A convenient form of carbon arc for experimental and other work may be constructed at small cost in the following manner. Two straight arc-carbons of suitable size are mounted vertically side by side and parallel and separated by two or three millimeters. These carbons are connected to a 110-volt circuit with a resistance of 5 to 10 ohms, more or less, in series. If now the gap between the upper ends of the carbons is momentarily bridged by another carbon rod, an arc will form and remain when the short-circuiting carbon rod is removed
