Page 75 - Teach Yourself Electricity and Electronics

Watt-hour meters 55

3-10 Power can be measured with a voltmeter and an ammeter.

much current for the house wiring to safely handle, and the fuse or breaker, detecting the
excess current, opened the circuit.
Specialized wattmeters are necessary for the measurement of radio-frequency
(RF) power, or for peak audio power in a high-fidelity amplifier, or for certain other spe-
cialized applications. But almost all of these meters, whatever the associated circuitry,
use simple ammeters as their indicating devices.

Watt-hour meters

The utility company is not too interested in how much power you’re using with one ap-
pliance, or even how much power a single household is drawing, at any given time. By
far the greater concern is the total energy that is used over a day, a week, a month or a
year. Electrical energy is measured in watt hours, or, more commonly for utility pur-
poses, in kilowatt hours (kWh). The device that indicates this is the watt-hour meter
or kilowatt-hour meter.
The most often-used means of measuring electrical energy is by using a small elec-
tric motor device, whose speed depends on the current, and thereby on the power at a
constant voltage. The number of turns of the motor shaft, in a given length of time, is di-
rectly proportional to the number of kilowatt hours consumed. The motor is placed at
the point where the utility wires enter the house, apartment or building. This is usually
at a point where the voltage is 234 V. This is split into some circuits with 234 V, for
heavy-duty appliances such as the oven, washer and dryer, and the general household
fines for lamps, clock radios and, television sets.
You’ve surely seen the little disk in the utility meter going around and around,
sometimes fast, other times slowly. Its speed depends on the power you’re using. The
total number of turns of this little disk, every month, determines the size of the bill you
will get—as a function also, of course, of the cost per kilowatt hour for electricity.
Kilowatt-hour meters count the number of disk turns by means of geared, rotary
drums or pointers. The drum type meter gives a direct digital readout. The pointer type
has several scales calibrated from 0 to 9 in circles, some going clockwise and others go-
ing counterclockwise.