IV. A Study of the Selenium Cell. By A. H. PFUND *. THE theory accepted to-day in explanation of the fact that a selenium cell changes its resistance when exposed to radiation, is due to Bidwell. Briefly, this theory attributes the sensibility of a cell to the presence of selenides from which selenium apparently never is free. The facts in support of this view are: 1. The conduction of a cell is electrolytic in character †. 2. Selenium is a very poor conductor, while a selenide is, comparatively, a good one. 3. Sunlight actually brings about a combination of metal and selenium, forming a stable selenide, capable of conducting an electric current‡. In enlarging upon his theory, Bidwell § says:-"If, as is commonly believed, electrolytic conduction involves a series of decompositions and recompositions throughout the electrolyte, any cause which will assist either the separation or recomposition (or both) of the components of the electrolyte might be expected to increase its conductivity; and it seems reasonable to suppose that the same influence which would assist the union of two substances when they have a tendency to unite would also be favourable to their separation when they have a tendency to separate. It is not impossible, therefore, that radiation, acting upon the surface of a thin layer of selenide of silver through which an electric current is passing, might, by facilitating the molecular rearrangement of the atoms of selenium and silver, exert a material influence upon the conductivity. As, in the above theory, the phenomenon is ascribed entirely to the selenide, it seemed worthy of interest to undertake experiments with cells containing different selenides. In the course of the work it soon appeared that the sensibility of the selenium cell reaches a maximum in the visible spectrum, and it was thought that the position of the maximum might be a function of the metal in the selenide. While it is possible, it is hardly probable, that light from the same region of the spectrum which is most favourable to the combination between copper and selenium, will also be most favourable to the combination between mercury, silver, lead, and other metals with selenium. Furthermore, in consequence of Bidwell's statement that * Communicated by Prof. Benjamin W. Snow. Adams and Day, Proc. Roy. Soc. 1876, P. 113. 1 Shelford Bidwell, Phil. Mag. vol. xl. p. 233 (1895). light produces an increase in the conductivity of a cell by facilitating the molecular rearrangement in the surface-layer of a selenide through which an electric current is passing, the question has presented itself, does the resistance of a cell undergo changes if it be exposed to light while no current is flowing? Selenium. It appears that, in spite of the most painstaking efforts, selenium cannot be obtained absolutely pure, although the process of purification may be carried to so high a degree as to make the amount of impurity still present very small indeed. As a preliminary series of experiments, cells were made with metallic electrodes and of crude selenium to which definite selenides had been added; these cells were subjected to light from various portions of the spectrum and the position of maximum sensibility was determined for each. It appeared that the position of the maximum was the same for all cells. On account of the impurity of the selenium some doubt was cast upon the trustworthiness of the results and it was thought advisable to repeat the experiments with purified selenium. As distillation does not effectively remove impurities, a chemical method* was resorted to. I wish to take this opportunity of acknowledging my indebtedness to Professor Victor Lehner, of the Department of Chemistry, for the details of this process, which yielded selenium of great excellence and purity. Briefly the method is as follows: Commercial selenium is dissolved in hot nitric acid and the resulting selenious acid is evaporated to hard dryness when the dioxide is formed. This is dissolved in distilled water and barium hydrate is added until a permanent precipitate is no longer formed. After filtering, the solution is boiled to dryness in an evaporating-dish, and the residue is covered by an inverted funnel whose base fits snugly upon the inside of the evaporating-dish. Continued heating brings about a sublimation of the dioxide upon the inner walls of the funnel in the form of white needles. As the slightest amount of impurity imparts to the selenium dioxide a reddish colour, it is necessary to repeat the process of sublimation until pure white needles are obtained. These are dissolved in distilled water, the solution is acidified with hydrochloric acid and acid sodium sulphite is added, bringing about a liberation of sulphur dioxide, which in turn precipitates the selenium in the form of a red powder. By boiling the mass for a few minutes the selenium forms into a hard black lump which, when washed and dried, is ready for use. * Victor Lehner, Journal Am. Chem. Soc. vol. xx. no. 8. Selenium prepared in this manner is deemed sufficiently pure for atomic and molecular weight determinations. Selenium Cell. A departure was made from the usual manner of making selenium cells in that carbon electrodes were used in place of metallic electrodes. The reason for this is, that when metallic electrodes are used a selenide is formed wherever metal and selenium come in contact, the amount formed being dependent upon the character of the metal and the length of time during which the cell is subjected to heat. The difficulty of knowing the exact amount of selenide in such a cell is obvious. As actual tests have proved that no conducting compound is formed when selenium is heated in contact with a carbon surface, it was thought that by using carbon electrodes the above difficulty would be avoided, or, in other words, that it would be possible to know the exact amount of selenide contained in a completed cell. Furthermore, when electrodes of silver and mercury are used, the strong affinity of these metals for selenium brings about such a copious formation of selenide that, after a few minutes of heating, the cell becomes worthless. The use of carbon electrodes overcomes this difficulty entirely. Fig. 1. selenium The above sketch (fig. 1) represents a full-sized selenium cell as actually used in these experiments. Two small labs of Atchison graphite, an especially pure form, were separated by a layer of mica, securely bound and shellacked together and then baked so as to form a compact mass. While the cell was still hot a very thin layer of molten selenium was spread over one side, as shown in the diagram. In the average the selenium layer was 15 mms. long, 1 mm. wide, and 0.08 mm. thick. Before being applied to the cell the selenium had been ground up to a fine powder in an agate mortar with 3 per cent. of a definite selenide *, for this proportion of selenide in selenium, as shown by Bidwell, is approximately that necessary to develop the highest sensibility in a cell. The selenides of copper, lead, mercury, and silver were used for the reason that they form with great readiness and conduct electricity well. The details followed out in the process of forming and annealing were similar to those described by Bidwell ‡, and, in general, this procedure yielded cells of fair, though not excessive sensibility. In the average a cell had a resistance of about 20 megohms, and, when new, increased its conductivity from three to twelve times upon being illuminated by a 32 candle-power incandescent lamp at a distance of 30 cms. Apparatus. The spectrometer was of the usual fixed-arm type, designed for work in the visible and infra-red spectrum. Light from a Nernst lamp L (fig. 2) was dispersed by a rock-salt prism P, the resulting spectrum being brought to a focus on a brass plate B which contained a vertical slit. By rotating the prism, light of any desired wave-length could be made to pass through the slit. Directly behind this a Rubens thermopile, T, and a selenium cell, S, were rigidly mounted on a vertical bar, pivoted near its lower end and working between stops so that the selenium cell and thermopile could be mutually interchanged in position. By the use of the slit at B there is no question but that light from the same region of the spectrum which fell upon the thermopile would also illuminate the selenium cell, thus avoiding numerous complications which are certain to arise if the one instrument does not replace the other with absolute exactness. K is a device placed in the path of the parallel beam to cut down the intensity of the energy. While a rotating sector would have * Obtained from Dr. Theodor Schuckhardt. Shelford Bidwell, Phil. Mag. vol. xl. p. 233 (1895). + Ibid. answered just as well, this device was chosen on account of its simplicity and ease of manipulation. The galvanometer, Gr, connected to the thermopile was of the four-coil Thomson type, being of low resistance and high sensibility; the other galvanometer, Gs, connected to the selenium cell was of the same type, but had a higher resistance and lower sensibility. Both were effectively shielded against magnetic disturbances by three concentric cylinders. of soft iron. On account of the large number of readings which must be taken in a work of this character, the changes in resistance of a cell were determined by using a method of direct deflexion rather than by using the Wheatstone's bridge. Although the data recorded are percentage-change in conductivity, the absolute value of this change in ohms may be determined readily as the initial dark resistance of a cell is always given. The specific nature of a cell determined the method of procedure which was to be used in the measurements of resistance, for the difference between various cells was quite |