QD UC-NRLF 7^7 EXCHANGE THE PRESSURE EFFECT IN SELENIUM CRYSTALS AND ITS RELATION TO THE LIGHT EFFECT By ERNEST OTTO IDIETERICH A.B., University oTTowa, 1912 M.S., University of Iowa, 1914 JUL 3 THE PRESSURE EFFECT IN SELENIUM CRYSTALS AND ITS RELATION TO THE LIGHT EFFECT By ERNEST OTTO UIETEIUCH A.B., University of Iowa, 1912 M.S., University of Iowa, 1914 THESIS Submitted in partial fulfillment of the requirements for the Degree of Doctor of Philosophy in Physics, in the Graduate College of the State ^University of Iowa, June, 1916 Iowa City, Iowa EXCHANGE THE PRESSURE EFFECT IN SELENIUM CRYSTALS AND ITS RELATION TO THE LIGHT EFFECT By E. O. DIETERIOH ABSTRACT The effect of pressure on selenium crystals is shown to be of a different nature from that produced by light. According to Brown '& theory of electric conduction in selenium, the change of resistance produced by light is due to a change in the rate of emission of electrons from the atoms during illumin- ation, while their coefficient of recombination is unaffected by the intensity of illumination. Tlus assumption is here verified by experiments. In the case of pressure, however, these experiments show that the coefficient of recombination is the factor that is altered, and the greater the pressure the smaller is the coefficient of recombination. A consideration of the various agencies capable of changing the conductivity of selenium, such as light, pressure, temperature and potential gradient, leads to the conclusion that the only one which modifies the rate of emission of conducting electrons is light, the others affecting only the coefficient of recombination. INTRODUCTION The discovery 1 of a method of producing large crystals of sele- nium provided additional means for the study of the interesting optical and electrical properties of this element in the crystalline or metallic modification. As a result of the investigation of some of these properties, Brown 2 has proposed an explanation of the nature of the electrical conductivity of selenium. According to this theory, which is essentially of the same nature as J. J. Thom- son's doublet theory of electrical conduction in metals, the con- ductivity of selenium may be altered in two ways, either by a change in the rate of liberation of conducting electrons from the atoms, or by a change in the rate of recombination of the electrons with the atoms. These assumptions were verified in certain experiments 3 in which the effect of light in changing the conductivity of selenium crystals under various conditions of temperature, pressure and potential gradient were studied. Since a change in pressure produces such a marked effect on the iF. C. Brown, Phys. Rev., 4, 85, (1915). zF. C. Brown, Phys. Rev., 5, 395, (1915). s P. C. Brown, Phys. Rev., 5, 395, (1915) and Phys. Rev., 7, 551, (1916). B74678 resistance of selenium it is of importance to investigate the pressure effect in more detail, in order to test the theory more completely in that direction. The experiments described below were so ar- ranged that the changes of conductivity produced by pressure changes could be studied immediately after the variation in pres- sure. THEORY Assuming that the conductivity of a selenium crystal at any instant is directly proportional to the number of conducting elec- trons present, it has been shown 4 that when the equilibrium con- ditions have been disturbed, the rate of change of conductivity may be expressed by di/dt = a i 2 (1) Here i represents the equilibrium conductivity of a crystal under fixed conditions of light intensity, temperature, pressure and poten- tial gradient, and a is the coefficient of recombination of the con- ducting electrons with the atoms. Equation (1) was tested in the form Ai/At = a i 2 . by a series of experiments in which the equilibrium value of the conductivity was altered by a change in the light intensity. Ai At was measured directly (At being .02 sec.) and a calculated. For light intensities over a wide range a was found constant, justifying the assumption made that a is independent of light intensity and has the same value in the dark as in the light. At equilibrium, in the dark, di/dt = a i 2 (2) Writing q for the rate of emission of conducting electrons in the dark, and M for the increase in the rate of emission due to light, we have, at equilibrium in light of constant intensity, di/dt == a i^ = M + q (3) in which i L is the equilibrium conductivity in the light. From the above equation we obtain i 1= = ^ (4) C. Brown, Phys. Rev., 5, 395, (1915). which shows that a change in conductivity may be produced by varying either M or a. The constancy of a in light of various wide- ly different intensities shows that M is the quantity modified by exposure to light. Similarly, if it be assumed that change of pressure alters the rate of emission of electrons in the same manner that change of illumin- ation does, we may write M ^ for the increase due to pressure alone, and obtain (5) The constants in Eq. (5) may be determined by the same method employed for Eq. (4), the change in conductivity being produced by a variation in the pressure upon the crystal. If the pressure and light effect are the same c^ will remain constant for different pressures. EXPERIMENTAL ARRANGEMENTS AND METHOD OF MEASUREMENTS The crystal under observation was held horizontally between platinum electrodes in one arm of a Wheatstone bridge. A weight of 500 grams, L, rigidly fixed to a hinged rod rested upon the upper electrode. This weight was connected to a shaft of a heavy timing pendulum by means of a cord passing over a pulley system. The pendulum served as a means of raising L at the instant of closing the key in the galvanometer circuit. A second key was also tripped by the pendulum and opened the circuit at the end of a pre-deter- mined short time interval, in these experiments, .04 sec. Different constant pressures were obtained by suspending weights W from a long shanked hook attached to the upper electrode and passing through a hole in the base. These weights were so chosen that the pressure upon the crystal ranged from 10 kg/cm 2 to 100 kg/cm 2 . At pressure higher than the latter the crystal resistance became too unsteady to obtain reliable readings. The electrodes and weights were enclosed in a light tight box provided with a slit directly in front of the crystal, which was opened when the light effects were studied. A Nemst glower, at a distance of 75 cm. from the crystal, was used, and the time of exposure of the crystal to light was regu- lated by means of a shutter attached to the timing pendulum. The method of Brown and Clark 5 for measuring rapid fluctua- 5 F. C. Brown and W. H. Clark, Phys. Rev., 32, 251, (1911) and Phys. Rev., 33, 53, (1911). tions in resistance was slightly modified to obtain the change of resistance of the crystal during the .04 sec. following a change in the equilibrium conditions. All factors besides light intensity and pressure were kept constant. Readings were made at room tem- perature which was maintained constant to a few degrees centi- grade. The potential drop across the crystal was at all times 14.2 volts, and since the crystals were all of approximately the same size the potential gradient may be assumed constant. A large number of crystals of several types was studied and satisfactory duplication of data obtained. For purposes of comparison the results given below were all chosen from the data referring to a single crystal which was a large hexagonal specimen about 1 mm. thick and 5 mm. long. RESULTS The Pressure Effect in the Dark and under Constant Illumination The crystal was allowed to reach equilibrium in the dark under the pressure due to W -f- 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. This procedure was followed rather than that of changing the conditions by the addition of L because of the danger of crushing the crystal con- tinuously exposed to light of constant intensity. Table I, below, summarizes the data. TABLE I PRESSURE EFFECT ON CONDUCTIVITY L = 500 grams; i = initial conductivity; ij = conductivity after .04 sec. W (X10T) i, (X107) Crystal Ai (X10T) in the Dark (xlO-r) oM (X10T) 500 gm. 1000 " 2000 '" 3000 6.71 8.55 11.86 15.79 4.23 6.91 10.97 15.00 2.48 1.64 .89 .79 1.37 .560 .158 .079 92.2 41.1 22.2 19.8 500 gm. 1000 " 2000 " 3000 " 20.00 24.90 34.10 44.00 Crystal 12.26 19.40 31.26 41.20 in the Light 7.74 5.50 2.84 2.80 .486 .222 .061 .036 194.1 137.7 71.0 69.8 Column 5 of the table shows the decrease of a with increasing pressure, the rate of decrease being a little greater in the light than in the dark when the factor by which the conductivity is altered by pressure is the same. In the next column are tabulated the varia- tiou of cd 2 with pressure, the value of the rate of emission of conducting electrons. These experiments show that part of the change, at least, is due to a change in a ; the variation in a is not sufficient to warrant the conclusion that the only effect of pressure is to vary the rate of recombination, and not the rate of emission. But considered in connection with the decrease in ai 2 and with experiments to be described in the next section, it may be concluded that such is actually the case. The Light Effect at Different Pressures, Intensity of Illumination Constant In these experiments, after equilibrium conductivity had been reached in the light the intensity was reduced by a constant amount, by means of a darkened photographic plate, and the rate of re- covery measured for different values of W. The values for a and for ai 2 can then be compared with those determined for a constant change of pressure above. TABLE II LIGHT EFFECT AS A FUNCTION OF PRESSURE W (X10T) ( X iV) Ai (X10T) (xlO-r) aio (X10T) 500 gm. 1000 J> 2000 " 3000 4000 15.36 19.20 28.47 38.15 47.35 14.36 18.05 28.70 35.90 44.40 1.00 1.15 1.77 2.25 2.95 .106 .078 .054 .039 .033 1.62 1.50 1.50 1.47 1.57 25.00 28.75 43.75 56.40 74.00 Table II represents the results obtained, a decrease in a but an increase in ai u 2 showing quite clearly that the action of pressure and light in producing a conductivity change are different. The pressure effect is shown in the decreasing value for a, the light effect, in the increasing values for od 2 (See equation 4). The Light Effect at Constant Pressure, Intensity of Illumination Varied Table III summarizes the results obtained, agreeing with those determined by previous work 8 and the constancy of a again points to the change in the rate of emission of electrons under the influence of illumination. . C. Brown, Phys. Rev., 5, 395, (1915). TABLE III Intensity W (X10T) (XIO*) Ai (xlOT) a (xlO-7) aio 2 (x!07) 1 2 3 1000 gm. 1000 " 1000 32.00 26.10 19.74 30.75 25.20 19.29 1.25 .90 .45 .031 .033 .029 31.9 22.6 11.2 The Effect of a Simultaneous Change in Pressure and Light In- tensity is shown in Table IV, and indicates the dissimilarity in the action of light and pressure. TABLE IV W = 500 grams. L = 500 grams. (XlOT) (XlOT) Ai (XlOT) a (XIO'T) (xYoT) Light Pressure Light & pressure 13.34 8.97 20.81 12.72 7.86 15.58 .62 1.11 5.23 .09 .34 .30 15.4 27.8 131.0 The values of a due to a change in pressure, and to a change in both 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. The behavior of the crystals in these experiments is worthy of note. When they were kept in the dark, and the conductivity altered by a change in pressure, they rapidly recovered their initial conduc- tivity on the restoration of original conditions, the recovery from the light effect 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 hours were required before the initial conductivity had been reached. CONCLUSIONS In all the results discussed above the same fact is evident, that is, that the effect of pressure is merely to decrease the coefficient of recombination of the conducting electrons with the positive resi- dues of the atoms 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 decreasing value of a was always observed. Now, Brown 7 has shown that with an increase in the potential drop across a crystal the value of a due to a change in the light intensity is decreased in the same manner 7 Loc. dt. as it is in the case of pressure. Further Mrs. Dieterich 8 showed that the temperature effect on a is similar to that observed in the case of pressure and voltage. The conclusion is obvious the only agency which changes the conductivity of selenium by causing an increase in 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. K. J. Dieterich, Phys. Rev. 2, Vol. VII, p. 551, (1916). G74678 UNIVERSITY OF CALIFORNIA LIBRARY