Fig. 206 shows two carbon rods which are joined to the two terminals of a dynamo. The upper, or positive, carbon gradually wears away and becomes slightly hollow. The heated _crater_, as it is called, is the hottest part. The negative carbon becomes pointed. The arc will pass in a vacuum, and even under water. [Illustration: Fig. 206.] As the electric arc is extremely hot, metals are easily vaporized in it; in fact, even the carbon rods themselves slowly melt and vaporize. This extreme heat is used for many industrial purposes. [Illustration: Fig. 207.] [Illustration: Fig. 208.] "The phenomenon of the electric arc was first noticed by Humphrey Davy in 1800, and its explanation appears to be the following: Before contact the difference of potential between the points is insufficient to permit a spark to leap across even 1/10000 of an inch of air-space, but when the carbons are made to touch, a current is established. On separating the carbons, the momentary extra current due to self-induction of the circuit, which possesses a high electromotive force, can leap the short distance, and in doing so volatilizes a small quantity of carbon between the points. Carbon vapor, being a partial conductor, allows the current to continue to flow across the gap, provided it be not too wide; but as the carbon vapor has a very high resistance it becomes intensely heated by the passage of the current, and the carbon points also grow hot. Since, however, solid matter is a better radiator than gaseous matter, the carbon points emit far more light than the arc itself, though they are not so hot. It is observed, also, that particles of carbon are torn away from the + electrode, which becomes hollowed out to a cup-shape, and some of these are deposited on the - electrode." [Illustration: 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. [Illustration: Fig. 210.] [Illustration: 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. [Illustration: Fig. 212.] [Illustration: Fig. 213.] [Illustration: Fig. 214.]