Electric power is measured by means of a wattmeter. This instrument is of the electrodynamic type.
It consists of a pair of fixed coils, known as current coils, and a movable coil known as the potential coil.
(See fig. 1-44.) The fixed coils are made up of a few turns of a comparatively large conductor. The
potential coil consists of many turns of fine wire. It is mounted on a shaft, carried in jeweled bearings, so
that it may turn inside the stationary coils. The movable coil carries a needle which moves over a suitably
marked scale. Spiral coil springs hold the needle to a zero position.
Figure 1-44.A simplified electrodynamic wattmeter circuit.
The current coil (stationary coil) of the wattmeter is connected in series with the circuit (load), and
the potential coil (movable coil) is connected across the line. When line current flows through the current
coil of a wattmeter, a field is set up around the coil. The strength of this field is proportional to the line
current and in phase with it. The potential coil of the wattmeter generally has a high-resistance resistor
connected in series with it. This is for the purpose of making the potential-coil circuit of the meter as
purely resistive as possible. As a result, current in the potential circuit is practically in phase with line
voltage. Therefore, when voltage is applied to the potential circuit, current is proportional to and in phase
with the line voltage.