1. eye, electro-magnetic An apparatus used in exploring a field of electro-magnetic radiations. It is a piece of copperwire 2 millimeters (.08 inch) in diameter, bent into an almost complete circle 70 millimeters (.28 inch) in diameter, with terminals separated by an air gap. This is moved about in the region under examination, and by the production of a spark indicates the locality of the loops or venters in systems of stationary waves.