Suppose your index stood like this:
Fig. 177A.—Gas-meter dial. It reads 38600 cubic feet.
Take the figure 3 on the 100 thousand circle, the figure 8 on the 10 thousand, and the figure 6 on the 1 thousand, and you have 30,000, 8000, and 600, or 38,600 feet. To ascertain the quantity of gas used in the time elapsing between the readings of the meter, subtract the quantity registered at the previous reading. Thus, if the previous reading was 38,600 feet, and the next reading 40,100 feet, the pointers standing thus:
Fig. 177B.—Gas-meter dial. It reads 40100 cubic feet.
| You have | 40,100 |
| Subtract your last reading | 38,600 and you find |
| ——— | |
| that your bill should be for | 1,500 feet |
When 100,000 feet have been passed, the index is at zero; that is, all the pointers stand at 0, and the registration begins all over again.
Prepayment Meters.
—In many places it is desirable to sell gas in small quantities and to prepay the amount for a given supply of gas. This is accomplished by a meter such as that of Fig. 179. The meter is constructed much the same as the former but provided with a mechanism such that when a coin—usually 25 cents—is deposited, according to the printed directions in the instrument, an amount of gas representing the proportional current rate is allowed to pass the meter. The supply is cut off as soon as the amount paid for is used; when in order to receive more gas, another coin must be deposited as before.