THE INDUCTION COIL
The voltaic cell and the accumulator provide us with currents of
electricity of considerable volume, but at low pressure or voltage.
For many purposes, however, we require a comparatively small amount of
current at very high pressure, and in such cases we use an apparatus
called the _induction coil_. Just as an electrified body and a magnet
will induce electrification and magnetism respectively, so a current
of electricity will induce another current; and an induction coil is
simply an arrangement by which a current in one coil of wire is made to
induce a current in another coil.
Suppose we have two coils of wire placed close together, one connected
to a battery of voltaic cells, with some arrangement for starting and
stopping the current suddenly, and the other to a galvanometer. As
soon as we send the current through the first coil, the needle of the
galvanometer moves, showing that there is a current flowing through
the second coil; but the needle quickly comes back to its original
position, showing that this current was only momentary. So long as we
keep the current flowing through the first coil the galvanometer shows
no further movement, but as soon as we stop the current the needle
again shows by its movements that another momentary current has been
produced in the second coil. This experiment shows us that a current
induces another current only at the instant it is started or stopped,
or, as we say, at the instant of making or breaking the circuit.
The coil through which we send the battery current is called the
“primary coil,” and the one in which a current is induced is called the
“secondary coil.” The two momentary currents in the secondary coil do
not both flow in the same direction. The current induced on making the
circuit flows in a direction opposite to that of the current in the
primary coil; and the current induced on breaking the circuit flows
in the same direction as that in the primary coil. If the two coils
are exactly alike, the induced current will have the same voltage as
the primary current; but if the secondary coil has twice as many turns
of wire as the primary coil, the induced current will have twice the
voltage of the primary current. In this way, by multiplying the turns
of wire in the secondary coil, we can go on increasing the voltage of
the induced current, and this is the principle upon which the induction
coil works.
We may now describe the construction of such a coil. The primary coil
is made of a few turns of thick copper wire carefully insulated, and
inside it is placed a core consisting of a bundle of separate wires of
soft iron. Upon this coil, but carefully insulated from it, is wound
the secondary coil, consisting of a great number of turns of very
fine wire. In large induction coils the secondary coil has thousands
of times as many turns as the primary, and the wire forming it may be
more than a hundred miles in length. The ends of the secondary coil are
brought to terminals so that they can be connected up to any apparatus
as desired.
[Illustration: FIG. 17.--Diagram showing working of Contact-Breaker for
Induction Coil.]
In order that the induced currents shall follow each other in quick
succession, some means of rapidly making and breaking the circuit is
required, and this is provided by an automatic contact breaker. It
consists of a small piece of soft iron, A, Fig. 17, fixed to a spring,
B, having a platinum tip at C. The adjustable screw, D, also has a
platinum tip, E. Normally the two platinum tips are just touching one
another, and matters are arranged so that their contact completes
the circuit. When the apparatus is connected to a suitable battery a
current flows through the primary coil, and the iron core, F, becomes
an electro-magnet, which draws A towards it. The platinum tips are thus
no longer in contact and the circuit is broken. Immediately this occurs
the iron core loses its magnetism and ceases to attract A, which is
then moved back again by the spring B, so that the platinum tips touch,
the circuit is once more completed, and the process begins over again.
All this takes place with the utmost rapidity, and the speed at which
the contact-breaker works is so great as to produce a musical note.
There are many other types of contact-breakers, but in every case the
purpose is the same, namely, to make and break the primary circuit as
rapidly as possible.
The efficiency of the coil is greatly increased by a condenser which
is inserted in the primary circuit. It consists of alternate layers of
tinfoil and paraffined paper, and its action is like that of a Leyden
jar. A switch is provided to turn the battery current on or off, and
there is also a reversing switch or commutator, by means of which the
direction of the current may be reversed. The whole arrangement is
mounted on a suitable wooden base, and its general appearance is shown
in Fig. 18.
[Illustration:
_By permission of_] [_Harry W. Cox, Ltd._
FIG. 18.--Typical Induction Coil.]
By means of a large induction coil we can obtain a voltage hundreds
or even thousands of times greater than that of the original battery
current, but on account of the great resistance of a very long, thin
wire, the amperage is much smaller. The induction coil produces a
rapid succession of sparks, similar to those obtained from a Wimshurst
machine. A coil has been constructed capable of giving sparks 42½
inches in length, and having a secondary coil with 340,000 turns of
wire, the total length of the wire being 280 miles. Induction coils are
largely employed for scientific purposes, and they are used in wireless
telegraphy and in the production of X-rays.
The principle of the induction coil can be applied also to the lowering
of the voltage of a current. If we make the secondary coil with less,
instead of more turns of wire than the primary coil, the induced
current will be of lower voltage than the primary current, but its
amperage will be correspondingly higher. This fact is taken advantage
of in cases where it is desirable to transform a high voltage current
from the public mains down to a lower voltage current of greater
amperage.