Selenium

Element, chemical symbol Se. A gray metal chemically similar to tellurium. Excellent conductor of electricity. Obtained as a by-product of the electrolytic refining of copper. Used chiefly in photoelectric cells, rectifiers and other electronic devices, and as a pigment for glass and ceramics. Selenium is added to steel to enhance machinability. It combines with manganese sulfide inclusions to modify their shape to be more globular.