A B C of television; or, Seeing by radio; a complete and comprehensive treatise dealing with the theory, construction and operation of telephotographic and television transmitters and receivers; written especially for home experimenters, radio fans and students.. (1929)

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SELENIUM CELLS 147 cubic centimeter. This is a factor which must be reckoned with in the design of cells and it is obvious that the selenium must be applied in such a way and upon such a structure that only a very short path with a comparatively large cross-section be traversed by the current. It is obvious, too, that we should design the cell in such a way that the selenium be spread out BRASS MICA PLAN END ELEV. O SLIT DETAIL OF STRIP FIG. 49.—Townsend selenium shown here is made up of two separate sets of brass plates properly insulated from each other. The spaces between the plates are filled with molten selenium. ill a very thin layer so that the ratio of surface to volume may be great. We must keep in mind the fact that we are desirous of having light act upon as large a surface of the selenium as is practical. It is also to be noted that the thinner the film of the selenium the less the inertia of the cell. There are at least twenty-five different types of selenium cells which have been due to the research work of as many dif ferent experimenters. Perhaps one of the simplest forms is