Chapter XXI

HOW LIGHT IS PRODUCED BY THE INCANDESCENT LAMP.

=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.

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.

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.

=150. Candle-Power.= A lamp is said to have 4, 8, 16 or more candle-power. A 16-candle-power lamp, for example, means one that will give as much light as sixteen standard candles. A standard sperm candle burns two grains a minute. The candle-power of a lamp can be increased by forcing a strong current through it, but this shortens its life.

_The Current_ used for incandescent lamps has to be strong enough to force its way through the filament and produce a heat sufficient to give a good light. The usual current has 50 or 110 volts, although small lamps are made that can be run by two or three cells. If the voltage of the current is less than that for which the lamp was made, the light will be dim. The filament can be instantly burned out by passing a current of too high pressure through it.

Even with the proper current, lamps soon begin to deteriorate, as small particles of carbon leave the filament and cling to the glass. This is due to the evaporation, and it makes the filament smaller, and a higher pressure is then needed to force the current through the increased resistance; besides this, the darkened bulb does not properly let the light out. The current may be direct or alternating.

=151. The Uses= to which incandescent lamps are put are almost numberless. Fig. 193 shows a decorative lamp. Fancy lamps are made in all colors. Fig. 194 shows a conic candle lamp, to imitate a candle. What corresponds to the body of the candle (see figure B to C) is a delicately tinted opal glass tube surmounted (see figure A to B) by a finely proportioned conic lamp with frosted globe. C to D in the figure represents the regular base, and thus the relative proportions of the parts are shown. Fig. 195 shows another form of candelabra lamp. Fig. 196 shows small dental lamps. Fig. 197 shows a small lamp with mirror for use in the throat. Fig. 198 shows lamp with half shade attached, used for library tables. Fig. 199 shows an electric pendant for several lamps, with shade. Fig. 200 shows a lamp guard. Fig. 201 shows a lamp socket, into which the lamp is screwed. Fig. 202 shows incandescent bulbs joined in parallel to the + and - mains. Fig. 203 shows how the lamp cord can be adjusted to desired length. Fig. 204 shows a lamp with reflector placed on a desk. Fig. 205 shows a form of shade and reflector.