Excerpt from Instruments and Methods Used in Radiometry, Vol. 3: The Photo-Electric Cell and Other Selective Radiometers Many substances are light sensitive, for example, copper oxide, sulphides of antimony, silver, etc. The best-known example is crystalline selenium, which will'be discussed presently. Group III includes substances which, when charged to a nega tive potential (in an evacuated chamber), lose their charge when exposed to light, especially violet and ultra-violet rays. When used in this manner they are commonly called photoelectric cells. The emission of electrons is. A surface phenomenon which is easily affected by oxidation of the surface. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.