In the purchase of electricity, the consumer pays a definite amount, say 10 cents per kilowatt. This represents an exact quantity of energy, that may be expended in light, in heat, or in the generation of power, all of which may be expressed as definite quantities. As light, it indicates in the electric lamp the number of candle-power-hours that may be obtained for 10 cents. At this rate a single watt costs 0.01 cent an hour. A 25-watt electric lamp will therefore cost 0.25 (1/4) cent for each hour of use; a 60-watt lamp costs 0.6 cent per hour; the ten 25-watt lamp mentioned above using 250 watts costs 2.5 cents per hour. As heat, it is expressed in English-speaking countries as British thermal units, 1 kilowatt-hour representing 3412 B.t.u. per hour. One cent’s worth of electricity at the rate given yields 341.2 B.t.u. of heat. As power, it represents an exact amount of work. So expressed, a watt represents 1/746 horsepower; therefore a kilowatt is represented in power as 1000/746 = 1.3 horsepower. Since the kilowatt purchased for 10 cents is a kilowatt-hour, the equivalent horsepower is for the same length of time. At the assumed rate, 10 cents buys 1.3 horsepower for one hour. When used as work it represents 2,544,000 foot-pounds or 255,400 foot-pounds of work for 1 cent. This work when expended in a motor, to do the family washing or perform any other household drudgery, represents the greatest value to be derived from its use. A 1/2-horsepower motor is amply large to operate a family washing machine. Even though the motor is only 50 per cent. efficient its cost of operation is less than 7 cents per hour. =Miniature Lamps.=--Miniature electric lamps include all that are not used for general illuminating purposes. The term applies more particularly to the form of the base than to the voltage or candlepower of the filament. There are three general classes of these lamps: candelabra and decorative, that operate on lighting circuits of 100 to 130 volts and are usually intended for decorative purposes; general battery lamps used for flash lights; and lamps for automobiles and electric-vehicle service. [Illustration: Candelabra screw base Miniature screw base Double-contact bayonet candelabra base Single-contact bayonet candelabra base FIG. 221.--Miniature lamp bases.] The term miniature lamp applies more particularly to the base than to the voltage or candlepower. The style of base is characteristic of the service for which the lamp is designed rather than the size or number of watts consumed. There are two general styles of bases: the screw type of the Edison construction of which there are two sizes; and the bayonet type of which there are two styles of construction.