=296. Water Meters.= The water meter (Fig. 159) is a device for measuring the number of gallons or cubic feet of water which pass thru a pipe. The reading of the meter indicates the total amount of water which has passed thru the pipe since the meter was installed. Water is paid for, unless purchased at a flat rate, at so many cents a thousand gallons or thousand cubic feet. One cubic foot is called in commercial transactions 7-1/2 gallons. [Illustration: FIG. 160. Electric meter.] =297. Prepayment Meters.= Prepayment meters are devices which will permit a certain amount of gas or water, as the case may be, to pass thru a pipe, and after this amount is used up, the pipe is automatically closed so that no more flows until more money is put into the meter. The weight of the coin works the valve. [Illustration: FIG. 160-_a_. Electric meter showing different readings.] =298. The Electric Meter.= Electricity is usually purchased by the kilowatt hour, and measured by the watt-hour meter (Fig. 160). This measures the current passing thru it, and the number of kilowatt-hours is shown by the indicators on the little dials. Start from left and read the number on the dial, such as in the illustration, 3 hundreds 4 tens 9 units, making 349 kilowatt-hours; the total kilowatt-hours used since the meter was installed. To find the number used between two dates, take the reading of the meter on the first date and subtract it from the reading on the second date. The difference is the amount used during the period. Good business women keep records of the readings of their meters. Care must be taken to read the meter correctly. The hand next higher than the one below may read too high. The higher hand may, if out of alignment, pass the figure when the lower hand approaches the ninth point in its dial, this causing the person to read the figures one, ten, hundred or thousand units too much. (Fig. 160-_a_.) CHAPTER XLIII THERMOMETERS AND THERMOSTATS =299. Mercury Thermometers.= There are two kinds of thermometers in use--the Fahrenheit and the Centigrade. Since the thermometer is used now in cooking, the housewife often has to meet the problem of translating temperatures from one to the other. The centigrade thermometer is marked on the assumption that the temperatures of boiling water and freezing water are constantly the same. The boiling point is marked 100, and the freezing point 0. The space in between is marked into even divisions and numbered 1 to 99. The Fahrenheit thermometer was made on the assumption that a mixture of ice and salt was the coldest temperature that could be reached, so this temperature of a certain proportion of ice and salt was marked zero.