use
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.

[Illustration: _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.

THE INCANDESCENT SYSTEM.--This method is entirely different from the arc
system. It has been stated that certain metals conduct electricity with
greater facility than others, and some have higher resistance than
others. If a certain amount of electricity is forced through some
metals, they will become heated. This is true, also, if metals, which,
ordinarily, will conduct a current freely, are made up into such small
conductors that it is difficult for the current to pass.

[Illustration: _Fig 118. Interrupted Conductor._]

In the arc method high voltage is essential; in the incandescent plan,
current is the important consideration. In the arc, the light is
produced by virtue of the break in the line of the conductor; in the
incandescent, the system is closed at all times.

Supposing we have a wire A, a quarter of an inch in diameter, carrying a
current of, say, 500 amperes, and at any point in the circuit the wire
is made very small, as shown at B, in Fig. 118, it is obvious that the
small wire would not be large enough to carry the current.

The result would be that the small connection B would heat up, and,
finally, be fused. While the large part of the wire would carry 500
amperes, the small wire could not possibly carry more than, say, 10
amperes. Now these little wires are the filaments in an electric bulb,
and originally the attempt was made to have them so connected up that
they could be illuminated by a single wir