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SELENIUM CELLS 143 the change of electrical resistance varied directly as the square of the illumination. Cells using this substance have been made showing a dark resistance as low as 300 ohms and a resistance when exposed to sunlight of but 150 ohms, a reduction of 50%. We must not make the mistake of confusing selenium cells with photoelectric cells inasmuch as we are dealing with two different effects, although it may be said, broadly speaking, that both types of cells operate on a photoelectric principle if we take photoelectric to mean the control of current by light. In the photoelectric cell we are dealing with pure electronic emission from photo-sensitive surfaces. In the selenium cell, on the other hand, we are dealing with changes in ohmic re sistance brought about by light. The mechanics of this change which takes place in selenium is still more or less of a mystery although many capable experimenters have added much illuminating data to our knowledge. The experimenter in phototelegraphy or television will do well to understand the limitations of selenium before he at tempts to use it in his work, and it is the object of this Chapter to point out the difficulties and the peculiar properties of this substance. Although a carefully designed selenium cell may be employed for amateur television transmission it is not as suitable for this service as substitute for a photoelectric cell. On the other hand, there are certain phototelegraphic systems where selenium performs much more satisfactorily than pure photoelectric devices. Of course, there is the advantage of the low cost of selenium cells as compared with photoelectric cells of the more sensitive type. Selenium cells may vary considerably in their quality and it is now possible to construct cells having approximately the same physical dimensions between 10 and 1,000,000 ohms. In every type of selenium cell there is always the variable factor