We introduced into the socket a "Hylo" lamp ([Fig. 96]). The filament, A, took half an ampere of electricity, gave 16-candle-power of light, and cost half a cent an hour. When the lamp was turned in its socket the current was switched off of the filament A, and on to the filament a. This took .03 of an ampere, gave one candle-power of light, and cost .03 of a cent an hour, or at the rate of about $3.00 a year, burning continuously day and night.
Fig. 96
The uses of such a lamp are apparent in rooms which have no daylight. However, a wall switch at the entrance of such a room, making it easy to throw on and off the light entirely, seems to be a more satisfactory arrangement. One of the boys connected a wattmeter in the circuit with a hylo lamp and found that the small filament did not pass current enough to move the armature of the wattmeter. Hence that may be burned alone without affecting the consumer's bills.
We took a 16-candle-power 220-volt lamp, and lighted it by a 220-volt current. The meter showed that it allowed only one quarter of an ampere to pass. The filament was very much smaller than that in the 110-volt, 16-candle-power lamp. The pressure was twice as great as before, but the resistance was four times as great, and hence only half as much current passed. We find that it costs just as much to generate one quarter of an ampere at 220-volt pressure as it does to generate half an ampere at 110-volt pressure.
We must, of course, pay for electricity according to the cost of producing it. To produce .5 ampere at 110-volt pressure costs the same as one ampere at 55-volt pressure, or .25 amperes at 220 volts. It will be noticed that the products of the two factors in each case are the same. The product of an ampere multiplied by a volt is a watt. In each of the above three cases the amount of electrical energy is 55 watts. This will produce a definite quantity of light—about 16 candle-power when the carbon filament is used, and this quantity does not vary as either volts or amperes, but as the product of these, namely, watts.
Each of these lamps is called a 55-watt lamp, and, since they each give 16 candle-power of light, a carbon filament lamp gives one candle-power of light for three and a half watts of electricity. Electricity for lighting purposes usually costs 10 cents per kilowatt hour, that is, 10 cents for 1000 watts for one hour, or one cent for 100 watts for one hour. Hence a 55-watt lamp costs a trifle more than half a cent for one hour, or exactly .55 cents, and a 32-candle-power lamp costs 1.1 cents per hour.
We introduced into the socket a 48-candle-power 110-volt tungsten lamp ([Fig. 97]), and turned on the 110-volt current. The ammeter showed 55 ampere. Hence the lamp is a 60-watt lamp, and requires one and a quarter watts per candle-power. That is, the metal tungsten is nearly three times as efficient as carbon for producing light from electricity.
