= too x = 5 per cent.

8.33

The' way in which the percentage ofthis form of distortion varies with theinput is also illustrated in Fig. 8,

Conclusions

Part II of this analysis has shownthat the leaky grid detector gives con-siderable harmonic distortion andthat the amount introduced is moreor less independent of the values ofthe grid leak and anode load,, beingdetermined chiefly by the value of theinput signal. There is one value ofinput which gives minimum totaldistortion and this occurs when thepeak value of the R.F. signal (whenunmodulated) is about one quarter ofthe value of the grid base (given bythe D.C. dynamic characteristic) forthe value of H.T. supply used. Inpractice it is best therefore to adjustthe value of the R.F. input so thatthe mean anode current as indicatedby a milliammeter, shows that thiscondition obtains. This critical valueof the anode current can be obtainedfrom the A.C. dynamic characteristicand will depend on the value of theanode load. For the AC/ HL., forexample, with 540 volts H.T. supplythe anode current should be 2.5o ma.for an anode resistance of 25,000ohms (from Fig. 6 for a grid leak of250,000 ohms and an ,A.C. input ofvolt peak value) and 1.30 ma. for ananode resistance of 5o,000 ohms (fromFigs. 7 and 4).

The optimum values of the gridcondenser and grid leak are bestchosen with reference to the condi-tions given at the end of Part I andthe optimum value of R1 is that valuewhich makes the triode most success-ful as a voltage amplifier (i.e., thevalue of R1 should be two or threetimes the R. of the valve).

Electronic Organ Design -concludedlarge-scale model was finished. Abackground of " roar " was detectedeven when no keys were depressed, andwas unmistakably recognised as thecomposite sound of all the tones in theinstrument, each at a very low level.These very weak tones were evidentlybeing generated by the vibrating reedsfrom the excitation produced by thecontact -potential. This is, or hasbeen, a problem in certain otherelectrostatic organs of the rotary type.

It is regretted that further investi-gations have had to be suspendedowing to the prevailing war conditions,but at least in the interim, the oppor-tunity may be taken to review thesuccesses and the failures of past workand to cast around for new and bettermethods.Tone Generation by Frequency -

ModulationIn conclusion, brief reference may

be made to some preliminary experi-ments along an alternative line ofapproach which showed definitepromise. Instead of using the undu-lating capacity to produce a corres-ponding current in a resistance, it canbe made to change the frequency of anoscillator, the 'resultant modulatedfrequency being detected in anysuitable frequency -modulation receiversA block diagram of the arrangement isshown in Fig. 9, the actual tonegenerators being represented by strings.Readers will recognise that the greaterpart of the electrical equipment is, ineffect, contained in a conventionalsuperhet receiver. A normal short-wave superhet was, in fact, found tobe satisfactory for experimental pur-poses and served to show that ampleoutput was obtainable from eitherstrings or reeds, especially at lowfrequencies. It may be remarked thatthe output from the rectifier will followthe variation of capacity even downto zero frequency -hence the highlysatisfactory performance in the bass.Keying must be performed by thecontrol of the capacity in the pickupsystem.

The outstanding advantages of thismethod of tone generation over theothers which have been described hereare the complete absence of mains -humfrom stray fields and absolute freedomfrom the troubles of contact -potential.In the author's opinion, this systemmerits further careful consideration. -

References1 G. T. Winch and A. M. Midgley, J.I .E.E., Vol. 86,

No. 622, June, 1940.2 British Patent 403444, 433050, 451798, 453846,

454502, 482284 and 499771.3 P. D. Merrill, Electronics, May, 1939, p. 30.4 British Patent 442379.



A New Electron Beam SwitchA new type of vacuum tube in which a flat radial beam of electrons in a cylindrical structure may be madeto rotate about the axis. Features of the tube are its absence of an internal focusing structure andresultant simplicity of design, its small size, its low voltages, and its high beam currents. This descriptionhas been taken from the article by A. M. Skellett in the Bell System Technical Journal*, to whom acknow-

ledgment is made.

IT has long been recognised that thesubstitution of electron beams formechanical moving parts would

offer decided advantages in manyapplications in the field of com-munications. The high voltages re-quired for the usual cathode-ray typeof tube and the very low currentsobtainable therefrom prevent their usein most such proposals ; their com-plicated guns aid their large sizes arealso undesirable features. The kind oftube described herein has no focusingstructure, is small in size, requires onlylow voltages, utilises the cathode powerefficiently, and produces beam currentsof the same order of magnitude as thespace currents of ordinary vacuumtubes.

Fig. i shows the elementary tubestructure. It consists, in the simplestcase, of a cylindrical cathOde of thesort in common use in vacuum tubes,surrounded by a cylindrical anodestructure. When this structure ismade positive with respect to thecathode and there is no magnetic fieldin the tube, the electrons flow to theanode structure in all directions aroundthe axis. ' When a uniform magneticfield is applied with its direction at right angles to the axis, the electronsare focused into two diametricallyopposite beams as shOwn. The beamsare parallel to the lines of force of themagnetic field so that if the field isrotated the beams move around withit. Thus the magnetic field serves bothto focus the electrons and to direct theresulting beams to different elementsof the anode structure.

If ordinary commercial cathodes areused with anode structures an inch ortwo in diameter, roo volts or less onthe anode will draw the full spacecurrent for which the cathode wasdesigned. The application of themagnetic field will then focus from 85to 90 per cent. of this electron currentinto the two beams, the remaining Toor 15 per cent. being lost at the cathodedue to an increase in the space chargewhich the magnetic field produces.Some of the smaller tubes producebeam currents of more than 5 milliam-peres with only 5o volts on the anode

* Vol. 23, April 1944, p. zgo.

CATHODE

ELECTRONBEAMS

Fig.

li

A NODE STRUCTURE -I . Elementary tube structure showing

focused beams.

structure, and in some of the tubes withlarger cathodes beam currents of 5omilliamperes or more are easily obtain-able. The magnetic field strengthsrange from 5o to 30o gauss. .

For some applications it is desirableto eliminate one of the two beams andthis may be accomplished by substitut-ing a uniform electrical field in thetube for the cylindrical one describedabove. The uniform field may beobtained by applying to the anodeelements a series of potentials thatvary according to the sine of the angletaken around the axis. The linejoining the maximum potentials (-1-

and -=) is maintained parallel to themagnetic field so that on one side ofthe cathode the potentials are allnegative and the beam on that side issuppressed. The remaining beam willhave somewhat less current than thecorresponding one in the cylindricalfield, but the magnetic field -strengthrequired for focus is reduced.

The photographs of Fig. 2 show thetrajectories obtained by introducingargon at a pressure of about a microninto the tube. The electrons areemitted from only two spots of activematerial located at the opposite endsof a diameter on the cathode sleeve.In Fig. 2a the line joining, the spots is

lined up with the magnetic field and in2b this line is at an angle of about 45°with respect to the field. This arrange-ment does not reproduce exactly thespace charge conditions in the tube asactually used; butk does serve to give apicture of the electron paths in aqualitative sort of way.Uniform Electric Field

As mentioned above the uniformfield is obtained by imposing potentialsaround the anode periphery varyingas .the sine of the angle. The cathodeis at a point of zero potential.. In thiscase a real electron optical image of thecathode is obtained.

In applications where the beam isrotated by means of a rotatingmagnetic field this electrostatic field ismade to turn by separating the anodestructure into four or six elements (orgroups thereof) and applying eithertwo- or three-phase alternating poten-tials to them.Magnetic Field Supply.

The stator of a two -pole polyphasealternating -current motor furnishes' anexcellent magnetic field for use withthese tubes. The tube is inserted inplace of the armature and when thepolyphase currents are applied thebeams are formed and rotate at thecyclic frequency. For applicationswhere the beams are not rotated con-tinuously, a two-phase stator may beused in which the currents through thetwo windings are adjusted to be pro-portional to the sine and cosine of thedesired direction angle of the beam.Permanent magnets of the horseshoedesign have also been found to besuitable.

Since a polyphase source of power isnot always readily available, it issometimes advantageous to splitsingle-phase power in the .stator itselfto produce the rotating field. Thismay be done by inserting a condenserin series with each winding so that thecurrent through one phase windinglags. by 45° and that through the otherleads by an equal angle. Polyphasepotentials for producing a rotatingelectrostatic field in the tube may thenbe taken from the windings of thestator if desired.



Tube DesignThe particular design of tube de-

pends on its application. The simpledesign shown in Fig. i has been foundadequate for some purposes, but moreelaborate designs which increase theversatility of the tube are also needed.

Fig. 3 shows a tube with 3o anodesthat incorporates various auxiliaryelements. This tube is 2.25 inches indiameter. Closely surrounding thecathode is a control grid that may beused for modulating the current densityof the electron beams. Farther out is acylindrical element with 3o windowsthat is maintained positive and whichby virtue of its similarity in position tothe third element of a tetrode is calleda screen. Immediately behind eachwindow there is a pair of paraxialwires which because of its similarity infunction to the fourth element of apentode is called a suppressor grid.In back of each suppressor grid thereis an anode. In this particular tubethere are projections like gear teeth onthe back of the screen element toprevent electrons, destined for oneanode, from reaching an adjacent one.

The control grid that is close to thecathode is biased negatively and con-trols the electron current in the sameway that it would if the magnetic fieldwere not present. Thus the tube maybe used for amplification in the usualway when the electrons are focused.The presence of this grid has noappreciable effect on the focusing ofthe electrons.

The suppressor grids are generallyoperated at cathode potential or at apotential that is negative with respectto the cathode. They may be used forthree purposes : to suppress secon-daries from the anodes, to modulatethe beam current to their particularanode, and to suppress one of the twobeams. For the first of these functionsthey are biased at cathode potential.For the second they are biasednegatively and have a modulationcurve similar to that of the suppressorgrid in a pentode.

When one beam is suppressed eitherby splitting the screen or by groupingthe suppressors, the currents to thedifferent anodes are not all exactly thesame. For instance, maximum currentwill be received by an anode back ofthe centre of one of the screenelements or one of the suppressorgroups and a minimum current will bereceived by an anode back of thejunction of two such elements orgroups. If two-phase supply is used(4 elements or groups) the ratio of

Fig. 2. Electron trajectories made visible with asmall amount of gas. (a) Magnetic field lined upwith active spots on the cathode. (b) Magneticfield at 45' with respect to the active spots.

maximum to minimum anode currentwill be 0.707 and for three-phasesupply this ratio will be o.866. Therewill be 4 or 6 maxima, respectively,around the tube. This variation maybe effectively eliminated by varyingthe individual anode load impedancesor in other ways.

The anode characteristics are similarto those of a pentode if suppressorgrids are used and to that of a tetrodeif these grids are not used.

Applications

The many possible combinations ofthe tube elements just described permita variety of applications. One of thesimplest and most obvious is that of anelectronic commutator which has theadvantages over the correspondingmechanical device of speed and free-dom from contact trouble. Thereis, however, a practical limitation tothe speed of this electronic commutatorthat is set primarily by the alternating -current losses in the stator. This isestimated to be in the neighbourhoodof I0,000 c/s. for ordinary stator andtube designs. The highest cyclicspeed for a stator that has been usedto date was 60o cycles per secondwhich with utilisation of both beamsgave an effective cyclic frequency of1,200 c/s.

Fig. 3 Radial beam tube with 30 anodes and unwound stator used with it.



The Multiplier Photo -CellIts Advantages and Limitations

By A. SOMMER, Dr.Phil.*

AN inherent difficulty in thepractical use of photoelectriccells of the emission type is the

fact that in most applications thephotoelectric current is extremelysmall, and in many cases photo -currents of less than to -s amp. haveto be dealt with. Therefore, in orderto reduce the necessary amount ofexternal amplification a multiplica-tion of the photocurrent within theactual photocell is desirable. Thishas been achieved by two methods,known as " gas -amplification " and"secondary emission multiplication."In_ gas -filled cells, which cannotbe here considered in greaterdetail, the fundamental process is asfollows : Each primary photo-electron suffers collisions with neu-tral gas molecules with the resultthat some of the latter are ionised.In this way additional electrons areproduced and by repeating the pro-cess of collision and ionisation theprimary current can be amplified bya factor of up to twenty times.

Gasfilled cells have found wideapplication, particularly in soundfilm projectors, relays, etc. Theyhave, however, two serious limita-tions : Firstly, in most cases to avoid` flashover " not more than a tenfoldamplification is obtainable and.secondly, the frequency responsedrops rapidly above 5,000 c/s., incontrast to the practically frequency -independent vacuum cells.

With the development of highdefinition television, photo -cells wererequired, with a frequency responseextending into the megacycle rangewhich obviously excluded the use of,gasfilled cells. On the other hand,extremely weak signals had to bedetected which made an internalamplification of the photocurrenteven more imperative than for pre-viously known applications. The de-mand for high frequency responsecombined with the highest possibleinternal amplification encouraged thedevelopment of the " MultiplierPhotocell."

The principle of this cell is that theelectrons released by light from thea Messrs. Cinema Television, Ltd.

photosensitive cathode are not col-lected by the anode as is the case inthe ordinary vacuum or gasfilled cell,but are directed towards an electrodewhich is provided with a surface ofhigh secondary emission factor. Eachprimary electron impinging on suchan electrode (which has, of course, tobe maintained at a positive potentialwith respect to the cathode) will re-lease several secondary electronswhich can then be collected by ananode. The great advantage of thisprocess is that it can be repeatedmany times by adding more elec-trodes of high secondary emissionfactor each of which is maintainedat a positive potential with respectto its predecessor. In this way anyrequired total factor of multiplica-tion can be obtained, factors ofi,000,000 and over are a practical pos-sibility.

The question may be asked, whywith this tremendous superiority inperformance the multiplier photocellhas not completely replaced the sim-ple vacuum and gasfilled cell. Thefact is that, although multiplier cellshave now been commercially avail-able in various types for about tenyears, they are still used in verysmall numbers only. As this may bepartly due to doubts on the part ofthe engineer whether the use of amultiplier cell would lead to an im-provement in' his apparatus it seemsworth while to consider in moredetail the fundamental facts whichdetermine if for a particular purposea multiplier photocell is preferableto a simple photocell.

In the following, four main pointswill be discussed :-

(z) Economic considerations.(2) Limited output current of mul-

tiplier photocell.(3) Detection of small D.C. sig-

nals.(4) Detection of small A.C. sig-

nals.

Economic ConsiderationsBy using a multiplier photocell

external valve amplification can becut down by several valves, which

represents a definite saving in out-lay and space. Unfortunately, thissaving is offset by two additionalexpenses : The multiplier photocellis more expensive than the simplevacuum- or gasfilled-cell and, fur-thermore, it requires a high voltagesupply (anything between 50o and15oo volts, according to type and de-sired factor of multiplication) whichin many cases means an additionalpiece of "equipment. From these factsthe conclusion can be drawn thateconomic considerations, warrant theuse of a multiplier cell only in appli-cations where a high voltage supplyis needed for other parts of the ap-paratus and has not to be speciallyincorporated for the multiplier cell.

Limited Output Current of MultiplierCellOwing to the detrimental effect of

heat on the stability of the multiplierthe output current in the final elec-trode of the multiplier cell shouldnot exceed the order of i milliampere.Assuming, for the sake of argument,a multiplication factor of zo,000, thismeans that the primary photocurrentmust not exceed o. i microamps, or,for an average cathode sensitivity of2oizA/L (microamperes per lumen)that the illumination must be below0.005 lumen. This limitation of theoutput current has two consequences:Firstly, if more than the maximumpermissible amount of light is likelyto fall on the cathode the multipliercell can only be used with reducedmultiplication factor (this factor canbe controlled by the number of stagesused and by the potential applied toeach stage) the consequence of whichis, of course, a correspondingly re-duced superiority over the simplecell. Secondly, it can be seen thatmultiplication factors above zo,000,although practically possible, wouldbe useless for most applications.

A good example is the use of thephotocell in sound film reproduction.Here the maximum illumination to bereckoned with is of the order of o.1lumen, corresponding to a primaryphotocurrent of 211A (if again acathode sensitivity of 2otzA/L isassumed). Hence, in order to keep



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the output current below 1mA, andmultiplication factor of not morethan 50o can be safely utilised. Whencompared with a multiplication of upto 20 in a gasfilled cell this relativelyminor additional factor of 25 seemsto explain why the multiplier photo-cell is still not widely used in sound -film reproduction.

It is clear from the above con-siderations that the useful range ofillumination for multiplier photocellsis that which corresponds to primaryphotocurrents of less than o.tiLA andin the two following sections we willbe concerned with such small lightsignals only.

Small D.C. SignalsFor the detection of light cor-

responding to photocurrents of to -9down to to -9 amp. a simple photo-cell can be used in connexion witha sensitive galvanometer. The ad-vantage of using a multiplier cell inthis range lies in the fact that thegalvanometer can be replaced by aless delicate current measuring in-strument. 11 the primary photo -current is smaller than to -9 amp. thetroublesome methods of D.C. ampli-fication have to be applied if only asimple photocell is available. Insuch a case the use of a multipliercell obviously represents a great im-provement. Examples of applica-tions of this nature are quantitativespectroscopy and photometry.Small A.C. Signals

In the detection of sinall A.C. sig-nals, two main points have to be con-sidered when comparing a simplephotocell followed by several stagesof valve amplification and a multi-plier cell: frequency response andsignal-to-noise ratio,(A) Frequency Response.

The response of the multiplierphotocell is practically as indepen-dent of frequency as that of a simplevacuum cell. In other words, verysmall signals over a wide frequencyband ale multiplied without distor-tion with the multiplier photocell,which is a favourable contrast to thedifficulties incurred if a simple photo-cell is used and the same degree ofmultiplication achieved by valveamplification. Hence, the multiplierphotocell is' definitely advantageousin connexion with high frequencysignals.(B) Signal-to-noise Ratio.

The determination of the signal-to-noise ratio in a particular system con-sisting of photocell or multiplierphotocell and valve amplifier is acomplicated problem because thereare so many factors which contribute

to the noise. The sources of noisecan be divided into two main groups,the first group (i) consisting ofvarious types of random electron

'emission (shot effect) from the photo-electric cathode,, while the secondgroup (ii) is connected with compo-nents and) conditions outside thephotocell.

(i) The random emission from thephoto cathode can consist of the fol-lowing three components (for a moredetailed explanation see below) :-(a) Fluctuation of photoelectric emis-

sion caused by the A.C. signal.(b) Fluctuation of the photoelectric

emission caused by constant(D.C.) background illumination.

(c) Fluctuation of the thermionicemission at room temperature.

(ii) The noise produced outside thecell can be subdivided as follows :-(a) Thermal agitation in the resistive

component of the coupling im-pedance.

(b)' Shot noise in the first amplifyingvalve.

(c) Various other causes, such asmicrophonic action, hum pick-up,erratic leakage currents, etc.

The noise produced by the firstgroup cannot be separated from thesignal and is obviously multipliedwith the signal if a multiplier photo-cell is used, hence it can be seen atonce that the signal-to-noise ratio isnot improved by using a multipliercell if the noise is chiefly due tocauses in group (i) above. On theother hand, if the noise is mainly oftype (ii) the multiplier increases thesignal to such a high level that thesignal-to-noise ratio will be greatlyimproved.

It is outside the scope of this paperto consider how the noise of type (ii)can be reduced. But if in a particularsystem, owing to preponderance ofgroup (ii) noise, the use of a multi-plier photocell is indicated, the im-provement will obviously be greaterthe smaller the group (i) noises canbe made. Therefore we have to con-sider how the most favourable con-ditions can be, obtained with respectto the above named types of randomemission.

(i)(a). The fluctuation due to ran-dom emission increases relative to thephotoelectric emission as the latterdecreases and therefore determinesthe lowest permissible signal fora given minimum signal-to-noiseratio and for a given sensitivity ofthe photo cathode. This funda-mental limitation to the signal is afurther reason why in most applica-tions a multiplication factor of more

than about 5,000 in the multiplier isof no real advantage.

(i)(b). If part of the photoelectricemission is due to constant (back-ground) illumination obviously thesignal-to-noise ratio will be adverselyaffected because the constant photo-electric emission will not contributeto the signal while the fluctuation Mrwhich it is accompanied will con-tribute to the noise. Hence it can beseen that it is advantageous to worka multiplier cell with a too per cent.modulated signal. The lower thedegree of modulation the greater thenoise relative to the useful signal andconsequently the smaller the advan-tage in using a multiplier cell ascompared with a simple photocell.

(i)(c). Even with no light fallingon the photo cathode there exists asmall constant emission of electronswhich is due to thermionic emissionat room temperature. The fluctuationof this thermionic emission contri-butes to the noise, in some cases toquite a large extent. This unwantedeffect could in principle be reducedor avoided by cooling the cell, but thisis in practice a most inconvenientmethod. Fortunately, the recentlydeveloped Antimony -Caesium photo-cathode has an extremely low ther-mionic emission as compared with themore generally used Silver -Oxygen -Caesium cathode and for this reasona multiplier cell with an Antimony -Caesium cathode should be used inall cases in which a required responseto infra -red radiation does not makea Silver -Oxygen -Caesium cathodeessential.

ConclusionSummarising all these considera-

tions it can be stated that the use ofa multiplier photocell is only indi-cated if the signal to be detected(D.C. or A.C.) is so small that itcorresponds to a primary photocur-rent of less than to -7 amp. Further-more it has been shown that in thecase of A.C. signals the superiority ofthe multiplier cell over the simplephotocell (followed by a valve am-plifier) is more marked,

(a) With increasing width of thefrequency band;

(b) With increasing percentagemodulation;

(c) With increasing noise in thesystem due to causes .outsidethe photocell itself.

Acknowledgment is made toMessrs. Cinema Television, Ltd., forpermission to publish the informa-tion, and to Mr, T. C. Nuttall ofMessrs. Cinema Television, Ltd., forvaluable advice.



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A Valve Voltmeter for Higher VoltagesUNLIKE the peak voltmeter with

diode previously described inthese columns' the valve volt-

meter developed by J. F. Tonnies'makes use of a triode and a diode isonly used additionally in order toenable the measuring method origin-ally devised for d.c. measurementsto be applied to a.c. circuits.

The principle of the method hasbeen known for nearly 3o years, butby certain modifications a valve volt-meter is obtained capable of measur-ing several hundreds of volts with ahigh degree of accuracy and the re-sults of the measurements are onlyslightly influenced by the propertiesof the valve and by the changes atvarious points of the characteristic.This is accomplished by avoiding theuse of transformers and ensuring acurrent -free measurement. By apply-ing the reflex principle' the upper

Fig. I. Fundamental circuit. Vgk voltagebetween grid and cathode of amplifier. Veinput voltage to be measured. V.1 anodevoltage. la anode current. Ra resistance of

the anode circuit.

limit of the d.c. or a.c. voltage to bemeasured is only determined by thevoltage resistivity of the amplifierwhich for ordinary broadcast receiv-ing valves lies at about I,000V.

The basic circuit diagram isshown in Fig. i in which Vgk desig-nates the voltage between grid andcathode of the amplifier, V. is the in-put voltage to be measured, V. theanode voltage, 1. the anode currentand R. the resistance of the anodecircuit.

The voltage drop I.R. counteractsthe voltage V. applied to the grid. itmay be as high as 8o per cent of theanode voltage so that with V. = 700,V. may be as high as sooV.

Fig.12 Relations between la and Vgk Ve and / iRa.

Calling Vg. that value of the gridvoltage which would be obtained atV. = 0 with a straight line character-istic, and v the active amplificationfactor of the valve the following rela-tions are found:

I.R.= V. + Vim (1)

Vgk = Vgo -

v +V. + Vgo = I.R.

(z)

(3)

(The minus sign given on the left sideof the last equation in the originalpaper is apparently a mistake).

The value (v + t)/v which for= zo is i.o5 characterises the

difference between the voltagechanges of the grid and cathode. Asmay be realised the cathode and notthe anode is forced to follow any com-mands given by the grid. The con-nexion described is similar to that ofone of the earliest proposals for avalve voltmeter or ammeter which re-ferred to the accurate measurement ofthe current changes in a photocell andwas the subject of an Americanpatent applied as early as 1914. In

-oVgo Vg

0

Fig. 3. Use of a boosting voltage (GermanPatent 690207) Vgo grid -cathode voltage for /a=0Vm voltage at an instrument connected as load

of the anode circuit.

this country it is also well knownunder the name of " cathode fol-lower " and in U.S.A. as " self bias "or " reflex voltmeter."

The relations between I. and Vgk,V. and I.R. are best understood fromthe diagram Fig. 2.

The constant value Vgo of equation(3) is taken account of in TOnnies'simprovement of this connexion by in-serting a separate boosting voltage in-to the negative lead for the voltageV. (Fig. 3) such that

V'. = 1.R.V + I

The anode resistance R. and am-meter may be replaced by an ordinarymoving coil voltmeter indicatingVm = I.R.. This will be proportional

Fig. 4. Counterconnexion of Vgk. By thechanges in cathod potential voltage changesat Rs are caused through Rk corresponding

to Vgk.

to V. and the factor (v+i)/v may betaken into account either by calcula-tion or by an appropriate reduction ofthe resistance connected in series tothe voltmeter.

Instead of using V,. for correctingthe measurement a voltage equal to,and varying as, Vgk may be generatedand used for boosting the input volt-age. Such a connexion using a volt-age divider is shown in Fig. 4 forwhich the equation

V".=/.R.+ Vgk - V'gk = ImR.= Vm

is valid Thus V. is identical withthe input voltage to be measured:

The connexion shown in Pig. 5 may



Fig. 5. Connexion of an additional appliancefor use with a standard moving coil voltmeterVoltage Vin measured by the latter corresponds

to the input voltage Ve

nerve as an additional appliance forusing any voltmeter as a current -freemeasuring instrument of high ac-curacy and which may be used to ad-vantage for connecting an oscillo-graph.

As may be seen from Fig. 2 the de-viations of the V. curve from astraight line are comparatively smalland they are still less if the amplifica-ti8n factor is about 20 (as is custom-ary) instead of 5 which value has beenchosen here in order to obtain aclearer diagram. This linear trendof the curve holds also for the valuesof /. and /k and thus also for V',k cor-responding to Figs. 4 and 5. The maincause of errors is therefore the devia-tion of Vet from a straight line.Changes in the value of v cause onlyvery slight errors as this value entersthe equation in the form (v + i)/v. Nor are the errors due to changes inthe cathode heating voltage and theanode voltage of great amount. By aproper dimensioning of the resist-ances R., R. and Rt the influence ofchanges of V. may be made'to counter-act those acting on V, or Vgt, especia-ally if R. or R. are large in compari-son with the internal resistance of thevalve. Taking into account allsources of errors an accuracy of -± 2per cent, at the.end value of the scalemay be obtained which may be con-sidered sufficient for operational

measurements in the range of audiofrequencies.

Fig. 6 shows a connexion suitablefor the measurement of a.c. or d.c.voltages. A diode is inserted betweenthe main valve and the measuring in-strument. The resistance Rd con-nected in series to it and parallel to a

conductor connecting one anode platewith the instrument serves for com-pensating the starting voltage of thecathode of the diode.

By inserting a resistance R1 shunt-ing the instrument the sensibility ofthe latter is so much reduced that thecalibration for d.c. is applicable fora.c. measurements also. Co and R,may be connected to the terminals formeasuring mixed voltages and formaking innocuous the influence of thed.c. potential when measuring thea.c. control at the anode of an ampli-fier. The earth connexion of pointc prevents errors which may be due tothe terminal b being capacitivelycoupled to the mains. The resistanceR". shunted by the condenser Ck pre-vents the entrance of destructivelylarge currents into the device duringoverloads.

A measuring outfit actually builtaccording to the principles describedand suitable for mains connexion has anearly linear scale for eight differentranges of measurement. For insert-ing these different ranges press but-tons are used which- are mutuallyblocked.

R. NEUMANN.

Fig. 6. Connexion for a.c. voltages rectifiedby a diode (German Patent 699719).

References1 BecoN, W., " Peak Voltage Measurements,"

Electronics, Television and Shortwave World, 13,p. 323, 1940.

2 Townies, J. P., " Rohrenvoltmeter filr holiereSpannungen," ETZ (Elehtrotechn. Z.) 63, p. 163,April 9, 1942.

3 RIDER, J. F., " Vacuum Tube Voltmeters," NewYork, 1941.

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CORRESPONDENCESelf -Testing Relays

DEAR SIR, -I read with interest Mr.W. Bacon's article on " Self -TestingRelays " in the June issue of " Elec-tronic Engineering." By the des-cription of the various circuits, it ap-pears that two instruments are re -

Oquired, one for relays with " make "contacts, and another for " break "contacts. It seemed to me that oneuniversal instrument with a change-over switch incorporated would bemore useful. Therefore, I append amodification of Mr. Bacon's circuitwhich appears as Fig. 4 in his article.

My suggestion is that, instead ofthe charging circuit including the" break " contacts of a relay undertest, it should be connected to thecommon terminals of a double -pole,double -throw switch. One side of theswitch is connected to the contacts ofa permanently fitted relay whichbreaks on operation. The operatingcoil of this relay is 'connected acrossthe power supply in series with thecontacts of a relay to be tested (iffitted to " make " contacts). Therelay contacts are connected to ter-minals ".T." Terminals T are alsoconnected to the remaining terminalson the change -over switch.

When testing " break " contactrelays, their contacts are connected toT and the switches arranged as in thediagram. S. isolates the test relayfrom Ry,. The circuit then acts asin Mr. Bacon's Fig. 4.

With " make " contact relays, S.is changed over, switching in the" break " contacts of Ry. and St isclosed putting the operating coil ofRy, in circuit with the " make " con-tacts of a test relay.

When the " operate " current of

the relay is reached, its contactsclose, energising Ry, which in turnopens its contacts and thus providesthe break in the charge circuit. Thereverse occurs when ascertaining the" release " current.

Yours faithfully,V. N. SPENCER.

Grid Bias Arrangements in Phase -SplittingAmplifiers

SIR,-With reference to the note onthe above subject by E.K.M. Bird,published in the August issue ofELECTRONIC ENGINEERING, the dangerof over -biasing by the flow of block-ing condenser leakage currentthrough the grid leak is a very realone. There is, however, an easyway of avoiding the difficulty by theuse of a few additional components.The method is essentially one ofbalancing the voltage which causesthe leakage current. A suitable cir-cuit is given below :-

The resistances R. R, form a poten-tiometer across the H.T. supply. Thetapping point is arranged to be at thesame potential as the junction of biasresistance and cathode load. Noleakage then occurs through C. andleakage through C, is immaterial.The A.C. conditions are unaltered.

At the input side, a well-known ar-rangement of double coupling con-densers and resistance removes the

worst effects of leakage. At D.0potentials, the resistance of the con-denser C is much higher than thatof the. resistance R,. and the P.D.across the latter is so low that objec-tionable leakage cannot occurthrough the coupling condenser C.'and the grid leak R,.

Yours faithfully,F. BUTLER.

Identifying ComponentsSIR, -I am in complete agreement

with Mr. Hurran's suggestions forconcise reference for circuit com-ponents. I have myself for some timeadopted a comparable method forconvenient reference in varied cir-cuits. My practice has had particularregard to annotations adopted byvarious draughtsmen and suffers bycomplication thereby.

In the case of resistors I use" VR " for variables and small let-ters following " R " to show thefunction, i.e., RL=Load RSD =ScreenDropper Rb = bias RCL = Cathode

Load and so on ; but this could wellbe obviated by some agreed table,which, however, should not omitwattage. It is a pity that valve basesare so numerous or the number of thepin might have been brought in-as Ihave done, being mostly limited to9 -pin and Octal bases.

It would, in the least, be helpfulif some inconsistencies could beironed out and uniformity insistedupon. For instance, I have a circuitin front of me which shows a capacityof .0003 µF. and another i,000 pf.feel that " µ " has so many uses intheoretical statements that it wouldbe preferable to use small " mf " andsmall " pf." I notice that ELECTRO-NIC ENGINEERING still retains " µµ "which, after all, is a rather cumber-some notation. " K " for i,000 isvery well established, but not com-pletely so. (Incidentally, I find thatits use in calculation simplifies arith-metic, e.g., a simple parallel pathmight be 0.5 M and IooK = Soo. loo

5ooKK2/600K = = 83.3K, i.e., mental

6.oarithmetic throughout).

Yours faithfully,A. J. PENNELL.

SEPTEMBER MEETINGSInstitute of Physics

The Organising Committee has de-cided to postpone the Conference on" Instruments for the AutomaticControlling and Recording of Chemi-cal and Other Processes," which wasto have been held in X.ondon next month.

The date of the postponed confer-ence will be given in due course.

Electronics GroupA meeting of the above Group will

be held on September 19, at 5.30p.m., at The Royal Institution, 21

Albemarle Street, London, W.i Apaper will be read by Dr. A. Sommerof Messrs. Cinema Television, Ltd.,on " Principles of Photo -electricEmission and Their -Application inPhoto -electric Cells."

Institution of ElectronicsN.W. Branch

The next meeting of the above willbe held on September 22, at 6.30p.m., at the Reynolds Hall, Collegeof Technology, Manchester. Dr.D. G. Drummond will lecture on" The Electron Microscope."

Tickets may be obtained on appli-cation' to Mr. L. F. Berry, 14Heywood Avenue, Austerlands, Old-ham, Lancs.



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ABSTRACTS OFELECTRONIC LITERATURE

CIRCUITSElectrical Hearing Aids

(T. S. Littler)An account is given of the problems

involved in the design of electricalhearing aids and components. De-tails of a number of circuits, bothportable and non -portable, typical ofmodern practice, are included, andmethods of measuring effective am-plifications are discussed. Finally,special adaptations, such as tone -control adjustment and automaticgain -control, are considered.

-Jour. I.E.E., Vol. 91, Part III.No. 14, June, 1944, p. 67.

Feed -back Amplifier for C.R. Oscillo-scopes

(G. R. Mezger)Practical application of feed -back for

impedance compensation in amplifierdesign, given wide -band performancewith small sacrifice of other charac-teristics. Both electrostatic and elec-tro-dynamic deflection applications arecovered.

Electronics, Vol. 17, No. 4 (1944),p. 126.

Standard Frequency Source(H. L. Clark and H. Johnson)

The standard frequency source des-cribed in this article was developedas a means of generating accuratefrequency, primarily for use withstroboscopic methods of measuringsmall changes of speed. The fre-quency output is accurate to within0.02 per cent. The equipment com-prises a tuning -fork controlled oscil-lator, the frequency of which is heldconstant by the natural frequency ofa magnivar tuning fork, an amplifierand a voltage regulator. With aninput voltage of 105 to 125 V. at from5o to 70 cycles, the output up to 25 W.is 115 V. at 6o cycles, with a totalharmonic content less than 2 per cent.The operation of the equipment isexplained and the tuning fork systemdescribed and analysed for free andforced vibrations.

-G.E. Rev., May,

MEASUREMENTPhotoelectric Torsiograph

(W. Spellmann)The torsiograph operates on the

stroboscope principle, the relativedisplacement of two flywheels beingmeasured by the amount of lighttransmitted through suitably spacedslots. For this purpose the two fly -

1944, P. 42.*

wheels are in the form of hollowdrums, which rotate one inside theother. The drums are closed at oneend and their supporting shafts arein line but face in opposite directions.The inner drum is very light and pro-vided with a belt drive to the shaftunder investigation, or can be drivendirectly from the end of this shaft.The second drum is heavy and actsas a reference inertia mass. It isdriven from the first drum by meansof a radial spring member, the rela-tive motion being limited to ± 5° bymeans of a stop. Both drums arefitted with. 12 slots inclined at 54° tothe axis, the external drum slotsbeing illuminated by a close fittingring carrying 11 miniature electriclight bulbs. The light entering theinner drum is caught by a mirror(paraboloid of revolution) attached tothe solid end of the drum andreflected on to a stationary photocell(caesium -gas filled) supported insidethe hub of the inertia drum. Thetime lag of the cell is so short that itcan be neglected.

The photoelectric current is ob-served by means of a, cathode rayoscillograph, a polar diagram beingpreferable on account of the simplerelationship between r.p.m. and har-monic order. The basic circle forthis diagram is obtained by generat-ing a sine vibration of shaft fre-quency which is then split up intotwo components at 900 phase dif-ference and which are passed to thefirst control grid of a special ampli-fier and then suitably applied to thetwo pairs of deflection plates of theoscillograph.

The photocell controls through asecond valve grid the anode currentin step with the torsional vibrations.

-Schweizer Archiv, Vol. 8, No. 8,August, 1942, pp. 252/255).t

Electronic Timer for MicrosecondIntervals

(P. B. Weisz)

This article describes an apparatuswhich,, working on the capacitorcharging principle, measures smalltime intervals. The method of opera-tion of the electronic circuit is des-cribed, together with problems asso-ciated with its design. The mechanicalmeans for overcoming leakage andphotoelectric effects in the V.T. volt-meter circuit are shown. Alternativemethods of calibration of the instru-

ment are indicated as well as the cir-cuit diagram of the apparatus forgeneration of a pair of pulses havinga given known time spacing.

-Electronics, April, 1944, p. 108*

INDUSTRYAn Electronic Indicator for Liquid

SeparationU. W. Broadhurst)

The presence of a non -conductingor conducting liquid in a pipe line isdetected electronically by means ofan oscillator, which is thrown in andout of oscillation according to theliquid in the line. Passage of a conducting liquid through the oscillatorinductance causes an eddy currenthiss, thus destroying the magneticfq,edback.

-Jour. Sci. Inst., June, 1944, p.to8.*

BerylliumCopper in Instrument Design(L. B. Hunt)

The physical characteristics ofberyllium copper are described withparticular reference to elastic proper-ties. The usefulness of this alloy forinstrument springs and similar partsis shown to be in its combination ofhigh elastic limit with relatively lowelastic modulus. The bearing of theseand of other properties of berylliumcopper upon the problems of arrivingat appropriate deflections and workingstresses is then discussed for the prin-cipal types of springs and pressureresponsive elements. Details aregiven of the technique required forthe fabrication and heat treatment ofberyllium copper.

-four. Sci. Jnst, VOL 21, No. 6.June, 1944, p. 97.

A photoelectric Device for RecordingVariations in the Concentration of a

Coloured Solution(T. B. Davenport)

An apparatus is described which iscapable of indicating by photoelectricmeans the concentration at any instantof a temperature controlled colouredsolution whose concentration is varyingwith time such that its optical densitychanges between 3 and o, the apparatusbeing calibrated with standard solu-tions.

-Jour. Sci. Inst., Vol. 21, No. 5(1944), P. 84.

Supplied by the courtesy of Metropolitan -VickersElectrical Co. Ltd., Trafford Park, Manchester.t By courtesy of R.T.P. Section, M.A.P.



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BOOK REVIEWSPractical Electrical Wiring and

ContractingEdited by A. C. Greenwood 384 pp. 400figs. (Odhams Press 8s. 6d. net).

Although the radio engineer is notoften actively concerned with electricalsupply matters there must be a numberof cases where the local supplier and ser-viceman is called on to make alterationsto supply circuits or install appliances.

This book should not therefore beconsidered as of interest only to theelectrical contractor, and its descrip-tion of wiring and installation methodsis so thorough that there will be nodifficulty in making a satisfactory jobwith its aid.

It is emphasised in the forewordthat all the wiring procedure conformsto the latest regulations and standardspecifications, and the specificationsfor lighting installations include themost recent developments in fluor-escent lighting fixtures. A chapter alsodeals with bell circuits, alarms, andelectric clocks.

The book concludes with a sectionon planning and contracting withtypical suggested layouts.

Altogether the book is an extra-ordinarily comprehensive treatise on thesubject and is well worth investment,particularly as the price is so moderate.

The Physics of Musicby Alexander Wood. 255pp. 110 figs(Methuen 21s.)This excellent text by Dr. Wood,

the well-known physicist and FacultyLecturer in Musical Acoustics atCambridge University, is an attemptto present to those concerned withmusic the knowledge available to -dayof the borderland between physics andmusic, in which so much new know-ledge has been acquired in recentyears.

The first five chapters cover thegeneral theory of sound, or, whatmay be called, classical acoustics,with many historical references, lead -

Books reviewed on this page or ad-vertised in this Journal, can be obtained

from

H. K. LEWIS & CO., LTD.136 Gower Street, W.C.I

If not in stock, they will be obtained from thePublishers when available.

ing up to a chapter on the ear andthe mechanism of hearing. The fol-lowing three chapters survey the pro-duction of sound by the vibration ofstrings, organ pipes, the human voice,and various orchestral instruments,both percussion and wind. The nexttwo chapters deal with dissonance andconsonance, and scales and tempera-ment, with the twelfth chapter re-viewing briefly the various methodsof recording and reproducing sound

Apart from the printer's errors,seemingly inevitable with any firstedition these days, serious errors arefew, although the passage in the firstchapter (p. 2o), referring to beatsis ambiguous, to say the least.

The publisher's note, inside thepaper cover, mentions a mathematicalindex, but this does not appear in thebook itself. The treatment through-out the book is admirably lucid, andalthough the presentation is designedprimarily for those whose chiefinterest is on the musical side, it isequally informative for those readersconcerned with the physical aspectsof the subject. D. W. A.

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The charge for miscellaneous advertisementson this page is 12 words or less 4/- and 4d.for every additional word. Box numbers(4 words) plus 1/- extra. Remittance shouldaccompany advertisement. Cheques andP.O.'s payable to Hulton Press Ltd., 43 ShoeLane, E.C.4. Press date -15th month forfollowing issue.

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Condenser Specialists for over 20 yearsDaly (Condensers) Ltd.,

West Lodge Works, The Green, Ealing, W.5.Telephone: Ealing 4841

-PHOTO -ELECTRIC CELLSSe/Te on gold -alloy, super -sensitive tolight, Argon Gas - filled, permanent,operate relay direct, or with ValveAmplifier, perfect reproduction ofSpeech and Music from sound trackof films ; large tube 3fin. from topto valve pin base, lin. diam., 38/-;small tube 2in. from top to ter-minal base, /in. diam., 30/- ; operateon 40-100 volts, connections blue printfree. Despatch by return.

CEFA INSTRUMENTS38A, York Street, Twickenham, Middx.

POPesgrove 6597

TRANSFORMERSto customers' specifica-tions or in accordancewith standard list.

tWestmoreland Rood. London. N W 9 Coltnda,e 7131

W. BRYAN SAVAGE LTD.

AFTER THE WAR IThe advance in Radio Technique after the warwill offer unlimited opportunities of high payand secure posts for those Radio Engineerswho have had the foresight to become technicallyqualified. How you can do this quickly andeasily in your spare time is fully explained in ourunique handbook.Full details are given of A.M.I.E.E., A.M.Brit1.R.E., City and Guilds Exams. and particularsof up-to-date courses in Wireless Engineering,Radio Servicing, Short Waves,TELEVIS1 N,Mathematics, etc., etc.Prepare for to -morrow's opportunities andpost-war competition by sending for your copyof this very informative 112 -page guide NOW-FREE.

British Institute of Engineering Technology(Dept. 337a) 17, Stratford Place, London, W.I

INSTRUMENTifee SOLDERING ,IIONIDEAL FOR DELICATE

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September, 1944 Electronic Engineering

VALVEBRITISH

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Telephone: Harrow 0895.

MIDGETVALVESHIVAC LIMITEDGreenhill Crescent.Harrow on the Hill. Middx.

ERNHALL ST.. WALTHAMSTOW. Eli. PHONE


iv Electronic Engineering September, 1944

7elephose;,-

GERRARD

2089

WEBB'Shands' are eng

As stockists and distributors of Eddystone Prod

instrument we are selling and servicing toda), is the highly practical and

358X RECEIVER, which can be examined at our London Showrooms.

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te or call whatever

Intelligent

requirementsWri.

tieahrecii npff euled.

serviceWEBB'S Radio.

14, SOHO STREET, OXFORD STREET, LONDON, W. I.HOURS:- 9 am. tin 6 p.m for OFFICIAL business. SHOP HOURS :-/Go m to 4 p.m. SM./0a m to 12 noon)

FrequencyRange 31,000 to90 Kc s. Sensi-tivity (above1,500 Kc s) -3 microvolts.Full circuit andperformancedetails from 30page instruc-tional book -2/6post free.

Printed in Great Britain by The Press at Coombelands, Ltd., Addlestone, Surrey, for the Proprietors and Publishers, Hutton Press, Ltd., 43-44, Shame Lane, London, E,C.'Sole Agents for Australia and New Zealand : Gordon and Gotch (A/sic), Ltd. South Africa: Central News Agency, Ltd.

Registered for Transmission by Canadian Magazine Post.


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INCORPORATING ELECTRONICS, TELEVISION …americanradiohistory.com/Archive-Television-UK/Electronic...n 11 -British product-tire result of excensive ... The coils are contained in enclosed