Fig. 2,561.—Circuit diagram of simple shunt Sangamo ampere hour meter. It is rated at 10 amperes, larger currents being measured by using shunts. In operation, the main or line current to be measured passes through the shunt, while a part proportional to the drop across the shunt, is shunted through the meter and measured. The only effect of reversing the current will be to reverse the direction of rotation of the meter. In battery installations it is never possible to take the same number of ampere hours from a battery as are put into it, hence, if the simple shunt ampere hour meter be used for repeated and successive charges and discharges, it will be necessary to reset the pointer to zero each time the battery is fully charged. When the shunt meter is equipped with a charge stopping device, the pointer is reset while charging, to allow for a predetermined overcharge.

Fig. 2,562.—Circuit diagram of Sangamo differential shunt type ampere hour meter for use in battery charging. Since a battery absorbs more energy on charge than it will give out on discharge, at its working voltage, it is usually given a certain amount of overcharge. This makes desirable a meter that automatically allows for the proper amount of overcharge. Such a meter indicates at all times the amount of electricity available for useful work without resetting the pointer every time the battery is charged. In other words, the battery and the meter would keep in step for considerable periods of time without readjustment. The Sangamo differential shunt meter is designed to meet these requirements, and it consists essentially of a Sangamo meter with two shunts connected as shown. The relative value of shunt resistance is adjustable by means of slider G, so that the meter can be made to run slow on charge or fast on discharge, whichever may be desired. The usual method is to allow the meter to register less than the true amount on charge and the exact amount on discharge, the difference representing the loss in the battery, or the overcharge. If the meter be provided with a charge stopping device, the battery can be given an amount of overcharge predetermined by the setting of the slider G. Therefore the amount of overcharge can be fixed in advance by a skilled man and the actual charging done by any unskilled person, since all there is to do is to make the connection.

Ques. How is a synchronous motor employed as a frequency indicator?

Ans. A small synchronous motor is connected in the circuit of the current whose frequency is to be measured. After determining the revolutions per minute by using a revolution counter, the frequency is easily calculated as follows:

frequency = (revolutions per second × number of poles) ÷ 2.