Electric Meter

Electric meter is a device that measures electric current. Electric utility companies use watt-hour meters to measure in kilowatt-hours the amount of electrical energy used by their customers. A kilowatt-hour is 1,000 watt-hours. Ten 100-watt light bulbs turned on for one hour will use 1 kilowatt-hour of electricity (see Kilowatt). The utility company connects the meters to the lines supplying electricity to a home or factory. The meter is read at regular intervals, and the customer's bill is based on these readings.

A watt-hour meter operates on the principle of electromagnetism (see Electromagnetism). It has a thin metal disk mounted so that it rotates in the field of an electromagnet. The electromagnet is connected to the line supplying electricity to the customer. The greater the load, or amount of electricity used by the customer, the stronger the electromagnetic field becomes. The stronger this field, the faster the disk turns. A system of gears registers the disk's rotations on five dials in kilowatt-hour units. A permanent magnet keeps the disk from revolving when no electricity is being used.

Each of the five dials has a pointer and is divided into 10 units, numbered from 0 to 9. The right dial registers kilowatt-hours in units of 1 up to 10. The next dial to the left indicates kilowatt-hours in units of 10 up to 100. The third dial records kilowatt-hours in hundreds up to 1,000. The fourth dial registers kilowatt-hours in units of 1,000 up to 10,000, and the fifth dial records kilowatt-hours in units of 10,000 up to 100,000. The entire meter is sealed inside a glass case which keeps out dust and moisture.

Utility companies usually use a watt-hour meter that has a demand indicator to measure the power used by industrial and commercial customers. This indicator has a pointer that shows on a graduated scale the highest number of kilowatts required by the customer in a given period. The pointer is usually reset to zero after the meter is read. Some demand meters record the varying kilowatt requirements of the customer on a chart.

Oliver B. Shallenberger made the first successful electric meter in 1888. In 1895, he patented a design for an induction watt-hour meter that is regarded as the basis of all watt-hour meters used today.

Other kinds of electric meters include galvanometers and ammeters to detect and measure the strength of a current (see Ammeter; Galvanometer). Voltmeters and wattmeters measure the voltage or power of a current (see Voltmeter; Watt).

Contributor: Raymond D. Findlay, Ph.D., Director, Engineering and Management, McMaster Univ.

Electric Meter

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