ESSUKE EFFECT IN SELENIUMS TALS AND ITS RELATION

TO THE LIGHT EFFECT

ERN E S T O T T O \D I E T E R I C H ,

A .B ., U niversity 0 owa, 1912M . S . , U niversity of I owa' 1914

Fa A N G E

' UL 3 1928

THE PRESSURE EFFECT IN SELENIUM

CRYSTALS AND ITS RELATION

TO THE LIGHT EFFECT

ERN E S T O T T o QI E T E BI O HA .B ., U niversity of Iowa, 1912

M .S ., U niversity of Iowa, 1914

T H ES I S

S ubmitted in partial fulfillment of the requirements for the D egree of D octorof Philosophy Physics, in the G

fiuate College of the

S tate niversity of Iowa, une, 1916I

IowaCity, Iowa

T H E P R E S S U R E EF F EC T I N S EL EN I U M

C R Y S T A L S A N D I T S R EL A T I O N T o

T H E L I G H T EF F EC T

By E . O . D IE T E R I O H

ABSTR AC T

T he efiect of pressure on selenium crystals is shown to be of adifferentnature from that produced by light . A ccording to Brown ’s theory of electricconduction i n selenium, the change of resistance produced by light is due toachange in the rate of emission of electrons from theatoms during illumination, while their coefi

'icient of recombination is unafi ected by the intensity of

illumination . T hisassumption is here verified by experiments. I n the case ofpressure, however, these experiments show that the coefi

‘icient of recombination

is the factor that is altered,and the greater the pressure the smal ler is thecoefficient of recombination . A consideration of the various agencies capableof changing the conductivity of selenium, suchas light, pressure, temperatureand potential gradient, leads to the conclusion that the only one which modifiesthe rate of emission of conduc ting electrons is light, the othersaffecting onlythe coefficient of recombination.

INTR OD U C TION

T he discovery1of a method of producing large crystals of sele

nium provided additional means for the study of the interestingoptical and electrical propert ies of this element in the crystallineor metallic modification . Asaresult of the - investigation of some

of these properties, Brown 2 has proposed an explanation of the

nature of the electrical conductivity of selenium . According tothis theory, which is essentially of the same natureas ' . ' . T hom

son’s doublet theory of electrical conduction in metals, the conductivity of selenium may be altered in two ways, either by a changein the rate of liberation of conducting electrons from the atoms, orby a change in the rate

.

of recombination of the electrons with theatoms . T hese assumptions were verified in certain experiments3 inwhich the cfiect of light in changing the conductivity of seleniumcrystals under various conditions of temperature, pressure andpotential gradient were studied .

S ince a change in pressure produces suchamarked efiect on the

1 F . c. Brown, P hys. R ev., 4, 85,2 F . C . Brown,Phys. R ev., 5, 395,ar . 0. Brown, P hys. R ev., 5, 395, (1915 ) and P hys. Bern, 7,551,

6 7 4 6 7 8

resistance of selenium it is of importance to investigate the pressureeffect in more detail, in order to test the theory more completely

in that direction . T he experiments described below were so ar

ranged that the changes oi conductivity produced by pressure

changes could be studied immediately after the variation in pressure .

TH E OR Y

Assuming that the conductivity of'

a selen ium crystal at anyinstant is directly proportional to the number of conducting elec

trons present, it has been shown' that when the equilibrium con

ditions have been disturbed, the rate of change of conductivity

may be expressed by

di/dt a12

H ere 5 represents the equilibrium conductivity of a crystal under

fixed conditions of light intensity, temperature, pressure and poten

tial gradient,and ais the coefl

‘icient. of recombination of the con

ducting electrons with the atoms .

E quation (1 ) was tested in-the form

A i/A t

by a series of experiments in which the equilibrium value of the

conductivity was altered by a change in the light intensity. A i'

A?

was measured directly (A t being .02 sec . ) andacalculated . F or

light intensities over a wide rangeawas found constant, justifyingthe assumption made that ais independent of light in tensity andhas the same value in the darkas in the light .At equilibrium, in the dark,

di/dt a (2 )

Writing q f or the rate of emission of conducting electrons in thedark, and M for the increase in the rate of emission due to light,we have, at equilibrium in light of constant intensity,

di/dt 2: aif = ’

M q

in which i, is the equilibrium conductivity in the light .F rom the above equation we obtain

4 F:0. Brown,Phys. Boy?

5,395,

which shows that a change in conductivity may be produced byvarying either M or q . T he constancy ofain light of various widely different intensities shows that M is the quantity modified by

exposure to light .

S imilarly, if it beassumed that change of pressure alters the rateof emission of electrons in the same manner that change of illumin

ation does,we may write M 1for the increase due to pressurealone,

and O btain

T he constants in E q . (5 ) may be determined by the same methodemployed for E q . the change in conductivity being produced

by a variation in the pressure upon the crystal . I f the pressure

and light effect are the same a, will remain constant for differentpressures .

E ' P E R IM E NTAL AR RANG E M E NTS A N D M E TH ODO F M E ASU R E M E NTS

T he crystal under observation was held horizontally between

platinum electrodes in one arm of a Wheatstone bridge . A weightof 500 grams, L ,

ri gidly fixed to a hinged rod rested upon the upper

electrode . T his weight was connected to a shaft of a heavy timingpendulum by means of a cord passing over a pulley system . T he

pendulum served as a means of raising L at the instant of clos ing

the key in the galvanometer circuit . A second key was also tripped

by the pendulumand opened the circuit at the end of a pre -deter

mined short time int er val,in these experiments, .04 sec . D ifferent

constant pressures were obtained by suspending weights W from

a long shanked hook attached to the upper electrode and passingthrough a hole in the base . T hese weights were so chosen that the

pressure upon the crystal ranged from10 kg/cm2 to 100 kg/cm

z.

At pressure higher than the latter the crystal resistance became toounsteady to obtain reliable readings . T he electrodes and weightswere enclosed in a light tight box provided with a slit directly infront of the crystal, which was opened when the light effects were

studied . A N ernst glower, at a distance of 75 cm . from the crystal ,was used, and the time of exposure of the crystal to light was regulated by means ofashutter attached to the timing pendulum .

T he method of Brown and C lark5 for measuring rapid fluctuaS E . C . Brownand W. II . Clark, P hys. R ev , 32, 251, (1911 ) and Phys. R ev.,

33, 53,

tiOns in resistance was slightly modified to obtain the change of

resistance of the crystal during the .04 sec . following a change inthe equilibrium conditions . All factors besides light intensity and

pressure were kept con stant . R eadings were made at room tem

perature which was maintained constant to a few degrees centigrade . T he potential drop across the crystal was at all times

‘

14 .2

volts, and since the crystals were all of approximately the same sizethe potential gradient may be assumed constant. A large number

of crystals of several types was ' studied and satisfactory duplication

of data obtained . F or purposes O f comparison the results givenbelow were all chosen from the data referring to a single crystalwhich was a large hexagonal specimen about 1 mm . t hick and 5

mm ' long .

R E SU L TS

T he P ressure E fleet in the Darkand under C onstant IllnminationT he crystal was allowed to reach equilibrium in the dark under

the pressure due to W L . Its recovery in the first .04 sec . im

mediately following a change'

in the pressure due to the removal

of L was determined for different values of W. T his procedure

was .followed rather than that of changing the conditions by the

addition of L because of the danger of crushing the crystal continuously exposed to

,

light of constant intensity . T able I, below,

summarizes the data .

TAB L E I

g

P R E S S U R E E F F E CT O N C O N D U C TI ' ITYL z 500 grams ' 10 initial conductivity ' il conductivityafter .04 sec.

i, A i

(x10? ) (x10? ) (x10? )Crystal in the Dark

.89

.79

Crystal in the L ight

C olumn 5 of the table shovvs the decrease of a”

with increas ing

pressure, the rate of decrease be ing a little greater in the light thanin the dark when the . factor by which the conductivity is alteredby pressure is the same . In the next columnare tabulated the varia

tion of ed,2 with pressure, the,value of (i i,

2 showing a decided decrease . N ow,

from E q . (4 ) it is evident that a change in i may be

the result of either a change inaor a change in M q, the rate of

emission of conducting electrons . T hese experiments show that

part of the change, at least, is due to a change in a' the variationin ais not sufficient to warrant the conclusion that the only effectof pressure is to vary the rate of recombination,and not the rateof emission . But considered in connection with the decrease in

ozi,2 and with experiments to be described in the next section, .

it

may be concluded that such is actually the case .

T he L ight E fi‘ectat D ifferent P ressures, Intensity ofIllumination C onstant

In these experiments,after equilibrium conductivity had been

reached in the light the intensity was reduced by a constant amount,bymeans of a darkened photographic plate,and the rate of re

covery measured for difierent values of W. T he values foraandfor o i,

” can then be compared with those determined for a constantchange of pressure above .

TA BL E IIL IG H T E F F E CT A S A F U N C TI O N or P R E S S U R E

i0 1, A i or. (l ioz

W (x10? ) (x107 ) (x10? ) (x10-7 ) (x10? )

500gm. .106

1000 078

2000 054

3000 .039

4 000 .033

T able II represents the results obtained,a decrease in abut anincrease in 011

0

2 showing quite clearly that the action of pressure

and light in producing a conductivity change are different . T he

pressure effect is shown in the d ecreasing value for a, the lighteffect, in the increasing values for (11

0

2(S ee equation

T he L ight E ffectat C onstant P ressure, Intensity ofIllumination 'aried

T able III summarizes the resul ts obtained,agreeing with thosedetermined by previous work6 and the

'

constancy of u again points

to the change In the rate of emission of electrons under the influence

of illumination .

o r . C . Brown, Phys. R ev, 5, 395,

TABL E I I I

io 11 A 1 0. (1102

W (x10? ) (x10? ) (x10? ) (x10-7 ) (x10? )

1000 gm. .031

1000 .90 .033

1000 .4 5 .029

T he E ffect ofaS imultaneous C hange in P ressureand L ight I ntensity is shown in T able

'

I ' , and indicates the dissimilarity in theaction of light and pressure .

TA BL E I '

W 500 grams.

io 12 A i a aiog(x10? ) (x10? ) (x10? ) (x10 ? ) (x10? )

L ight .62 .09

Pressure .34

L ight 85 pressure .30

T he values ofadue to a change in pressure, and to a change inboth light and pressure are practically the same, showing that the

change in conductivity produced by the action of light is caused by

a change in the rate of emission of electrons by the atoms . T he

behavior of the crystals in these experiments is worthy of note .

When they were kept in the dark, and the conductivity altered bya change in pressure, they rapidly recovered their ini tial conduc

tivity on the restoration of original conditions, the recovery from

the light eHect was somewhat slower, a matter of a quarter of an

hour, but the return to equilibrium conditions when disturbed -by

both light and pressure was very slow—frequently several hourswere required before the initial conductivity had been reached .

C ONC L U SIONS

Inall the results discussed above the same fact is evident, thatis, that the effect of pressure is merely to decrease the coefficientof recombination of the conducting electrons with the positive residues of theatoms from . which they have escaped . In no

’

case can

the experimental results be taken to indicate an increased rate of

emission of electrons, for with increasing pressure, a decreasingvalue ofawas always observed . N ow,Brown

7 has shown that withan increase in the potential drop across a crystal the value ofadueto a change in the light intensity is decreased in the same manner

0

7 L oc. cit.

as it is in the case of pressure . F urther M rs. D ieterich8 showed thatthe temperature efiect onais ‘

similar to that observed in the caseof pressure and voltage . T he conclusion is obvious—the on lyagencywhich changes the conductivity of selenium by causingan increasein the rate of emission of conducting electrons is light ' pressure,temperature and potential difference affect the coefficient of recom

bination of the conducting electrons with the atoms .

8 K . ' . D ieterich, Phys. R ev. 2, ' ol . ' I I , p . 551,

TH I S B O O K I S D U E O N T H E L A S T D A TES T A M P E D BE L OW

A N I N I T IA L F IN E O F 2 5 C E N T SW I L L B E ASS E S S E D F O R F A I L U R E TO R E T U R NTH IS BOOK ON TH E D ATE D U E . TH E P E NAL TYW I L L IN C R EAS E TO 5 0 C E NTS ON T H E F O U R THD A Y A N D TO ON THE S E ' E NTH D A Y

O' E R D U E .

MAR 2 8 1935