Fig. 209.
153. Arc Lamps. As the carbons gradually wear away, some device is necessary to keep their ends the right distance apart. If they are too near, the arc is very small; and if too far apart, the current can not pass and the light goes out. The positive carbon gives the more intense light and wears away about twice as fast as the - carbon, so it is placed above the - carbon, to throw the light downwards.
| Fig. 210. | Fig. 211. |
Arc lamps contain some device by which the proper distance between the carbons can be kept. Most of them grip the upper carbon and pull it far enough above the lower one to establish the arc. As soon as the distance between them gets too great again, the grip on the upper carbon is loosened, allowing the carbon to drop until it comes in contact with the lower one, thus starting the current again. These motions are accomplished by electromagnets. Fig. 207 shows a form of arc lamp with single carbons that will burn from 7 to 9 hours.
Fig. 212.
| Fig. 213. | Fig. 214. |
Fig. 208 shows the mechanism by which the carbons are regulated. Fig. 209 shows a form of double carbon, or all-night lamp, one set of carbons being first used, the other set being automatically switched in at the proper time.