dinary carbon-filament incandescent lamp. This is used largely in
the circuits of batteries, generators, and other sources of supply to
prevent overload in case of short circuits on the line. These are
cheap, durable, have large current-carrying capacities, and are not
likely to set things afire when overheated. An additional advantage
incident to their use for this purpose is that an overload on a
circuit in which they are placed is visibly indicated by the glowing
of the lamp.

[Illustration: Fig. 117. Mica Card Resistance]

[Illustration: Fig. 118. Iron-Wire Ballast]

Obviously, the carbon-filament incandescent lamp, when used as a
resistance, has, on account of the negative temperature coefficient of
carbon, the property of presenting the highest resistance to the
circuit when carrying no current, and of presenting a lower and lower
resistance as the current and consequent heating increases. For some
conditions of practice this is not to be desired, and the opposite
characteristic of presenting low resistance to small currents and
comparatively high resistance to large currents would best meet the
conditions of practice.

_Iron-Wire Ballast._ Claude D. Enochs took advantage of the very high
positive temperature coefficient of iron to produce a resistance
device having these characteristics. His arrangement possesses the
compactness of the carbon-filament lamp and is shown in Fig. 118. The
resistance element proper is an iron wire, wound on a central stem of
glass, and this is included in an exhausted bulb so as to avoid
oxidation. Such a resistance is comparatively low when cold, but when
traversed by currents sufficient to heat it considerably will offer a
very large increase of resistance to oppose the further increase of
current. In a sense, it is a self-adjusting resistance, tending
towards the equalization of the flow of current in the circuit in
which it is placed.

CHAPTER XII

CONDENSERS

Charge. A conducting body insulated from all other bodies will
receive and hold a certain amount of electricity (a charge), if
subjected to an electrical potential. Thus, referring to Fig. 119, if
a metal plate, insulated from other bodies, be connected with, say,
the positive pole of a battery, the negative pole of which is
grounded, a current will flow into the plate until the plate is raised
to the same potential as that of the battery pole to which it is
connected. The amount of electricity that will f