SELENIUM CELLS. The light-sensitive property of sel enium was discovered in 1873 by Willoughby Smith, who, as a pioneer of submarine telegraphy, had selected that substance for some experiments requiring the use of a high electrical re sistance. It is interesting to notice that 56 years had elapsed since the isolation of selenium by Berzelius, and that Riess, throughout his work upon its electrical conductivity had missed, in 1845, the important fact that light is capable of modifying the actual effect he was studying. In 1875 Siemens suggested the use of selenium in photometry and, from that time till the present day, many other proposals relating to the subject have been forthcoming. Thus in 188o Ayrton and Perry describe what they called "electric vision" and in the following year Shelf ord Bidwell constructed several improved selenium cells.
Prof. Minchin, in 1891, used selenium for measuring the light from the stars and a few years later Giltay showed that Röntgen rays also changed the electrical resistance of selenium just as light does. Work on the electrical transmission of pictures was carried out during 1902 by Korn, who, five years later, succeeded in thus sending a picture from Munich to Berlin. Dr. Fournier d'Albe invented the "optophone" in 1912 and Graham Bell had made the first "photophone" some 32 years earlier. A greatly improved photophone, however, was designed by Prof. A. 0. Rankine in 1916 and is referred to more in detail later.
It will be understood, from what has preceded, that these cells or bridges depend for their action on the change in electrical resistance of selenium when illuminated. A variety of forms of them have been devised but in practically all the aim is similar, namely to arrange two or more conductors so that they are separated by and in close contact with crystalline selenium. The great interest attaching to the subject arises from the fact that no completely satisfactory explanation of the action of light on selenium has been discovered.
amorphous form of selenium, in which it is sold by dealers, is so poor a conductor of electricity that it may be classed as an in sulator, it is necessary when preparing light-sensitive cells to convert this into one of the grey crystalline forms, which conduct comparatively well.
The simplest cells can be made by winding two thin copper wires side by side and one mm. apart upon a strip of mica, say 3" long by 1" wide. The strip should then be placed on a hot plate and a thin layer of selenium melted over the wires. On lustrous selenium will rapidly become dull and finally grey. In that state it will be found to be sensitive to light and a fair con ductor of electricity. With two adjacent ends of the wires sepa rated and a potential difference of 5o volts applied to the others, a current of perhaps 5o to ioo micro-amperes may be passed through such a cell in the dark, and in the ordinary light of a room this current should immediately rise to 30o micro-amperes or even more. In a modification due to Dr. Pfund, a piece of glass i cm. wide and 3 cm. long is coated with selenium which is smoothed out by a hot glass rod. Four strands of bare copper wire are then wound round the whole so that the selenium sur face is covered. After fixing the ends of the wires, the first and third strands are unwound and removed so as to leave a gap be tween the second and fourth and the bridge is reheated and annealed to grey.
Dr. Fournier d'Albe recommends preparing selenium cells by coating slate with graphite and then scratching away a zig-zag line as shown in fig. 1, so that a potential difference applied at the regions A and B would tend to cause a current to flow across the scratch. Owing to the high in sulation of the slate, however, no current can pass until selenium is melted over it and allowed to turn grey. In practice it is found necessary to grind the slate surface as smooth as possible by means of pumice powder and subse quently to rub in the graphite very thoroughly to ensure a suffi ciently conducting surface.