Fig. 99
23. Arc Lamp.—We fastened two electric light carbons to the ends of copper wires connected for the 110-volt current. A rheostat, R ([Fig. 99]), in circuit, was set at 6.5 ohms. One lower carbon was fastened into a clamp, and the other was touched to it, and then drawn away about three-eighths of an inch. A very brilliant light was produced. Probably about 1800 candle-power. The ammeter A showed 10 amperes, and the volt meter V showed 45 volts. 45 volts × 10 amperes = 450 watts, 1800 candle-power, 25 watts per candle-power.
The arc light is the cheapest of all lights but is too dazzlingly bright for household purposes. It is used for outdoor lighting chiefly, and particularly for large search-lights. The temperature is over 6000 degrees, which boils the carbon and fills the gap between the two pencils with a stream of carbon vapour. This conducts the current like the filament in an incandescent lamp. The air gap between the carbon pencils would have a resistance of many thousand ohms if it were not for the presence of the carbon vapour. The hot carbon vapour reduces the resistance of this space to 4.5 ohms.
(45 volts)/(4.5 ohms) = 10 amperes.
or
(110 volts)/(6.5 + 4.5 ohms) = 10 amperes.
The carbon pencils account for part of this resistance—not more than a third of an ohm however.
It is evident that arc lamps in use must have an automatic mechanism which shall permit the carbons to touch whenever the current is not passing, but which shall draw them apart to the proper distance after the carbon vapour has been formed, or, as we say, after the arc has been established. This mechanism is nothing else than electro-magnets which are operated by the lighting circuit itself. It may require thoughtful examination to recognize these as electro-magnets, in every case, but that is what they are. Sometimes they are coils of wire, which do not have iron cores and armatures separate to be sure—but nevertheless they have both of these united in one movable rod, and they produce magnetic fields.
Suppose I pass an electric current around this coil A ([Fig. 100]). The region about the coil becomes a magnetic field with its north pole situated at a point in space, say N. The influence of this field causes the iron rod to become a magnet with its south pole uppermost, and if the current is strong enough, and the field which it produces is strong enough, it will lift the iron rod up into the coil. By varying the strength of the current you see I may make this rod dance up and down in space touching nothing—a veritable ghost dance.