Selenium

—A rare non-metallic element, closely resembling sulphur in its behavior, and found in conjunction with that substance and with tellurium. The electrical resistance of selenium changes remarkably under the action of light, consequently selenium cells are much employed in certain cla es of apparatus, notably the photophone, where sound is transmitted along a beam of light which is directed upon a selenium cell, the variation in the light altering the current and reproducing the sound.