The carbon filament lamps were, as has been said, the invention of Mr. Thomas A. Edison in 1879. Such a statement must, however, be qualified by the assertion that this, like nearly all invention, was but the consummation of a long line of researches made by many men for many years. The early filaments were made of bamboo thread, charred, but now they are drawn like spider's web out of a sticky liquid and carbonized at a high temperature. They are attached in the lamp to short pieces of platinum wire which are sealed through the glass walls of the bulb. One wire connects with the brass collar of the bulb, and the other with the central piece of brass at the base of the bulb. We dissected a socket and found that when the lamp is placed in the socket, the collar of the lamp is screwed into the collar of the socket, and the base of the lamp comes in contact with a brass spring in the bottom of the socket (Fig. 98). The spring is connected with one copper wire bringing electricity from the dynamo. The collar is connected with the other wire from the dynamo. This connection is made and broken by turning the key of the socket. The wires are made of copper since copper is a particularly good conductor of electricity. No electricity can flow unless this circuit is complete. Socket keys and wall switches make or close gaps in this circuit. No copper wires for carrying electric-lighting current are smaller than No. 12, which has a diameter of .08 or about one twelfth of an inch. The intention is to have as little resistance to the current as possible, except in the filament of the lamp itself. There resistance is purposely introduced in order to convert electricity into light, light without heat if that were possible, but since that has not yet been found possible, heat for the sake of the accompanying light. Unhappily only 4 per cent. of the electrical energy goes into light and 96 per cent. goes into useless, or even harmful, heat. The tungsten lamps, which are now coming into use, are nearly three times as efficient in the production of light as are the carbon filament lamps. The dynamo exerts its entire pressure upon the lamp and furnishes current as follows: A dynamo of 110-volt pressure gives: 1 ampere = 110 watts, through a 32-candle-power lamp, cost one cent an hour, or .5 ampere = 55 watts, through a 16-candle-power lamp, cost half a cent an hour, or .25 ampere = 27-1/2 watts, through an 8-candle-power lamp, cost a quarter of a cent an hour. A dynamo of 220-volt pressure gives: .5 ampere = 110 watts, through a 32-candle-power lamp, cost one cent an hour, or .25 ampere = 55 watts, through a 16-candle-power lamp, cost half a cent an hour, or .125 ampere = 27-1/2 watts, through an 8-candle-power lamp, cost a quarter of a cent an hour.