Selenium is prepared by heating it to a temperature of 120° C., keeping it there for some hours, and allowing it to cool slowly, when it assumes a crystalline form and changes from a bluish grey to a dull slate colour. A selenium cell in its simplest form consists merely of some prepared selenium placed between two or more metal electrodes, the selenium acting as a high resistance conductor between them. The form given by Bell and Tainter to the cells used in their experiments is given in Figs. 53 and 53a. It consists of a number of rectangular brass plates P, P', separated by very thin sheets of mica M, the mica sheets being slightly narrower than the brass plates, the whole being clamped together in the frame F by the two bolts B. { 110} By means of a sand-bath the cell is raised to the desired temperature, and selenium is rubbed over the surface, which melts and fills the small spaces between the brass plates. All the plates P are connected together to form one terminal, and the plates P' to form the other. By using very thin mica sheets, and a large number of elements, a very narrow transverse section of selenium, together with a large active surface, can be obtained.
The cell used for commercial purposes is usually constructed as follows. A small rectangular piece of porcelain, slate, mica, or other insulator, is wound with many turns of fine platinum wire. The wire is wound double, as shown in Fig. 54, the spaces between the turns being filled with prepared selenium. A thin glass cover is sometimes placed over the cell to protect the surface from injury.
A strong light falling upon a cell lowers its resistance, and _vice versa_, the resistance of a cell being at its highest when unexposed to light; the light is apparently absorbed and made to do work by varying the electrical resistance of the selenium. Selenium cells vary very considerably as regards their quality as well as in their electrical resistance, it being possible to obtain cells of the same size for any resistance between 10 and 1,000,000 ohms, and also, a cell may remain in good working condition for several months, while another will become useless in as many weeks.
The ability of a cell to respond to very rapid changes in the illumination to which it is exposed is determined largely upon its inertia, it being taken as a general rule { 111} that the higher the resistance of a cell the less the inertia, and _vice versa_, and also, that the higher the resistance the greater the ratio of sensitiveness. Inertia plays an important part in the working of a cell, slightly opposing the drop in resistance when illuminated, and opposing to a [Illustration] much greater degree the return to normal for no-illumination. The effects of inertia or "lag," as it is termed, can readily be seen by reference to Fig. 55. It will be noticed that the current value rapidly increases when the cell is first illuminated, but if after a short time _t_ the light is cut off, the current value, instead of returning at once to normal for no-illumination, only partially rises owing to the interference of the inertia, and some time elapses before the cell returns to its normal condition; the time varying from a few seconds to several minutes, depending upon the characteristics of the cell and the amount of light to which it is exposed. An actual curve is given in Fig. 55a. The inertia or "lag" of a cell produces upon an intermittent current an effect similar to that produced by the capacity [Illustration] of a line, as was noted in Chapter I., preventing the incoming signals from being recorded separately, and distinctly. To obtain the best results in photo-telegraphy, the resistance of a cell should only be decreased to an extent sufficient to pass the current required to operate the recording apparatus, and the illumination should be regulated so that this condition of the cell takes place.
