selenium n a nonmetallic element that exists in several allotropic forms. It occurs free in volcanic areas and in sulphide ores, esp. pyrite. The common form is a grey crystalline solid that is photoconductive, photovoltaic, and semiconducting: used in photocells, solar cells, and in xerography. Symbol: Se; atomic no.: 34; atomic wt.: 78.96; valency: --2, 4, or 6; relative density: 4.79 (grey); melting pt.: 221°C (grey); boiling pt.: 685°C (grey) (C19: from New Latin, from Greek selene moon; named by analogy to tellurium (from Latin tellus earth))
n a photoelectric cell containing a strip of selenium between two metal electrodes
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