Page:Elementary Text-book of Physics (Anthony, 1897).djvu/401



318. The Electric Arc.—If the terminals of an electric circuit, in which is an electromotive force of forty or more volts, be formed of carbon rods, a brilliant and permanent luminous are will appear between the ends of the rods if they be touched together and then withdrawn a short distance from each other. The temperature of the arc is so high that the most refractory substances melt or are dissipated when placed in it. The carbon forming the positive terminal is hotter than the other. Both the carbons are gradually oxidized, the loss of the positive terminal being about twice as great as that of the negative. The arc is, however, not due to combustion, since it can be formed in a vacuum.

The current passing in the arc is, in ordinary cases, not greater than ten amperes, while the measurements of the resistance of the arc show that it is altogether too small to account for this current when the original electromotive force is taken into account. This fact has been explained by Edlund and others on the hypothesis that there is a counter electromotive force set up in the arc, which diminishes the effective electromotive force of the circuit. The measurements of Lang show that this counter electromotive force in an arc formed between carbon points is about thirty-six volts, and in one formed between metal points about twenty-three volts.

319. The Spark, Brush, and Glow Discharges.—When a conductor is charged to a high potential and brought near another conductor which is joined to ground, a spark or a series of sparks