[Illustration: Fig. 660.--Interior of Thompson prepayment watt hour meter. The actuating force is a large flat coil spring enclosed in a barrel or drum to which its outside end is attached. The operating knob winds this main spring by turning the drum. The spring has many turns and as the operation of the device never equals one whole turn, the spring always exerts a practically constant force. The rate device consists of a small train of gears secured to the front of the frame directly back of the register. Each device is marked with the price per kw-hr. for which it should be used. The switch is of the double pole double break type with leaf contacts. The coin receptacles are placed at the back of the meter. To make an advance payment, the winding knob is turned so that the arrow points upward. A quarter dollar is then inserted in the slot and the knob turned to the right, the coin serving as a key which operates the mechanism within the device, turning the registering wheel and placing the coin to the credit of the customer. If the circuit be open when the coin is deposited the same motion of the knob which moves the registering mechanism closes the circuit switch contained within the case. The dial contains a scale marked in plain figures over which a pointer passes indicating the number of coins remaining to the credit of the depositor. When the first coin is deposited and the knob turned closing the main switch, the pointer rests opposite the first division on the scale. If a second coin be deposited before the current purchased with the first coin has been consumed, a second motion of the knob will bring the pointer opposite the second division on the scale. Twelve coins can thus be deposited consecutively, after which the slot is automatically closed and further prepayment cannot be made until the value of one or more coins has been consumed. Whenever energy to the value of one coin has been delivered through the meter, the escapement is mechanically released turning the pointer back one division. This process continues until all the energy has been delivered for which payment has been made. Thus the depositor can ascertain at any time how much energy can be obtained without further payment. When all energy has been delivered, the circuit switch is opened so that no more current can be obtained until one or more coins have been deposited. The indicating mechanism shows only the number of coins which stand to the credit of the customer, but, by consulting the meter dial, one can determine what fractional part of the prepayment next to be cancelled remains to the credit of the customer. A coin or washer larger than the coin for which the device is designed cannot be introduced into the receiving slot and a smaller one will not operate the device.] How to test a meter.--A simple test for ascertaining whether a customer's meter is fast or slow[C], may be applied as follows: