gong. Just before the stroke occurs, the
spring B leaves the tip of the screw, and the circuit is broken, so that
the magnet no longer attracts. H is carried by its momentum against the
gong, and is withdrawn by the spring, until B once more makes contact,
and the magnet is re-excited. The hammer vibrations recur many times a
second as long as the push is pressed in.

[Illustration: FIG. 52.--Sketch of an electric-bell circuit.]

The electric bell is used for so many purposes that they cannot all be
noted. It plays an especially important part in telephonic installations
to draw the attention of the subscribers, forms an item in automatic
fire and burglar alarms, and is a necessary adjunct of railway
signalling cabins.

THE INDUCTION OR RUHMKORFF COIL.

Reference was made in connection with the electrical ignition of
internal-combustion engines (p. 101) to the _induction coil_. This is a
device for increasing the _voltage_, or pressure, of a current. The
two-cell accumulator carried in a motor car gives a voltage (otherwise
called electro-motive force = E.M.F.) of 4.4 volts. If you attach a wire
to one terminal of the accumulator and brush the loose end rapidly
across the other terminal, you will notice that a bright spark passes
between the wire and the terminal. In reality there are two sparks, one
when they touch, and another when they separate, but they occur so
closely together that the eye cannot separate the two impressions. A
spark of this kind would not be sufficiently hot to ignite a charge in a
motor cylinder, and a spark from the induction coil is therefore used.

[Illustration: FIG. 53.--Sketch of an induction coil.]

We give a sketch of the induction coil in Fig. 53. It consists of a core
of soft iron wires round which is wound a layer of coarse insulated
wire, denoted by the thick line. One end of the winding of this
_primary_ coil is attached to the battery, the other to the base of a
hammer, H, vibrating between the end of the core and a screw, S, passing
through an upright, T, connected with the other terminal of the battery.
The action of the hammer is precisely the same as that of the armature
of an electric bell. Outside the primary coil are wound many turns of a
much finer wire completely insulated from the primary coil. The ends of
this _secondary_ coil are attached to the objects (in the case of a
motor car, the insulated wire of the sparking-plug and a wire projecting
from its outer iro