When the electrodynamometer is used as an ammeter, a special type of construction must be used.
This is because the large currents that flow through the meter cannot be carried through the moving coils.
In the ammeter in view B of figure 3-17, stationary coils a and b are wound of heavier wire to carry up to
5.0 amperes. An inductive shunt (XL) is wired in parallel with the moving coils and permits only a small
part of the total current to flow through the moving coil. The current flowing through the moving coil is
directly proportional to the total current flowing through the instrument. The shunt has the same ratio of
reactance to resistance as the moving coil does. Therefore, the instrument will be reasonably correct at
frequencies at which it is used if ac currents are to be measured.
Electric power is measured by means of a wattmeter. This instrument is of the electrodynamometer
type. As shown in figure 3-18, it consists of a pair of fixed coils, known as current coils, and a moving
coil, called the voltage (potential) coil. The fixed current coils are wound with a few turns of a relatively
large conductor. The voltage coil is wound with many turns of fine wire. It is mounted on a shaft that is
supported in jeweled bearings so that it can turn inside the stationary coils. The movable coil carries a
needle (pointer) that moves over a suitably graduated scale. Coil springs hold the needle at the zero
position in the absence of a signal.
Figure 3-18.Simplified electrodynamometer wattmeter circuit.
The current coil of the wattmeter is connected in series with the circuit (load), and the voltage 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 in phase with and proportional to the line current. The voltage
coil of the wattmeter generally has a high-resistance resistor connected in series with it. The purpose for
this connection is to make the voltage-coil circuit of the meter as purely resistive as possible. As a result,
current in the voltage circuit is practically in phase with line voltage. Therefore, when voltage is