rrangement of the first is shown in Fig. 45. A disc, D, of some
insulating material--fibre or vulcanite--is mounted on the cam, or
half-speed, shaft. Into the circumference is let a piece of brass,
called the contact-piece, through which a screw passes to the cam shaft.
A movable plate, M P, which can be rotated concentrically with D through
part of a circle, carries a "wipe" block at the end of a spring, which
presses it against D. The spring itself is attached to an insulated
plate. When the revolution of D brings the wipe and contact together,
current flows from the accumulator through switch S to the wipe; through
the contact-piece to C; from C to M P and the induction coil; and back
to the accumulator. This is the _primary, or low-tension, circuit_. A
_high-tension_ current is induced by the coil in the _secondary_
circuit, indicated by dotted lines.[10] In this circuit is the
sparking-plug (see Fig. 46), having a central insulated rod in
connection with one terminal of the secondary coil. Between it and a
bent wire projecting from the iron casing of the plug (in contact with
the other terminal of the secondary coil through the metal of the
engine, to which one wire of the circuit is attached) is a small gap,
across which the secondary current leaps when the primary current is
broken by the wipe and contact parting company. The spark is intensely
hot, and suffices to ignite the compressed charge in the cylinder.

[Illustration: FIG. 46.--Section of a sparking-plug.]

ADVANCING THE SPARK.

We will assume that the position of W (in Fig. 45) is such that the
contact touches W at the moment when the piston has just completed the
compression stroke. Now, the actual combustion of the charge occupies
an appreciable time, and with the engine running at high speed the
piston would have travelled some way down the cylinder before the full
force of the explosion was developed. But by raising lever L, the
position of W may be so altered that contact is made slightly _before_
the compression stroke is complete, so that the charge is fairly alight
by the time the piston has altered its direction. This is called
_advancing_ the spark.

GOVERNING THE ENGINE.

There are several methods of controlling the speed of
internal-combustion engines. The operating mechanism in most cases is a
centrifugal ball-governor. When the speed has reached the fixed limit it
either (1) raises the exhaust valve, so that no fresh charges are dr