=148. Incandescence.= We have just seen that the electric current produces heat when it flows through a conductor that offers considerable resistance to it. As soon as this was discovered men began to experiment to find whether a practical light could also be produced. It was found that a wire could be kept hot by constantly passing a current through it, and that the light given out from it became whiter and whiter as the wire became hotter. The wire was said to be _incandescent_, or glowing with heat. As metal wires are good conductors of electricity, they had to be made extremely fine to offer enough resistance; too fine, in fact, to be properly handled. =149. The Incandescent Lamp.= Many substances were experimented upon to find a proper material out of which could be made a _filament_ that would give the proper resistance and at the same time be strong and lasting. It was found that hair-like pieces of carbon offered the proper resistance to the current. When heated in the air, however, carbon burns; so it became necessary to place the carbon filaments in a globe from which all the air had been pumped before passing the current through them. This proved to be a success. [Illustration: Fig. 194.] [Illustration: Fig. 195.] [Illustration: Fig. 196.] Fig. 192 shows the ordinary form of lamp. The _carbon filament_ is attached, by carbon paste, to short platinum wires that are sealed in the glass, their lower ends being connected to short copper wires that are joined to the terminals of the lamp. When the lamp is screwed into its socket, the current can pass up one side of the filament and down the other. The filaments used have been made of every form of carbonized vegetable matter. Bamboo has been largely used, fine strips being cut by dies and then heated in air-tight boxes containing fine carbon until they were thoroughly carbonized. This baking of the bamboo produces a tough fiber of carbon. Various forms of thread have been carbonized and used. Filaments are now made by pressing finely pulverized carbon, with a binding material, through small dies. The filaments are made of such sizes and lengths that will adapt them to the particular current with which they are to be used. The longer the filament, the greater its resistance, and the greater the voltage necessary to push the current through it. [Illustration: Fig. 197.] [Illustration: Fig. 198.] After the filaments are properly attached, the air is pumped from the bulb or globe. This is done with some form of mercury pump, and the air is so thoroughly removed from the bulb that about one-millionth only of the original air remains. Before sealing off the lamp, a current is passed through the filament to drive out absorbed air and gases, and these are carried away by the pump. By proper treatment the filaments have a uniform resistance throughout, and glow uniformly when the current passes. [Illustration: Fig. 199.] [Illustration: Fig. 200.]