Maker's Name: 
General Radio Co, Cambridge, Mass.
Where made: 
c. 1940
27.5 × 17 × 16 cm

A sensitivity high input impedance impedance AC voltmeter.

FSD ranges are 0.3V, 1V, 3V, 10V, 30V, 100V, 300V.

It uses an acorn 957 valve (tube) as a AC rectifier, and two 1S5 valves in a DC amplifier, the second one as a cathode follower with a meter in the cathode circuit forming one arm of a bridge. The bridge has to be balanced manually before a reading is taken.

A single 1,5V cell is used to heat the filament of each valve. Each 1S5 has a separate 30V supply for its anode.

In this case the 30V supplies have each been converted to three 9V batteries