Electric Meter, or Watt-hour Meter, an instrument that measures the amount of electric energy used by a consumer. The meter is calibrated in kilowatt-hours. One kilowatt-hour is the amount of electric energy required to provide 1,000 watts of power for a period of one hour. (Ten 100-watt light bulbs left on for one hour consume one kilowatt-hour of electric energy.)

An electric power company uses electric meters to measure the amount of electricity consumed by each of its customers. The power company installs an electric meter near where its power lines enter a customer's building. It reads the meter periodically and charges the customer for the amount of electricity used.

The most common type of electric meter is essentially an electric induction motor that drives a series of geared wheels connected to indicators on the meter's face. This type of meter is designed for use with alternating current. It contains two electromagnets and a metal disk that is free to rotate between them. One electromagnet is powered directly by current from the incoming power lines; the other, by current drawn through the building's electrical circuits. The interaction of the magnetic fields produced by the coils causes the disk to rotate. Two permanent magnets near the disk's edge brake the disk in such a way that the speed of rotation is proportional to the amount of current drawn. As the disk rotates, it turns the series of geared wheels connected to the indicators on the meter's face.

Electronic watt-hour meters use solidstate circuits that produce electrical signals whose frequency or strength is proportional to the voltage and current being used. These signals are converted into energy measure ments recorded by mechanical or electronic indicators. Electronic watt-hour meters are generally more expensive than electromechanical models, but are more accurate. They can provide such features as the ability to record separately the energy consumed during different times of day and the ability to report meter readings by means of signals sent through the power lines to the power company.