Harper's Electricity Book for Boys

by Joseph H. Adams

The Electric Light

In the [chapter] on Electrical Resistance we learned that an electric current always encounters a resistance in passing through a conductor, and that when the current is strong enough the conductor is heated up. The electric light is produced by the heating of a conductor of one kind or another to incandescence by the electrical friction of the current passing through it.

The first electric light was made by Sir Humphry Davy over a hundred years ago. He discovered that when a current from a great many cells of battery was interrupted the spark did not simply appear for an instant and then go out, as it does when only a few cells are used, but remained playing between the terminals of the circuit. He found by experiment that if pieces of carbon are used as the terminals—or “electrodes,” as they are called—the electricity passes between them in an intensely hot flame, or “arc.” The latter, which is due to the electrical resistance of the vapor of carbon, heats up the carbon-points so that they give a brilliant white light.

Fig. 1

Fig. 2

Before the dynamo came into use, the electric light was rarely seen, except as a philosophical experiment; but as soon as cheap electricity became available, commercial electric arc-lamps were made by many inventors and have been continually improved. [Fig. 1] shows one form of modern arc-lamp, with its case removed to show the interior mechanism. In most arc-lamps the lamp itself consists of a pair of carbon or other electrodes in the form of long rods arranged vertically, with their tips normally in contact. When the current is turned on, the mechanism lifts the upper electrode away from the lower one. The interruption of the circuit thus caused “strikes the arc” between the tips, and the mechanism keeps the arc-distance unchanged as the carbons burn away. Some arc-lamps are made to burn on continuous-current, and others on alternating-current circuits. When continuous current is used, the upper (or positive) carbon burns away about twice as fast as the lower one, forming a cup, or “crater,” from which most of the light comes.

Uses of the Arc-Light

The first commercial use of the arc-light on a large scale was for street-lighting, to replace the old-fashioned gas-lamps. But another important use is in search-lights, in which the arc-lamp is fitted with a powerful reflector for throwing a very bright light to a distance. [Fig. 2] is a view of a search-light arranged to go on top of a ship’s pilot-house. In war-time the ships carry search-lights to help them find the enemy’s ships and repel attack; and they are used in the army also, by having a portable dynamo and engine drawn by horses. The arc is also employed in projectors for lecture-rooms, and sometimes for the headlights of steam and electric locomotives and interurban electric cars.