Voltmeter, US Army, M-433, 1944

Genuine WWII US Army voltmeter IS-185, model 433 dated 1944. The voltmeter is a magnetically, A.C. iron-vane type equipment incorporating a frequency-compensated movement. It permits accurate voltage measurments on alternating voltages of nonsinusoidal wave forms. 

Also used in the US Army Air Force to test heating equipments.