APPENDIX A
SELENIUM CELLS
Selenium is a non-metallic element, and was first discovered by Berzelius in 1817, in the deposit from sulphuric acid chambers, which still continues the source from which it is obtained for commercial purposes, although it is found to a small extent in native sulphur. Its at. wt. is 79.2, and its sp. gr. 4.8. Symbol, Se.
In its natural state selenium is practically a non-conductor of electricity, its resistance being forty thousand million times greater than copper. Its practical value lies in the property which it possesses, that when in a prepared condition it is capable of varying its electrical resistance according to the amount of light to which it is exposed, the resistance decreasing as the light increases.
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.
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.
P, P', plates; M, mica; S, selenium.
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
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
The comparative slowness of selenium in responding to
any great changes in the illumination offers a serious difficulty to its use in photo-telegraphy, but various methods have been devised whereby the effects of inertia can be counteracted. In the system of De' Bernochi (see Chapter I.) the changes in the illumination are neither very rapid nor very great, and the inertia effects would therefore be very slight; but in any photo-telegraphic system in which a metal line print is used for transmitting, where the source of illumination is constant and the resistance of the cell is required to drop to a definite value and return to normal instantly, many times in succession, the inertia effects are very pronounced. The most successful method of counteracting the inertia is that adopted by Professor Korn of always keeping the cell sufficiently illuminated to overcome it, so that any additional light acts very rapidly. Another method worked out and patented by Professor Korn, and known as the "compensating cell" method, gives a practically dead beat action, the resistance returning to its normal condition as soon as the illumination ceases. The arrangement is given in the diagram Fig. 56.
Light from the transmitting or receiving apparatus, as the case may be, falls upon the selenium cell S1, which is
placed on one arm of a Wheatstone bridge, a second cell S2 being placed on the opposite arm. The selenium cell S1 should have great sensitiveness and small inertia, the compensating cell S2 having proportionally small sensitiveness and large inertia. Two batteries B, B', of about 100 volts, are connected as shown, B being provided with a compensating variable resistance W; W' is also a regulating resistance. When no light is falling upon the cell S1, light from L is prevented from reaching the second cell S2 by a small shutter which is fastened to the strings of the Einthoven galvanometer (described in Chapter III.), and the piece of apparatus C—relay or galvanometer as the case may be—remains in a normal condition. When, however, light falls upon the cell S1, the balance of the bridge is upset, and light from L falls a fraction of a second later upon the second cell S2, and the current flowing through C completes the circuit. Needless to say it is necessary that the two cells be well matched, as it is very easy to have over-compensation, in which case the current is brought below zero.
It is also stated that by enclosing the cells in exhausted glass tubes, their inertia can be greatly reduced and their life considerably prolonged. The sensitiveness of a cell is the ratio between its resistance in the dark and its resistance when illuminated. The majority of cells have a ratio between 2:1 and 3:1, but Professor Korn has shown mathematically that by conforming to certain conditions regarding the construction the ratio of sensitiveness may be between 4:1 and 5:1. Thus a cell of R = 250,000 ohms can be reduced to 60,000 ohms from the light of a 16 c.p. lamp placed only a short distance away; the resistance may be still
further decreased by continuing the illumination, but this produces a permanent defect in the cells termed "fatigue," the cells becoming very sluggish in their action and their sensitiveness gradually becoming less, the ratio between their resistance in the dark and their resistance when illuminated being reduced by as much as 30 per cent.
Excessive illumination will also produce similar results. The inertia of a cell is practically unaffected by the wavelength of the light used, but the maximum sensitiveness of a cell is towards the yellow-orange portion of the spectrum.
In addition to light, heat has also been found to vary the electrical resistance of selenium in a very remarkable manner. At 80° C. selenium is a non-conductor, but up to 210° C. the conductivity gradually increases, after which it again diminishes.