CHAPTER XXII.
HOW LIGHT IS PRODUCED BY THE ARC LAMP.

Fig. 206.

152. The Electric Arc. When a strong current passes from one carbon rod to another across an air-space, an electric arc is produced. When the ends of two carbon rods touch, a current can pass from one to the other, but the imperfect contact causes resistance enough to heat the ends red-hot. If the rods be separated slightly, the current will continue to flow, as the intensely heated air and flying particles of carbon reduce the resistance of the air-space.

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.

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.

"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."

Fig. 207. Fig. 208.