s 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.

[Illustration: _Fig. 116. Parallel Carbons._]

This device is admirable where the alternating current is used, beca