To determine the amount of electricity used by consumers, each circuit is provided with some form of wattmeter. These meters might be more correctly called watt-hour meters since they register the watt-hours of electrical energy that pass through the circuit.

In the common type of meter, the recording apparatus in composed of a motor and a registering dial. The motor is intended to rotate at a rate that is proportional to the amount of passing current. An example of this device is the Thompson induction meter of Fig. 252. The motion of the aluminum disc seen through the window in front indicates at any time the rate at which electricity is being used. This constitutes the rotating part of the motor. It is propelled by the magnetism, created by the passing current, and is sensitive to every change that takes place in the electric circuit. Each lamp, heater or motor that is brought into use or turned off produces a change of current in the conducting wires and this change is indicated by the rate of rotation of the disc. Each rotation of the disc represents the passage of a definite amount of electricity that is recorded on the registering dials.

The shaft on which the disc is mounted is connected with the recording mechanism by a screw which engages with the first of a train of gears. These gears have, to each other, a ratio of 10 to 1; that is, ten rotations of any right-hand gear, causes one rotation of the gear next to the left. The pointers on the dial are attached to the gear spindles. One rotation of the right-hand dial will move the pointer next to the left one division on its dial. Each dial in succession will move in like ratio.

The meters are carefully calibrated and usually record with truthfulness the amount of electricity used. They are, however, subject to derangement that produces incorrect registration.

To Read The Meter

First, note carefully the unit in which the dial of the meter reads. The figures above the dial circle indicate the value of one complete revolution of the pointer in that circle. Therefore, each division indicates one-tenth of the amount marked above or below the circle.

Second, in reading, note the direction of rotation of the pointers.

Fig. 252. - Recording watt meter.

Commencing at the right, the first pointer rotates in the direction of the hands of a clock (clockwise); the second rotates counterclockwise; the third, clockwise; etc., alternately. The direction of rotation of any one pointer may easily be determined by noting the direction of the sequence of figures placed around each division. The arrows (shown above) indicate the direction of rotation of the pointers when the meter is in operation.

Third, read the figures indicated by the pointers from right to left, setting down the figures as they are read, i.e., in a position relative to the position of the pointers. Note: One revolution of the first or right-hand pointer makes one-tenth of a revolution of the pointer next to it on the left. One revolution of this second pointer makes one-tenth of a revolution of the pointer next to it on the left, etc. Therefore, if, when reading the dial, it is found that the second pointer rests very nearly over one of the tenth divisions and it is doubtful as to whether it has passed that mark, it is only necessary to refer to the pointer next to it on the right. If this pointer on the right has not completed its revolution, it shows that the second pointer has not yet reached the division in question. If it has completed its revolution, that is, passed the zero, it indicates that the second pointer has reached the division and the figure corresponding is to be set down for the reading.

The foregoing also applies to the remaining pointers. When it is desired to know whether a pointer has passed a tenth division mark, it is necessary to refer only to the next pointer to the right of it.

Fourth, see if the register is direct-reading, i.e., has no multiplying constant. Some registers are not direct-reading in that they require multiplying the dial reading by a constant such as 10 or 100 in order to obtain the true reading. If the register bears some notation such as "Multiply by 100," the reading as indicated by the pointers should be multiplied by 10 or 100 as the case may be to determine the true amount of energy consumed.

Some of the earlier forms of meters were equipped with what is known as a "non-direct-reading register." In this case, the reading must be multiplied by the figure appearing on the dial as just explained, but the dial differs from those just described in that the multiplying constant is generally a fraction such as 1/2, etc., and the dial has five pointers. This older style of register reads in "watt-hours" of "kilowatt-hours."

Fifth, the reading of the dial does not necessarily show the watt-hours used during the past month. In other words, the pointers do not always start from zero. To determine the number of watt-hours used during a certain period it is necessary to read the dial at the begining of a period and again at the end of that period. By subtracting the first reading from the second, the number of watt-hours or kilowatt-hours used during the period is obtained.

The meter man, having in his possession a record of the readings of each customer's meter for the preceding months, is thus able to determine the amount of energy consumed monthly.