It is to be noticed that this flasher is an automatic switch which is opened or closed according to temperature. Remove the fine wire from a and we have precisely the device which regulated the temperature in our electric incubator. Suppose the "thermostat" (as it is called in that case) is placed within the egg chamber which is to be kept at 103 degrees. A screw in the metal strip c underneath the end of a may be set so that it will normally touch a. Suppose now the brass strip is underneath the strip of iron in a. As the hot plate warms up the egg chamber, the brass will expand more than the iron, and the bar will curve upward and break the connection with c. As soon as the current stops the temperature of the chamber begins to fall, and the bar curves downward again until connection is made. This device is capable of adjustment so as to keep the temperature constantly at 103 degrees or any other desired degree. The device is in use for scores of different purposes, including the regulation of temperature in school rooms.
17. Electric Car Heaters.—Ten or fifteen years ago there were no heated street cars in New York City. Now they are all heated by electricity and their maximum and minimum temperatures are regulated by law. The resistance wire may be seen in coils underneath the car seats. Electric street cars usually operate on a 500 or 600-volt current. The amount of current used for heating varies from 2 to 12 amperes. Perhaps 3 amperes may be taken as an average.
500 V × 3 a = 1500 w = 1½ kilowatts.
It costs the large electric railway companies about 1.5 cents per kilowatt hour to generate their supply of current. Eighteen hours is considered a car day.
1½ kilowatts × 18 hours = 27 kilowatt hours.
27 kilowatt hours at 1.5 cents = 40 cents per car day.
18. Heating Apartments by Electricity.—For heating apartments by electricity the same sort of apparatus is used as that already described for heating cars. A family of four adults, living in an eight-room apartment with at least 120 cubic feet of fresh air admitted per minute, will use on an average ten amperes of the 110-volt current. The cost will be about two dollars and fifty cents per day or seventy-five dollars per month. Although this is as much as the entire rental of a perfectly comfortable apartment, the novelty and the convenience attract tenants and the extra cost of rent does not deter them.
Fig. 88