Maker's Name: 
A.J. William
Where made: 
18.5 × 31.2 × 16.5 cm

This meter would have been used for testing aircraft power systems that include 400 Hz 115 VAC, the higher frequency allowing weight to be saved on transformers and inductors.

This meter uses phase discrimination circuits to generate DC voltages to drive the moving coil meter movement. 115 VAC is fed through a large value inductor. Two parallel tuned circuits, one resonant below 360 Hz and the other resonant above 450 Hz, vectorially output a signal proportional to the frequence difference from the central reference of 390 Hz. The rectified signal drives the meter needle up or down from the central value.