Need for a means of determining whether a thermal battery had been accidentally activated prior to its installation in a piece of equipment led to an investigation of thin metal films that melt at temperatures between 100 and 200 C, and have high resistances after and low resistances before melting. Indium films in thicknesses ranging from 200 to 5300 angstroms were vacuum-deposited onto mica substrates. Prior to melting at 156 C, these films were continuous and exhibited low resistance (less than 1 ohm). On melting, the films agglomerated into discrete particles, because indium will not wet mica, and high resistances (less than 1 ohm) were obtained. Thus, satisfactory elements were obtained on mica. (Author).