The mother of all meters ..

TRIVECTOR

Measures kWh, kVAh, kVArh

The left side is a 3Ø 4 wire kWh meter.
The right side is a 3Ø 4 wire kVArh meter.

By means of an ingenius gear box underneath the kVAh dial (middle) the highest readings weather active or reactive power are transferred to the KVA dials.

These meters were used from around the 1950's, 1960's till ousted by solid state metering in 1990 here in New Zealand.

This one is ex NZ 754 AM radio transmitter station, The Concourse Henderson.

[Linked Image from static.flickr.com]

The product of rotation, excitation and flux produces electricty.