Watt-hour meters

The most often-used means of measuring electrical energy is by using the small electric motor, whose speed depends on current, and on the power at a constant voltage. The number of turns of motor shaft, in the given length of time, is directly proportional to the number of kilowatt hours consumed. The motor is placed at point where utility wires enter house or apartment. This is at a point where the voltage is 234 V usually. This is split into some circuits having 234 V, for heavy-duty appliances like the washer, oven, and dryer, clock radios and the household fines for lamps, and, TV sets.

You have seen the little disk in utility meter going around, sometimes fast, and at times slowly. Its speed depends on power you are using. The total number of turns of this disk, every month, determines the size of bill you will get-as a function, certainly, of the cost per kilowatt hour for the electricity.

Kilowatt-hour meters count number of disk turns by means of geared, rotary drums or pointers. The drum type meter gives direct digital readout. The pointer type has many scales calibrated from 0 to 9 in circles, some going clockwise and others going anticlockwise.

Reading a pointer type utility meter is a bit tricky, because  you should think in whatever direction  the scale goes. An instance of a pointer type utility meter is shown in the Figure given below. Read from left to right. For each little meter, take down the number which the pointer has passed recently. The meter in the figure reads 3875 kWh. If you want to be precise, read it as 3875-1/2 kWh.

Figure--   An example of a utility meter. The reading is a bit more than 3875 kWh.