of what is called a
choking coil. It is merely a coil of wire, wound upon an iron core, and
the current to be choked passes through the coil. To illustrate this,
let us take an arc lamp designed to use a 50-volt current. If a current
is supplied to it carrying 100 volts, it is obvious that there are 50
volts more than are needed. We must take care of this excess of 50 volts
without losing it, as would happen were we to locate a resistance of
some kind in the circuit. This result we accomplish by the introduction
of the choking coil, which has the effect of absorbing the excessive 50
volts, the action being due to its quality of self-induction, referred
to in the foregoing.

[Illustration: _Fig. 114._ CHOKING COIL]

In Fig. 114, A is the choking coil and B an arc lamp, connected up, in
series, with the choking coil.

THE TRANSFORMER.--It is more economical to transmit 10,000 volts a long
distance than 1,000 volts, because the lower the pressure, or the
voltage, the larger must be the conductor to avoid loss. It is for this
reason that 500 volts, or more, are used on electric railways. For
electric light purposes, where the current goes into dwellings, even
this is too high, so a transformer is used to take a high-voltage
current from the main line and transform it into a low voltage. This is
done by means of two distinct coils of wire, wound upon an iron core.

[Illustration: _Fig. 115._ A TRANSFORMER]

In Fig. 115 the core is O-shaped, so that a primary winding (A), from
the electrical source, can be wound upon one limb, and the secondary
winding (B) wound around the other limb. The wires, to supply the
lamps, run from the secondary coil. There is no electrical connection
between the two coils, but the action from the primary to the secondary
coil is solely by induction. When a current passes through the primary
coil, the surging movement, heretofore explained, is transmitted to the
iron core, and the iron core, in turn, transmits this electrical energy
to the secondary coil.

HOW THE VOLTAGE IS DETERMINED.--The voltage produced by the secondary
coil will depend upon several things, namely, the strength of the
magnetism transmitted to it; the rapidity, or periodicity of the
current, and the number of turns of wire around the coil. The voltage is
dependent upon the length of the winding. But the voltage may also be
increased, as well as decreased. If the primary has, we will say, 100
turns of wire, and has 200 vo