An "electric eye" alarm detects a break in the beam of light not with a simple "electric" cell but with this type of cell
Cells are composed of two different types of electrodes interacting with 
an... [DOC] [TLE] Patent US3653021 - Universally adjustable and focusable alarm ... [PAR] The apparatus has an alarm circuit which is energized when the light beam ... 
electric element of a light beam transmitting and receiving circuit is mounted in 
the holder.