Jewell Electrodynamic Wattmeter Disassembly

[JackOfVA]

Before digital AC mains wattmeters, there were electrodynamic wattmeters.Unlike a typical d'Arsonval meter with a permanent magnet and a moving coil, electrodynamic wattmeters operate with two coils and no permanent magnet. One coil is fixed and the second is coupled to the meter pointer and is free to rotate. One coil is powered from the AC mains voltage and the second carries a sample of the current. The net magnetic force against the retaining spring is proportional to voltage x current, or watts. This arrangement compensates for power factor and thus reads true power, not volt-amperes.

I've had an old wattmeter in the junk box for years - it was acquired at a ham radio swap and shop for a few dollars - but it was not in good shape. The pointer would stick and it was in generally poor shape. It's now disassembled into a pile of parts in the trash bin, as illustrated in the photos below.

The meter also has two resistors, 4.37K and 411 ohms and a 0.25uF capacitor and a current transformer.

The meter is a marvel of mechanical construction, particularly the fixed and rotating coils.