A WATT-HOUR METER is basically a small motor whose instantaneous speed is proportional to
the power through the motor. The total revolutions in a given time are proportional to the total energy, or
watt-hours, used during that time. Watt-hour meters measure energy.
FREQUENCY METERS are used to measure the frequency of an ac signal. The two basic types
are the vibrating-reed frequency meter which is usually used as an in-circuit meter, and the moving-disk
frequency meter which is usually used as an out-of-circuit meter.