Arc lights. The current may be used to produce an arc light in the following manner:—Two carbon rods, A and B, are held by suitable means in the position shown in [Fig. 15], and the two wires from a dynamo are joined respectively to A and B, the upper one always being the positive lead when a continuous current is used. When the current is sent through the circuit, it passes through the carbons A and B, which are conductors. Immediately this occurs, suitable mechanism in the lamp, being acted on by the current, or by hand in the case of search-lights, or by clock-work, moves the two carbons a small distance apart, with the consequence that a dazzling arc of light is formed between them. Mechanism to regulate carbons. If the carbons get too far apart, the mechanism brings them nearer together again, and on the delicacy with which it acts, depends the steadiness of the light. It would be useless to explain how this mechanism acts, as it is in a different form in each maker’s lamp. Some lamps suitable for alternating current. Some lamps have been constructed for use with an alternating current, but with the majority a continuous current is used. While an arc light is burning the carbons waste away, the upper one more rapidly than the lower, and the mechanism has to approach them constantly to make up for this waste.
When carbons are consumed light goes out. When the carbons are consumed as far as convenient, an automatic arrangement cuts off the current, and the light goes out; or it diverts the current to another set of carbons, which at once light up. The carbons are made in suitable lengths to last a certain number of hours, four, six, eight, &c. In [Fig. 16] is shown an arc lamp complete.
Fig. 16.
Arc Lamp Complete.
Arc lamp very complicated. An arc lamp is of necessity a complicated affair, which it is not advisable to have on board ship, except where an electrician is engaged permanently.
Jablochkoff candle. Another way of producing light is to use the current in what is called an electric candle, of which a familiar type is the Jablochkoff candle.
Fig. 17.