*:• The Color-Sensitiveness of Photo-Electric Cells ELEANOR FRANCES SEILER A.B., Uni-versily of Den-ver, igiy yi.M., Unix'ersity of Deni'tr, ig/g A. M . , Umi'tnity of lllinoh, ig/6 THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN PHYSICS IN THE GRADUATE SCHOOL OF THE UNIVERSITY OF ILLINOIS > 92 ^- r UNIVERSITY OF ILLINOIS THE GRADUATE SCHOOL -192- 1 HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY ^SUPERVISION BY_ ELEANUR FRAXCES-SEXLEB ENTITLED_ THR. COLOR SECSITIUEUR SS OF P - ] BE ACCEPTED AS FULFILLING THIS PAR'T OF THE REQUIREMENTS FOR THE DEGREE OF DOCTO R ■■OF, -PHILOSOPHY lU PHYSICg- In Charge of Thesis Head of Department Recommendation concurred in* [^. Q , , / Committee on Final Examination* •Rei]uired for doctor’s degree hut not for master's Digitized by the Internet Archive in 2015 https://archive.org/details/colorsensitiveneOOseil The Color-Sensitiveness of Photo-Electric Cells BV ELEANOR FRANCES SEILER A. B. , Un 'vvenity of Denver, /gij A.M., University of Denver, igi4 A.M., University of Illinois, igib THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN PHYSICS IN THE GRADUATE SCHOOL OF THE UNIVERSITY OF ILLINOIS 192^ Reprinted from tlie Ahtuopii ysicai, Joi k.\al, Vol. LM, nuiiilicr 3, October 1920, page 129. Se.A THE ASTROPHYSICAL JOURNAL AN INTERNATIONAL REVIEW OF SPECTROSCOPY AND ASTRONOMICAL PHYSICS VOLUME Lll OCTOBER 1920 NUMBERS COLOR-SEXSmVEXESS OF PHOTO-ELECTRIC CELLS By ELEANOR FRANCES SEILER ABSTRACT Color-sensitivcntss curves of thirty photo-electric cells, including all the alkali metals and hydrides of Xa, K, Rb, and Cs. — As great care was used in mounting the apparatus rigidly, in calibrating the thermo-couple and wave-length scale, and in maintaining a constant source of light, the curves were accurately determined and this enabled the wave-lengths of maximum sensitiveness to be located within about i nfi. It was found that as the atomic weight of the alkali metal increases, the maximum sensi- tiveness decreases, the resonance peak becomes broader, and Xmax shifts toward the red. The author suggests that these changes may be associated with the increase in atomic volume. For glass cells filled with argon at low pressure, Xmax was 405, 419, 440, 473, and 539AIAI for Li, Na, K, Rb, and Cs respectively, while for the cor- responding hydrides Nall, KH, Rbll, and CsII, the values were larger; 427, 456, 481, and S40/i|i respectively. With neon instead of argon, in the case of Nall and KH, Xmax was about 20 mm shorter. And while, in the case of KII, pyrex cells gave the same curve as glass ones, quartz cells showed a longer Xmax for both KII and Rbll, contrary to what one would expect from differences in absoq>tion. Since the effects of gas and of cell wall were not eliminated and the normal and selective photo- electric effects were not difTerentiated, the curves are characteristic of the particular cells rather than of the various metals alone; nevertheless it is of interest to note that the products of Xmax with (i) resonance potential, (2) ionization potential, and (3) absolute melting temperature of the corresponding metals are each roughly constant. The hydride cells were somewhat more sensitive than the corresponding metal cells. Fatigue tests of two photo-electric cells. The effect of illuminating a K cell for 525 hours was to increase its sensitiveness by about 70 per cent, whereas a KII cell remained constant in sensitiveness cluring 1000 hours’ illumination. Preparation of a lithium photo-electric cell. It was found possible to dissolve lithium in aethylamine if absolutely dry and if a trace of ammonia was present. uniformly distributed layer of lithium was obtained by evaporating the solution. Method of preparing pure aethylamine is described. I 2Q 130 ELEANOR FRANCES SEILER I. INTRODUCTION In 1918 T. Shinomiya' carried out an experimental determi- nation of the maximum wave-length color-sensitiveness point (Xmax) of photo-electric cells of the pure alkali metals sodium, potassium, and rubidium, and of the colloidal modification of the alkali hy- drides of these same metals. He found that the Xmax was not the same for the pure metal and the hydride, but that a shift occurred, the nature of which was not fully determined. The object of this investigation was threefold: 1. To obtain more consistent results on the shifting phenome- non and to secure conclusive experimental proof of its direction and amount by the use of many more cells and the extension of the work to include all the alkali metals. No satisfactory theory has been advanced to explain the cause of this shifting of the wave- length of maximum sensitiveness. However, the results of this investigation prove the existence of such a phenomenon, and furthermore they show the shift to be consistently always in the same direction, namely, toward longer wave-lengths when the alkali metals were sensitized by means of hydrogen. 2. To complete the study of the color-sensitiveness for the whole group of alkali metals. When the photo-electric cell is to be used in making photometric measurements, it is necessary to have a complete knowledge of its curve of sensitiveness. In scientific literature it is found that the curves given contain only a few points, which hardly suffice to determine the point of maximum sensitive- ness or the general nature of the curves with much accuracy. The ideal photo-electric cell would have the same color-sensitiveness as the human eye. 3. To furnish data for theoretical purposes in the application of the quantum theory to chemistry. No general agreement has yet been reached regarding the relation of the selective to the nor- mal photo-electric effect. Wiedemann^ found that hydrogen was essential for the selective effect of potassium. The results of this experiment gave a decided selective effect in cells where the potas- sium was repeatedly distilled in vacuo. According to Xlillikan Astro physical Journal, 49, 303, 1919. ^ Berichle der deutschen physikalischen Gesellschaft, 18, 333, 1916, COLOR-SENSITIVENESS OF PHOTO-ELECTRIC CELLS 131 and Souder* there is an “essential identity” of the two effects. This investigation was not carried on with the purpose of getting either effect to the elimination of the other. Primary interest was centered in the photometric properties of the cell and not of the alkali metals as such. Apparently there is considerable difference in sensitiveness when the absorption of light through the glass is taken into account and when gas is present in the cells. II. THE .APPARATUS AND METHOD A. THE CALIBRATIONS In order to measure the energy of illumination throughout the visible spectrum and the corresponding wave-lengths of light, it was necessary to make four calibrations, about every month during the course of the investigation. We will call these calibrations: (i) thermo-couple, (2) scale-energy, (3) spectrometer, and (4) scale- wave-length. I. Thermo-couple. A thermo-couple, furnished by W. W. Coblentz from the Bureau of Standards, was calibrated by means of the light from a Hefner lamp. Chemically pure amyl acetate was put into the lamp, which was placed exactly one meter from the thermo-couple and in the same horizontal plane with it. Both were placed in a large two-compartment box, which was completely blackened. The partition contained a shutter, by means of which the light could be allowed to fall u])on the thermo-couple, and which acted as a partial diaphragm. A hooded chimney was placed above the lamp to take care of the heat from the llame. A small glass window, with a horizontal line on it, was set in the box directly behind the image of the llame, so that the flame could be accurately adjusted to the proper height and observed from the outside, d'he box being tight and free from air currents, the llame soon came to an equilibrium condition. 'Fwo line wires led from the thermo- couple to an insulating plug in the back of the box. d'here they were soldered to heavy double silk-wound copper wires, which led through the plug down a heavy-walled wooden tube to a galva- nometer. 'Fhis tube served the purpose of keeping the temperature ' Proceedings of the National Academy of Science, 2, 19, 1916. 132 ELEANOR FRANCES SEILER constant and of preventing air currents. The lead wires in the tube were insulated by passing down the center over strips of hard rubber. The galvanometer was a Leeds and Northrop high sensitivity type, whose figure of merit was 2.7X10”®. It was inclosed in a box about which tin foil was wrapped for earthing. These precautions made the thermo-galvanometer deflections exceedingly steady, and a reading could be repeated time and time again with not as much as i per cent of variation. The mean of twenty readings gave a deflection of 1 2 1 mm. The energy radiating from a Hefner lamp, according to K. Angstrom,'' is 1089 ergs per sec. per sq. cm at a distance of one meter. Hence a deflection of I mm corresponded to a flow of energy of 9 ergs per sec. per sq. cm. The slit of the thermo-couple was a trifle narrower than the slit L (Fig. 2) admitting light to the photo-electric cell, so that the former was always covered by light. The area of L was 0.2842 sq. cm, and hence the energy passing through that slit per mm of deflection of the galvanometer of the thermo-couple was 9-0.2842 or 2.558 ergs. 2. Scale-energy. The thermo-couple was placed with its slit directly behind L and held there rigidly. It w'as connected to the same galvanometer through another tube, which was completely wrapped with tin foil and earthed. By means of a fine screw R (Fig. 2), the thermo-couple was moved along the millimeter scale S through the entire spectrum, and the mean of three readings taken as the deflection at each point. Then by plotting a curve with scale-readings as abscissae and energy in ergs as ordinates, the energy incident upon every point in the spectrum was exactly determined. 3. Spectrometer. Since the Hilger constant-deviation spectrom- eter was to be used in determining the wave-length incident at every corresponding point on the scale S, it, too, had to be cahbrated. This was done by means of a Plucker discharge tube. The prism of the spectrometer was adjusted so that the corrections were very slight. 4. Scale-wave-length. The wave-lengths corresponding to the points along the scale were then obtained with this instrument. ‘W. W. Coblentz, Bulletin of Bureau of Standards, 11, 05, igi4. COLOR-SENSITIVENESS OF PHOTO-ELECTRIC CELLS 133 The lines seen in the spectrometer were almost as fine as those seen when the discharge tube was used, showing that the light was very nearly monochromatic. They became somewhat broader toward the red end of the spectrum, but this was unavoidable and, as the experiment did not require great accuracy in that region, accuracy was sacrificed to get better definition in the other end of the spec- trum. By plotting a curve with wave-lengths as abscissae and scale-readings as ordinates, the wave-length corresponding to any position on the scale was obtained. B. THE EXPERIMENT I. The optical system. The optical system used in this inves- tigation was an improvement over that used by T. Shinomiya in that, by the use of two large prisms and a condensing lens L4 (Fig. i). greater dispersion and better monochromatism were obtained. As the sensitiveness of the photo-electric cell changes in a con- tinuous way along the spectrum, the very slight deviation from the ideal condition of absolute monochromatism had little efiect on the final curves. 'I'he set-up is shown diagrammatically in Figure i. The source of light, F, was a specially constructed tungsten 17-ampere nitrogen- filled lamp made by the Nela Research Laboratory. The bright filament was a straight, narrow fiat strip which acted as a line source. A Weston direct-current ammeter. A, measured the cur- rent through the lamp, d'his had to be kept very constant at 17 amperes, for the slightest variation in the intensity of the light made itself felt in both the thermo and photo-electric galvanometers, d’he current was adjusted by a tin resistance. Hi, and a fine adjust- ment sliding resistance Rj in parallel. Two sets of storage bat- teries connected in parallel served as the source of current. They were kept well charged and the resultant current was steady. 134 ELEANOR FRANCES SEILER The light was condensed by lenses Li and and brought to a focus on the slit S, which was o . 7 mm in width. It was then rendered parallel by the lens The diaphragm was adjusted so that the front face of the prism Pi was just covered by light. The position of minimum deviation for green-blue light was chosen for both prisms. After passing through the prisms the various colors of the spectrum were brought to a focus and made pure by the lens P4. The spectrum, 10 cm in length, was beautifully distinct and bright. The reflected and stray light was eliminated by the diaphragms D^, D2, and D^, and further by placing the entire optical system in a box made of thin compo-board, completely blackened on the inside. The slit L (Fig. 2) was accurately located COLOR-SEX SIT IV EN ESS OF PHOTO-ELECTRIC CELLS 135 in the focal plane of the lens L^. It was mounted on the table T, which could be moved through the spectrum by means of the fine screw R. This table was inside a heavy black box K, rigidly attached to a solid pier. The entire optical system was made purposely heavy, so that not even the slightest displacement could take place and hence change the calibrations. One can readily see that a very small change in any part of such a com- plicated system would throw the whole arrangement out of order. The scale, S, on the outside of the box was rigidly connected with the slit L. It was the sliding of this scale past a fine reference mark M that was calibrated, first in ergs and then in wave-lengths of light. The mounting for the slit L was made to exactly fit the mounting of the thermo-couple used in the energj- calibration. On the top of the table were bored four holes, a, b, c, d (Fig. 2). Into these four holes fit the legs of a light-tight metal box, specially constructed for containing the photo-electric cell under test. The shutter on this box for admitting and cutting oil the light to the cells was situated so that it was directly behind the slit L. This shutter was manipulated by the operator at his observing position by means of pulleys and strings. The apparatus was so arranged that at the same place where these strings were situated the operator could adjust the resistance R2, read the ammeter A , and note the photo-electric galvanometer deflections. 'I'he metal box container hafl two hard rubber insulators for supporting the photo-electric cell and two insulating plugs and binding posts for the lead wires, d'he positive lead was made of sulphur, and the negative one of hard rubber. A binding post attached directly to the box served the purpose of grounding the box and the outside of the photo-electric cells. 2. I'he pholo-elcctric cells. 'Fhere were thirty cells used in the investigation, including all of the alkali metals and the colloida modification of the alkali hydrides of sodium, potassium, rubidium, and cassium. d’hey all contained argon at a low pressure, except two cells where neon was used. Three kinds of glass — common, pyrex, and quartz — were investigated. .All of the cells were made 136 ELEANOR FRANCES SEILER by Dr. Jakob Kunz in this laboratory. A full description of their construction, with the exception of lithium, has been given.^ The making of the lithium cell was somewhat of a task, because of the danger encountered in trying to distil lithium, and of the difficulty in obtaining a solute for it. At the suggestion of Dr. A. G. Loomis, of the chemistry department, we tried dissolving it in aethylamine, and were eventually successful. The necessary condition for its solution is that the aethylamine be absolutely dry and that it be in the presence of a trace of ammonia. The aethylamine was made according to the following directions, the chemical reaction being (C.HsNH,+HBr) +NaOH = NaBr +C,HsNH,+H,0. Seventy grams of aethylamine hydrobromide were placed in a 500-cc pyrex glass flask, fitted with a tight cork stopper holding a separatory funnel and a reflux condenser kept at about 30° C. To the top of the reflux condenser was attached a drying column of solid NaOH, and this connected to a downward condenser of the spiral type, cooled with ice and salt. The receiver was also cooled with a mixture of crushed ice and salt and had a NaOH tube attached. All stoppers were of cork to prevent the product from becoming yellow because of attack of rubber by the amine. About 200 grams of a 30 per cent solution of NaOH was slowly dropped into the flask through the separatory funnel, and when all had been added, the mixture was first heated with warm water and later with a flame, refluxing until no more amine condensed into the receiver. The crude amine was then redistilled three or four times, using metallic lithium as a drying agent. The final product was a pure, colorless, dry fluid. Metallic lithium was put into it, in a vessel connected with a cell. After standing ten hours in the aethylamine in the presence of a trace of ammonia, the lithium was dissolved and the clear liquid became a deep-blue color. This solution was poured into the cell and the aethylamine pumped out, leaving a uniformly distributed surface of grayish blue lithium. The cell was then partly filled with argon at low pressure and sealed off. ^Physical Review, 7, 62, 1916. COLOR-SENSITIVENESS OF PHOTO-ELECTRIC CELLS 137 3. The set-up. A diagrammatic sketch of the connections is shown in Figure 3. The source of potential E was a set of new dry cells. The voltage ranged anywhere from 100 to 310 volts, depending upon the kind of cell used. The potential was measured by a Weston direct-current voltmeter, V. All of the connecting wires, as well as the battery itself, were insulated by parafi&n. The galvanometer, G, which measured the photo-electric current, was a Leeds and Xorthrup high-sensitivity type. It had a sen- sitiveness of 8X io~'° amperes for a scale-distance of 1.25 m, and a critical damping resistance of 9600 ohms. To safeguard against short circuit, a resistance of 100,000 ohms was inserted in the circuit. The outside of the cell, the positive terminal of the bat- tery, and the box B, Figure 3, containing the photo-electric cell, were all earthed. When working with the lithium cell, a quadrant electrometer had to be used to measure the photo-electric current, because of the small light intensities in the region of its maxi- mum sensitivity, and the rather low sensitiveness of the cell. 4. Method of procedure. After the calibrations were made, great care was taken that no part of the system was disturbed, d'hen a characteristic curve was made of the first cell to be tested, after which a preliminary sensitiveness-curve was run and finally the real sensitiveness-curve data obtained, d'his procedure was repeated for all the cells used. 'Fhe process will now be discussed a little more in detail. ELEANOR FRANCES SEILER 138 The cell was mounted in the box K (Fig. 2), the approximate point of maximum galvanometer deflection was found by referring to the work of T. Shinomiya, and the cell placed at that particular wave-length. Starting with a low voltage, around 80, until a deflection of about 40 mm was obtained, the cell was moved back and forth until the exact maximum was found by trial. Keeping this position, the deflections for varying voltages were noted. The upper limit of the voltage was determined by the unsteadiness of the galvanometer deflections. This unsteadiness indicated that the arcing potential was being reached. If the cell was allowed to glow much, its sensitiveness decreased. A slight glowing often- times increased the sensitivity. Glowing could easily be detected by the galvanometer deflections suddenly going off the scale. Whenever this happened, an entirely new characteristic had to be run. The Na and NaH cells were the most difficult to handle, for the characteristic curve rises so rapidly that a variation of 4 volts (one dry cell) often caused great unsteadiness in the deflections, and sometimes glowing. Barring this unsteadiness, the other readings were very steady and could readily be repeated at the time when the data were taken. If, however, a characteristic was run shortly after a cell was made, and again a day or two later, it was found to change somewhat. After a week’s time very little change took place. All the data were taken on cells more than a week old. This variation was more marked in the hydrides of the alkali metals than in the pure metals. Immediately after running a characteristic, the curve was drawn, plotting volts against photo-electric current. Some characteristic curves are shown in Shinomiya’s paper. A constant potential difference was then chosen from the horizontal portion of the curve, or at most up only a short distance on the vertical part. In no case was such a voltage used as to make the galvanometer deflections unsteady, which occurs when the critical potential is approached. Having selected a safe potential, one that would give reasonably large deflections (100 mm or more), the cell was started at the extreme violet end of the spectrum and by steps of a centimeter was taken up through the red end. This was the preliminary COLOR-SENSITIVEXESS OF PHOTO-ELECTRIC CELLS 139 run to determine approximately the position, not of maximum galvanometer deflection, but of maximum photo-electric sensitive- ness. This point varied somewhat from the former and was found by dividing the deflections by the corresponding energ}' at the given scale-reading, .\fter a lapse of about fifteen minutes, during which time the cell was left in the dark, a second final run was made, again starting at the violet end. If no lapse of time is allowed to take place between the first and second sets of data, the later readings will be higher, because for a short time the cell becomes more sensitive when it has been under the influence of light. Two readings, rarely differing, were taken at each point, and this time they were taken only 3 mm apart in the region of maximum sen- sitivity. Thus the maximum point was accurately determined to within one nfj.. Finally curves were plotted with wave-lengths of light e.xpressed in nfjL as abscissae and specific photo-electric sensitiveness as ordi- nates, the units being o.6iXio~‘‘ coulombs per erg. III. EXPERLMEXT.VL RESULTS The question as to how the colloidal modification of the alkali hydrides affect the color-sensitiveness has been studied. It was found that the results of sensitizing by hydrogen were twofold: a) It shifts the maximum sensitiveness, Xmax, to longer wave- lengths by an amount depending upon the alkali used. As can be seen from 'I'able 1 below, where X^ax and the applied voltage of all the cells investigated are listed, Nall shifted 8 yu/x, KII 16 /n/x, Rbll 8 fjLfi, and CsM gave only the slight change in one mm- Figures 6, 7, and 8 show this shifting graphically. b) d’he colloidal modification of the alkali hydrides also in- creases the .sensitiveness; that is, a greater jihoto-electric current is obtained with a given applied voltage from a hydride cell than from the pure metal. 'I'he sensitiveness depends upon the applied voltage, but on the whole the results indicate a larger ordinate for the hydride cells. l*ohl and I’ringsheim' suggest that this increased sensitiveness is due to the greater ease with which the electrons ' Vcrhandltiugen dcr deutschen physikalischen Gcscllschafl^ 13 , 211 , 1911 ; 15 , 179 , 191 1 . 140 ELEANOR FRANCES SEILER TABLE I Type of Cell ^max Applied Voltage Li 405 246 Na I 419 149 Na 2 419 116 Mean for Na. . . 419 NaH I 427 304 NaH 2 426 182 NaH 3 427 308 NaH 4 428 171 NaH 5 429 140 Mean for NaH . 427 NaH Neon 407 285 K I 440 167 K 2 440 172 Mean for K ... . 440 KH I 457 119 KH 2 457 136 KH3 454 140 Mean for KH . . 456 KH Neon 438 127 Type of Cell ^max Applied Voltage KH Quartz 462 145 KH Pyrex i 457 154 KH Pyrex 2 457 127 Rb I 473 146 Rb 2 473 123 Pb3 473 136 Rb 4 473 III Mean forRb. . 473 RbH I 479 140 RbH 2 483 lOI RbH 3 481 123 Mean for RbH 481 RbH Quartz 507 179 Cs I 540 131 Cs 2 538 II7 Mean for Cs . . 539 CsH I 539 148 CsH 2 541 135 Mean for CsH. 540 TABLE II Calibration Data Giving the Wave-Lengths and Energy Along the Arbitrary Scale 5 Scale-Reading Wave- Length in fi/a Energy in Ergs 40.0. .. . 40.5 41.0. ... 41.3, .. . 41.5. .. . 41.7. . 42.0. ... 42.3 42.5 42.7. , 43.0. . . 43-3 - ■ • 43- S- ■ • 43 ■ 7 ■ ■ ■ 44.0. . . . 44- 3 44- 5 44.7.. .. 45- 0 393 o 397-3 401 .0 403.2 404.9 406.6 409-5 412.0 414.6 416.5 419.6 423-3 425-4 428.5 433-0 438.0 440.8 445-0 451-0 1-53 2 .04 3- 84 4- 59 5- 89 6- 55 8.68 10.48 12.56 14.67 17-35 21 . 22 23-58 26.85 32.70 41-30 45-20 51-75 61.80 Scale-Reading Wave- Length in /iM Energy in Ergs 45-3 456.7 73-8 45-5 462.0 84-4 45-7 466.7 94-3 46.0 473-1 109. 1 46.3 483-2 127.3 46.5 488.7 137-9 46.7 496.6 151-5 47-0 506.3 172.0 47-3 516.7 191-9 47-5 524-3 205.7 47-7 535 - 2 221 .0 48.0 550-6 239-0 48.3 567-7 259.2 48-5 580.2 270. 1 48-7 593-5 288.2 49-0 616.6 305.3 49-5 660.7 338.5 50-0 718.0 366.7 COLOR-SENSITIVENESS OF PHOTO-ELECTRIC CELLS 141 can emerge from the tiny globules of metal which characterize the colloidal state. The curv^es of color-sensitiveness have been obtained for all the pure alkali metals and for all the hydrides with the exception of Li. The cur\'es resemble resonance cur\*es, and Pohl and Pringsheim have suggested that the selective effect is a molecular resonance phenomenon in which the electron follows the electric force. The curves, however, are not pure resonant ones because of lack of perfect symmetry. If a center line is drawn from the peak of a curve perpendicular to the wave-length axis and then about half- way between the peak and the axis another line is drawn perpen- dicular to this center line, the distances measured along the latter from the center to the two points where it cuts the curve is a measure of the curve’s symmetry. In the pure metal as well as in the hydride of the sodium and potassium cells, the distance from the central line to the right side of the curve was greater than that to the left; Cs and CsH, on the other hand, showed dissymmetry in the opposite direction. The Rb and RbH cells are almost symmetrical. A sufficient number of points are located on each curve so that its exact nature is determined. Many of the points have been omitted from the drawings for the sake of clearness. They all, however, fell upon the smooth curve. An e.xamination of the curves and Table I clearly shows how consistently the cells operated. The position of X^ai for individual cells of the same metal fell in almost exactly the same position. Figures 6 and 7 show how the gas in the cells affects the jihoto- electric effect. A sodium cell and a potassium cell, each filled with neon instead of the argon of the other cells, showed a decided shift of the Xman toward shorter wave-lengths. The amount of shift was 20 fifjL for sodium and 18 fxjj. for potassium. That there should be such a difference between the Xma:t for the cells filled with neon and with argon, both being inert gases, is a rather surprising phe- nomenon. It is probably connected with the fact that the maximum sensitiveness of sodium and potassium in these cells deviates con- siderably from the values obtained for pure metals without gas. This question requires further study. 142 ELEANOR FRANCES SEILER Figures 6 and 7 indicate the effect that the kind of glass used in the photo-electric cell has on the wave-length of maximum sensitive- ness. Most of the cells were made of common glass, but two cells of pyrex (KH Pyrex i and KH Pyrex 2) and two of quartz (KH Quartz and RbH Quartz) were constructed. Both pyrex cells TABLE III Variation in All Alkali Metals* Scale in Centimeters 40.0. 41.0. 41. 5. 42.0. 42.3. 42.5. 43-0- 43 - S- 44- 0. 44- 3- 44.5. 44.7. 45- 0- 45.3. 45-5. 45.7. 46.0. 46.3. 46.3. 46.7. 47-0. 47-3-. 47. 5- . 47-7- 48.0. . 48. 3 -. 48. 5- . 48.7.. 49.0. . 49- S-- 50- 0-. Li Elec- trical Deflec- tion mm 14.8 43-8 70.7 loi . 2 133-3 166.5 302.5 321.7 307.0 230.0 21.3 Deflec- tion Energy 1265.0 1194.0 IS 74-0 1527.0 1300.0 1256.0 877.0 681.5 476.8 299.7 79.8 ir .8 8.0 ■4'^ Na 2 Galva- nometer Deflec- tions mm 1 . 2 4.8 8.9 16. 1 21.4 26.1 36.6 48.3 57.0 60.4 56.8 53-3 51.0 4^7 44.8 41.5 36.6 31.0 "1S.8 10.7 7.8 Specific Photo- Electric Sensi- tiveness 62.2 99.8 120.9 148.1 163.2 166.2 168.5 163.6 139.2 116.9 87.8 68.9 47 -S 35-6 25-9 19.3 14.2 10.4 3.6 3.1 1.8 K 2 Galva- nometer Deflec- tions 2.0 3.4 6.6 11.9 21.2 36.8 55.8 78.6 88.7 96.7 107.2 113.3 113-3 111.4 103.7 93.9 81 . 1 57.9 33.2 21 . 2 6.9 Specific Photo- Electric Sensi- tiveness Rb 3 41.7 46.2 60.7 75.8 97.6 124.7 136.4 139.0 137.0 124.9 116 . 1 107.4 96.0 81.6 63.2 34.4 42.8 27.0 13.9 8.3 2.3 0.6 0.3 Galva- nometer Deflec- tions 33 123 59 17 Specific Photo- Electric Sensi- tiveness 52 Cs 2 Galva- nometer Deflec- tions 1.8 3.9 9-0 47.4 69.7 104.7 123.9 145. 0 166.2 169.0 144.2 117.2 94.7 48.2 II . I 3.8 Specific Photo- Electric Sensi- tiveness 16.6 17.9 22.0 27.51 31.2 34.7 40.4 48.7 51. 6 56.4 60.1 56.6 44.5 34.7 26.2 12.6 2.6 0.9 *The sensitiveness column in each case but Li is multiplied by lo sensitiveness in coulombs per erg. giving the specific photo-electric gave excellent curves, whose Xmax varied only one /x/x from that of KH cells made from common glass. The two cells constructed of quartz, however, showed a definite shift in Xmax toward longer wave-lengths, by an amount equal to 6 /x/x for the KH quartz and 26 mm for the RbH quartz cell. If this shift due to the quartz was caused by absorption of light in the wall of the cell, one should expect it to be in the opposite direction. COLOR-SEXSITIVEXESS OF PHOTO-ELECTRIC CELLS 143 Fig. 4a Wave-L ength in jj- ]x Fig. 4/1 5ca/e for L 144 ELEANOR FRANCES SEILER A further experiment was carried on for the purpose of detecting a fatigue effect in a K and a KH cell. The temperature, intensity of light, and applied voltage were kept constant and the galvanometer deflections observed at intervals of from ten to twenty-four hours. The voltage continuously applied to the K cell was no and after TABLE IV Variations in the Hydride Cells* Scale in Centi- meters NaH I KH I RbH 2 CsH 2 Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness mm mm mm mm S-8 22.9 26.8 53-0 32-3 394-0 58-7 2.8 25-7 59-0 449-5 43.0 108.3 500.2 17-5 80.6 7-3 33-6 2.3 10.6 44.0 203.0 496.0 50.0 122.0 23-5 57-4 II. 9 12.0 IQS -5 345-9 38-7 68.4 45-0 183.0 236-4 127.3 164-5 62.9 81.2 13-0 16.8 88.6 45 -S 161.6 153 -2 172.4 163-4 97.0 91.9 21.0 19-9 46.0 I 34 -S 98-3 190.2 139 -5 137-S 100.8 35-3 25.9 46 s 107.0 62-0 183.6 107.6 170.6 99.0 57-3 33-2 47-0 82.2 38-1 161.9 75-2 171 .0 79-5 95-3 44.3 47.5 54-5 21 . 2 126.4 49 1 138-2 53-7 147. I 57-2 48.0 30-3 10. I 85-3 28.5 88.9 29.7 176.4 58-9 46.8 48.5 14.0 41 -5 45-8 13.6 46-3 13.7 106. s 31-4 49 ■ 0 7.0 1.9 20.1 5.3 20.1 5-3 31-6 8-3 4 Q -5 4.6 2.0 10. 0 2.4 7.7 1.8 5-6 1-3 50.0 3-3 0.7 7.0 I-S 3.3 0.7 1.7 0.4 *The sensitiveness column in each case is multiplied by lo — ”, giving the specific photo-electric sensitiveness in coulombs per erg. five hundred and twenty-five hours’ exposure to strong light the cell had increased in sensitiveness by quite a large amount. The galvanometer deflections began with 176 mm and gradually in- creased up to 300 mm at the end of the e.xposure. The KH cell, on the other hand, under the same conditions, except that the applied voltage was 36, showed an exceptional constancy. Slight COLOR-SENSITIVENESS OF PHOTO-ELECTRIC CELLS 145 fluctuations occurred during the one thousand hours’ exposure; the initial and final galvanometer deflections were 194 mm and 191 mm, respectively, showing that the fatigue is negligible. These same two cells were exposed for 60 hours without con- tinuously applying a voltage. At the times when the photo- electric galvanometer deflections were taken, a voltage of 100 was applied to the potassium cell and of 27 to the potassium hydride cell. All the conditions remained constant throughout the experi- Wai/e-Length In jijji Fig. s ment. The results showed an increase in sensitiveness for both cells, the initial and final deflections for the potassium cell being 206 mm and 274 mm, and those for the potassium hydride cell being 196 mm and 316 mm. It seems strange that merely the exposure to light, without the passage of a jihoto-electric current, should increase the sensitiveness so much more rapidly than when the applied voltage caused a current to flow constantly through the cells, both investigations, however, point to a negative con- clusion in regard to a fatigue effect. 146 ELEANOR FRANCES SEILER TABLE V Variations in Potassium Cells* Scale in Centimeters K I KH 2 KH Quartz KH Pyrex I KH Neon Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness mm mm mm mm nun 41.0 3.8 79-0 2.5 $2.0 1 .0 20.8 1.7 34.7 5.8 120.7 41.5 42.0 9.8 90. 2 6.0 5 S -2 2.6 23 -9 9.6 42.2 16.9 148.1 42.5 IS-2 96.7 10. 0 63.6 4.2 26.7 9.0 57.4 27.2 173.0 43-0 23 s 108.2 16. S 76.0 6.9 31.8 151 69.4 44.0 202.6 43 S 36.1 122.4 28.4 96.2 II .8 39.9 32.2 109.3 70.7 239.6 44.0 53 0 129.4 49.0 119.6 20.1 49.0 53-2 130.2 105.0 256.5 44 -S 74-3 131-4 80.2 141.8 33.8 59-8 145.3 256.9 44-7 84.1 129.8 98.7 152.4 42.7 66.0 g6.6 149.0 161.8 249-8 45-0 98.0 127.4 I3I.3 169.8 59.8 77-3 122.7 158.3 187.0 241.9 85.8 4 S-S 124.8 118.3 178.0 168.6 92.0 87 . 2 171.3 162.3 213.0 201.8 129.5 109.7 102.5 86.8 46.0 128.9 94.4 194.8 142.6 112.8 82.6 200.0 146.4 204.0 149.5 119.7 7 S .2 117.0 73.4 46.5 108. s 62.9 177.9 103.0 117.7 68.2 210.5 122.0 170.0 98.5 41.0 66.8 31. 1 144.8 ■ 67.4 120.0 SS.8 212.0 98.4 II8.O 54.8 47.5 27-3 10.6 90.6 32.2 no. 7 43.0 184.0 71 . 5 65.1 25-3 48.0 10.4 3-5 38.3 12.8 72.0 24.0 126.1 42.1 30.0 10. 0 48. S 6.7 1 .9 14.0 4.2 26.3 7.7 36.2 16.6 14.8 4.4 49.0 5-0 1-3 7-3 1.9 6.1 1 .6 20.0 5.2 7-5 2.0 49.5 3-9 0.9 5.2 I . 2 1-7 0.4 II-S 2.7 3.5 0.9 50.0 3.0 0.7 4.0 0.9 0.8 0.2 9.4 2.1 1.9 0.4 *The sensitiveness column in each case is multiplied by 10 — ”, giving the specific photo-electric sensi* tiveness in coulombs per erg. Wai/e-L ength in Fig. 6 COLOR-SENSITIVENESS OF PHOTO-ELECTRIC CELLS 147 TABLE VI V.\RL\TIONS IX SODItm CELLS* Scale in Centi- meters Na I XaH 4 NaH 5 NaH Neon Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness mm 1.2 8.2 16.8 29.0 mm 1-5 4.0 6.5 78.2 832 88.3 mm 0.9 2.9 4.9 8.2 46.3 60.4 66.6 75-4 .mm ’2.3 6.4 10.2 15. 2 17.8 119.9 133.3 138.7 139.0 135.7 170.5 228.3 267.1 42.3 13-4 102.2 42. s 46.0 292.6 13-2 20.1 25.6 29.9 35-0 84.0 92.6 96.4 101.4 104.2 24.9 31. S 36.8 42.0 49.9 114.5 118.5 124.7 124.9 121.8 27 . 2 3 X.S 125.3 118,5 43.5 83.8 284.0 43.7 44.0 106.0 238.9 41.2 100. 5 I 2 S -0 134-0 146.9 157-2 162.8 163.0 148.1 107 -3 222.4 207.1 189.9 170.1 154.2 120.8 85.8 49.8 62.8 m.i 57.1 64.9 100.8 96.7 48:8 50.0 53-9 86.2 77.2 69.7 44.7 45.0 76.3 98.6 45.3 45.5 91.2 100.0. 96.0 71.8 34.3 n .3 86.4 73-4 55-6 33-4 13-4 37-8 97.2 Si .2 38.0 42.5 46.5 96.0 79-8 ss-s 37.1 47 45-2 29.9 13-2 21.0 II. 7 4.4 47.5 18.2 9.2 6.4 4-5 30 6.1 2.8 1.7 1 .0 0.7 21 .0 48.5 2.6 0.7 0.8 0.2 I . I 0.3 50.0 r -5 0.3 0.3 0. 1 •The sensitiveness column in each case is multiplied by lo — ", giving the spccihc photo-electric sensitiveness in coulombs per erg. 148 ELEANOR FRANCES SEILER TABLE VII Variation in Rubidium and Caesium Cells* Scale in Centimeters Rb 2 RbH3 RbH Q Cs I Cs H I Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness Galva- nometer Deflec- tions Specific Photo- Electric Sensi- tiveness mm I .o 20.8 mm mm 0-5 10.7 mm mm 2.8 25.7 5-3 30.4 1 . 2 n .0 0.7 6.3 1 .0 9.2 7.8 35-9 7.2 33-1 2.6 12.0 I-S 6.9 2.1 9-7 22.3 54-5 18.8 45-9 6.1 14-9 3.4 8.3 6.9 16.8 3 S -0 6r.Q 29.9 52-3 lo-S 18.6 SS-S 72.8 85.5 99.6 119. o 134.5 138.3 138.6 126.6 71.8 78.8 81.0 84.4 87.2 84.4 80.1 73-2 58.2 8. . 50.0 67.2 80.9 97.9 122 . 1 145.5 155.9 162.0 157.9 64.6 72.8 76.6 83.0 89.5 91.3 90.4 85.4 73.4 18.3 25. 5 31.8 41.4 51.2 77.2 91.6 no. 6 131.8 144.0 146.2 147.7 129.3 23.7 27.6 30.1 35-8 41.2 48.5 53-1 58.3 61 . 2 59-9 56.8 53-4 43-3 8.6 11 . 1 20.1 26.0 14-7 13-9 31.6 30.0 24.8 18.2 48.7 35.7 41 .0 51.2 69.8 90.0 108.7 126.2 132. I 122. 5 93-8 23-7 27.0 32.5 37.5 42.2 45.6 44.2 37-8 27.7 71.6 84.1 106.0 126.5 150.4 171.5 171 .0 147.6 103.9 75.8 29.0 4.3 1-3 41.5 44- 4 49-3 52.6 590 62.0 57.2 45- 5 30.7 21 . 1 7.7 1 .0 0.3 84.4 32.. 8. . II7.O 45 . 5 40.5 I 3 -- 6.. 63.4 21 . 2 17.0 5.. 27.7 8.2 87.1 25-7 6.1 2 . 1 0.9 I . . 0.6 0.5 12.4 6.0 4.1 3-2 1-4 0.9 48.5 25-9 8.9 12.7 6.1 2.3 36.5 6.8 2.8 95-8 1.6 0.6 50.0 *The sensitiveness column in each case is multiplied by 10 — giving the specific photo-electric sensi- tiveness in coulombs per erg. 0 ^ ■ ■ ■ i 1 ^ -nr- 410 440 470 500 530 560 590 620 650 680 7/0 Wai/e-L ength in juju- Fig. 8 COLOR-SENSITIVENESS OF PHOTO-ELECTRIC CELLS 149 IV. THEORETIC.VL CONSIDERATIONS The results here obtained cannot be used directly for theoretical considerations, because of the effect which absorption of light in the glass wall and the presence of gas in the cell have upon them. INIore- over, these results are due to both the normal and the selective effects, for they have not been separated in the present study. Conclu- sions which have been drawn from previous results are only approximately true. Certain equations to follow, are deduced from theoretical considerations, and when the values from this study are substituted in them, the results show a coincidence of about the same degree of approximation as those previously obtained. Since the curves resemble resonance curves, it is natural to assume that the frequency = :r— , at which the maximum ''max sensitiveness takes place, is connected with the resonance potential and the resonance frequency of the alkali vapors, where the quan- tum relation IV = and = //«,- holds for resonance and ioniza- tion potentials respectively. All photo-electric phenomena seem to be governed by the quantum relation, so that one might expect proportionality between hiph and hn, or //h,-. Thus propor- tionality may exist between npi, and the resonance and ionization potentials, i.e., the ratios and may be constants. These values are calculated both from the Xmi« found in Pohl and Prings- heim' and from this study, and are given in Table VIII. A glance will show that the latter values are more nearly constant, in spite of the fact that the values of Xmax for Na and Li are greatly at variance with the former. In this work they were found to be in the visible spectrum and others have located them in the ultra-violet region. 'I'he ionization and resonance potentials are those determined by d’ate and Foote for alkali vapors. The greatest variation from constancy in these ratios is in the case of ^ for Na. It seems i)robable that the value of V, for Na is a little high. Why should it be higher than that for Li, for F, and Vr both seem to decrease as the atomic weight increases with this one exception ? ' Die lichlelektrischcn Erschcimingen, j). 33, Tabic IX. 15 ° ELEANOR FRANCES SEILER It was thought that possibly the product of and the melting-point temperature of the alkali metals (Wien’s law) might be_^constant. This calculation shown in Table IX gives fairly good values for the constant C. The results as graphed in Figures 4 and 5 show clearly that as the atomic weight of the alkali metal decreases, (a) the position of Xmax shifts toward the violet, (b) the sensitiveness in general TABLE VIII Metal Ionization Potential L- Resonance Potential X max Frequency «ph "ph ”ph Li 5-36 1 .84 280 10.68 1.99 5.80 Na S-ii 2.09 340 8.81 I . 72 4.21 K 432 1 .60 43 s 6.88 1-59 4 ' 3 ° Rb 4. 16 I- 5 S 480 6.23 1-50 4,02 VALUES OF ^ AND FROM THIS INVESTIGATION Vr Li 5-36 1 .84 40s 7-41 1.38 4-03 Na S-ii 2.09 419 7 . 16 I .40 3-43 K 4-32 1 .60 440 6.82 1.58 4. 26 Rb 4. 16 I-SS 473 6.34 I 52 4.09 Cs 3.88 I- 4 S 539 S' 56 1-43 3 83 TABLE IX Element ^max T ^ m CXios Li 405 458.0 1.86 Na 419 370.5 1-55 K 440 335-3 1 .48 Rb 473 311-0 1-47 Cs 539 299.0 I .61 increases, (c) the resonant point, if such it may be called, becomes more marked, that is, the lithium cuiA^e is narrower than that of Cs. In order to show this latter point, an extra Figure 4b has been made, where the curves are so drawn that the maximum ordinates are all equal. In regard to the shifting of Xmax as we go from Cs to Li, it is known that a similar relation holds for the “convergence fre- quency” of the principal spectral series for these alkali metals. COLOR-SENSITIVENESS OF PHOTO-ELECTRIC CELLS 151 It may be connected with the increase in atomic volume. The colloidal state increased the volume and shifted X^ax toward longer wave-lengths, so it is natural to suppose that a large atomic volume means that the electron is held less firmly and consequently can be set in motion by a quantum of relatively small magnitude — that is, at a longer wave-length — than in the case of a substance with small atomic volume. Therefore Cs, having an atomic volume five times larger than Li, would have a Xn,a.x of a much longer wave- length. Two formulas, i and 2, have been suggested by Lindemann' which are partly of an empirical character. e I nph=~-\i , or 27T \mO e 'dfh , . nph = \ 2 \ — ... (i) 7 T \niM where nph is the characteristic violet frequency or the maximum color-sensitiveness frequency, e the charge on the electron in electrostatic units, the valency, m the mass of the electron, r one-half the distance between the centers of two neighboring T.XBLE X Element - .-V- . ' .'^ - '’■■vHv.,'»'^"S ."'7 • ^’7 A .1 /. >. '*■ ■' ^ ."i - •■ * ■ ‘,'sS’-''- .,' .->.-'^V -V ■ •' • ' . ' ^ '' *s' •«* ' •7'’ ‘•'^■^ S’"'' •'’»-' ’ *' Sl!!:|'!'i!i!| ' ilils