e, as with the arc system above
explained, one following the other as shown in Fig. 117.

[Illustration: _Fig. 119. Incandescent Circuit._]

It was discovered, however, that the addition of each successive lamp,
so wired, would not give light in proportion to the addition, but at
only about one-fourth the illumination, and such a course would,
therefore, make electric lighting enormously expensive.

This knowledge resulted in an entirely new system of wiring up the lamps
in a circuit. This is explained in Fig. 119. In this figure A represents
the dynamo, B, B the brushes, C, D the two line wires, E the lamps, and
F the short-circuiting wires between the two main conductors C, D.

It will be observed that the wires C, D are larger than the cross wires
F. The object is to show that the main wires might carry a very heavy
amperage, while the small cross wires F require only a few amperes.

This is called the _multiple_ circuit, and it is obvious that the entire
amperage produced by the dynamo will not be required to pass through
each lamp, but, on the other hand, each lamp takes only enough necessary
to render the filament incandescent.

This invention at once solved the problem of the incandescent system and
was called the subdivision of the electric light. By this means the cost
was materially reduced, and the wiring up and installation of lights
materially simplified.

But the divisibility of the light did not, by any means, solve the great
problem that has occupied the attention of electricians and
experimenters ever since. The great question was and is to preserve the
little filament which is heated to incandescence, and from which we get
the light.

The effort of the current to pass through the small filament meets with
such a great resistance that the substance is heated up. If it is made
of metal there is a point at which it will fuse, and thus the lamp is
destroyed.

It was found that carbon, properly treated, would heat to a brilliant
white heat without fusing, or melting, so that this material was
employed. But now followed another difficulty. As this intense heat
consumed the particles of carbon, owing to the presence of oxygen, means
were sought to exclude the air.

This was finally accomplished by making a bulb of glass, from which the
air was exhausted, and as such a globe had no air to support combustion,
the filaments were finally made so that they would last a long time
before being finally di