The most important step in the electric field, after the dynamo had been brought to a fairly workable condition, was its utilization to make light. It was long known prior to the discovery of practical electric dynamos, that the electric current would produce an intense heat.

Ordinary fuels under certain favorable conditions will produce a temperature of 4,500 degrees of heat; but by means of the electric arc, as high as six, eight and ten thousand degrees are available.

The fact that when a conductor, in an electric current, is severed, a spark will follow the drawing part of the broken ends, led many scientists to believe, even before the dynamo was in a practical shape, that electricity, sooner or later, would be employed as the great lighting agent.

When the dynamo finally reached a stage in development where its operation could be depended on, and was made reversible, the first active steps were taken to not only produce, but to maintain an arc between two electrodes.

It would be difficult and tedious to follow out the first experiments in detail, and it might, also, be useless, as information, in view of the present knowledge of the science. A few steps in the course of the development are, however, necessary to a complete understanding of the subject.

Reference has been made in a previous chapter to what is called the Electric Arc, produced by slightly separated conductors, across which the electric current jumps, producing the brilliantly lighted area.

This light is produced by the combustion of the carbon of which the electrodes are composed. Thus, the illumination is the result of directly burning a fuel. The current, in passing from one electrode to the other, through the gap, produces such an intense heat that the fuel through which the current passes is consumed.

Carbon in a comparatively pure state is difficult to ignite, owing to its great resistance to heat. At about 7,000 degrees it will fuse, and pass into a vapor which causes the intense illumination.

The earliest form of electric lighting was by means of the arc, in which the light is maintained so long as the electrodes were kept a certain distance apart.

To do this requires delicate mechanism, for the reason that when contact is made, and the current flows through the two electrodes, which are connected up directly with the coils of a magnet, the cores, or armatures, will be magnetized. The result is that the electrode, connected with the armature of the magnet, is drawn away from the other electrode, and the arc is formed, between the separated ends.

As the current also passes through a resistance coil, the moment the ends of the electrodes are separated too great a distance, the resistance prevents a flow of the normal amount of current, and the armature is compelled to reduce its pull. The effect is to cause the two electrodes to again approach each other, and in doing so the arc becomes brighter.

It will be seen, therefore, that there is a constant fight between the resistance coil and the magnet, the combined action of the two being such, that, if properly arranged, and with powers in correct relation to each other, the light may be maintained without undue flickering. Such devices are now universally used, and they afford a steady and reliable means of illumination.

Many improvements are made in this direction, as well as in the ingredients of the electrodes. A very novel device for assuring a perfect separation at all times between the electrodes, is by means of a pair of parallel carbons, held apart by a non-conductor such as clay, or some mixture of earth, a form of which is shown in Fig. 116.

The drawing shows two electrodes, separated by a non-conducting material, which is of such a character that it will break down and crumble away, as the ends of the electrodes burn away.

Fig. 116. Parallel Carbons. Fig. 116. Parallel Carbons.

This device is admirable where the alternating current is used, because the current moves back and forth, and the two electrodes are thus burned away at the same rate of speed.

In the direct or continuous current the movement is in one direction only, and as a result the positive electrode is eaten away twice as fast as the negative.

This is the arc form of lamp universally used for lighting large spaces or areas, such as streets, railway stations, and the like. It is important also as the means for utilizing searchlight illumination, and frequently for locomotive headlights.

Arc lights are produced by what is called the series current. This means that the lamps are all connected in a single line. This is illustrated by reference to Fig. 117, in which A represents the wire from the dynamo, and B, C the two electrodes, showing the current passing through from one lamp to the next.

Fig. 117. Arc Lighting Circuit. Fig. 117. Arc-Lighting Circuit.

A high voltage is necessary in order to cause the current to leap across the gap made by the separation of the electrodes