Special Treatment of the Selenium.
“The first point to which we devoted our attention was reducing the resistance of crystalline selenium within manageable limits. The resistance of selenium cells, employed by former experimenters, was counted in millions of ohms; there is no record of a cell measuring less than 250,000 ohms in the dark. We have succeeded in producing cells measuring only 300 ohms in the dark and 150 in the light. Our predecessors all seemed to have used platinum for the conducting part of their cells, excepting Werner Siemens, who found that iron and copper would do. We have discovered that brass, although chemically acted upon by selenium, forms an excellent material; indeed, we are inclined to believe that the chemical action between brass and selenium has contributed to the lowness in resistance of our cells, an intimate union taking place between the two substances. In brass we have constructed many cells of diverse forms. One of them (two are described by Professor Bell), is cylindrical so that it may be used with a concave reflector instead of with a lens. It is composed of many metallic disks separated by mica disks slightly smaller in diameter. The spaces between the brass disks over the mica are filled with selenium, and the alternate brass disks are metallically connected. The selenium is applied to the cell duly heated: next comes annealing. To effect this an oven is inserted in a pot of linseed oil standing upon glass supports in another similar pot of linseed oil. The whole is then heated to about 214° C., and kept there for twenty-four hours, then allowed to cool down during forty to sixty hours until the temperature of ordinary air is reached.