Fig. 55.—Magnetism from Electricity
The principle employed is illustrated in [Figure 56]. A is a coil of wire wound around one end of an iron bar and connected with a battery; B is an entirely separate coil of wire wound around the other end of the bar, with its ends separated by a short distance. By closing the battery switch the current will be permitted to flow in coil A, and the bar will become magnetized; the magnetism that it throws out will be felt by coil B. When the switch is opened the current stops flowing and the magnetism dies out of the bar; these changes in strength will create an electric current in coil B, which will form a spark as it passes across the space between the ends.
Fig. 56.—Principle of Spark Coil
In ignition coils, coil B is wound on top of coil A. Coil A, called the primary winding, consists of a few layers of coarse wire. The more turns of wire there are in coil B, called the secondary winding, the more intense will be the current that it produces, and the intensity is also increased by keeping the windings close to the iron core. The secondary winding is, therefore, made of exceedingly fine wire, and has a very great number of turns.
To obtain a spark, a current is permitted to flow through the primary winding to create magnetism, and the flow is then stopped to cause the magnetism to die away. The secondary winding is affected by each of these changes in magnetic strength. The bar loses magnetism more rapidly than it gains it, however; it is therefore the dying out of the magnetism that has the greater effect on the secondary winding, and that causes it to produce a sparking current.
To use this principle for ignition, the engine is fitted with a revolving switch, which closes the circuit as a piston is on the compression stroke, and then breaks the circuit at the instant when a spark is desired. Combined with the revolving switch, or timer, is a distributor like the distributor of a magneto, which passes the sparking current to the cylinder that is ready to receive it.
