The other terminal of the primary has a wire F leading to the insulated contact plug G of the contact maker. This completes the generating circuit. The cam H is on a shaft I, which travels one half the speed of the engine shaft.
One side of the secondary coil J has a wire K leading to the spark plug, while the other terminal of the secondary has a wire L which is grounded on the engine M.
When the nose of the cam pushes over the spring finger and closes the cam, the circuit through the finger flows through the primary coil and excites the secondary. When the cam again immediately breaks the circuit a high tension current is momentarily induced in the secondary, so that the current leaps the gap in the spark plug and makes the spark.
Metallic Core for Induction Coil.—In the previous description of the induction coil it was stated that the spool might be made of wood. These coils are also provided with metal cores, which can be used to make what is called a vibratory coil.
The Condenser.—A necessary addition to the circuiting provided by an induction coil, is a condenser. This is used in the primary circuit to absorb the self-induced current of the primary and thus cause it to oppose the rapid fall of the primary current.
The condenser is constructed of a number of tinfoil sheets, of suitable size, each sheet having a wing at one end, and these sheets are laid on top of each other, with the wings of the alternate sheets at opposite ends. Very thin sheets of waxed paper are placed between the tin foil sheets so that they are thus insulated from each other.