While the greater number of tractor engines use magneto ignition, many use battery and coil systems, which are the same in general principle as magneto systems, but produce magnetism in a different manner.

Copper is a nonmagnetic metal; that is, magnetism will not flow through it, nor can it be magnetized. If a pile of iron filings is stirred with a copper wire there will be no effect, as might be expected; but if a current of electricity flows through the wire, the iron filings will cling to it, as shown in Figure 54, as if it were a real magnet.

It is one of the principles of electricity that when a current flows through a wire, the wire is surrounded by magnetism, which continues as long as the current flows; when the circuit is broken and the current stops flowing, the magnetism dies away. The magnetism produced is feeble and can be very greatly increased by winding the wire around an iron bar. The magnetism produced by the current then flows into the bar, and that, like the core of the winding of a magneto, throws out magnetism of its own. This is indicated in Figure 55. By changing the intensity of the electric current, or by cutting it off, the strength of the magnetism can be made to change, and this change of strength can produce a sparking current.

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.

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.

To produce an intense sparking current, it is necessary to break the circuit as abruptly as possible, in order to cause the magnetism to die away suddenly. Figure 57 shows how this is done in the Atwater-Kent system.

The parts of the circuit breaker are carried on a plate, in the center of which revolves a shaft with a notch in it. Against the side of this shaft rests the hooked end of the sliding catch; as the notch comes under this hooked end, the sliding catch is drawn forward, only to be snapped back by its spring as the notch moves from under it. The lifter is a bit of metal, pivoted at one end, with its free end lying between the sliding catch and the flat steel spring that carries one of the contact points.

A, Figure 57, is a diagram of the system. B shows the position of the parts as the notch carries the sliding catch forward, and C shows their positions as the spring snaps the sliding catch back to its place. It will be seen that in thus moving back it strikes the lifter, which in turn moves the contact spring, and so closes the circuit; but the circuit is instantly broken as the parts spring back to position. The movement of the parts is so rapid that to the eye they seem to be standing still. The circuit is closed only for an instant, but that is sufficient to magnetize and demagnetize the coil, and to produce a sparking current.

The operation of this system depends on the very great swiftness with which the circuit is made and broken; there is not sufficient time for the core to get thoroughly magnetized, but such magnetism as is produced changes strength so quickly that it gives a sufficiently intense current to create an ignition spark.

In other battery systems of like principle, the circuit is closed for a long enough time to allow the core to become fully magnetized, the circuit then being suddenly broken. In some of these systems the timer breaks the circuit, while in others it is broken by the magnetism, through a vibrator.

A vibrator coil system is illustrated in Figure 58. The timer is a ring made of some kind of insulating material, with a plate of metal set in it and forming one of the timer contacts. The other contact is the revolving brush, driven by the engine; the circuit is closed when the brush touches the metal plate.

Opposite the end of the core is a flat steel spring, or vibrator blade, resting against the tip of a screw; when the core is magnetized it draws the end of the blade to it, and separates it from the screw. The battery current flows from the timer contact to the screw, then to the vibrator blade and to the primary winding of the coil. The core then becomes magnetized, and draws the blade away from the screw, which breaks the circuit; this causes the magnetism to die away, and a sparking current is produced in the secondary winding of the coil. The vibrator blade, no longer held down by the magnetism, springs back against the screw; the circuit is again made, and the action is repeated. The movement of the vibrator blade is very rapid, being some hundreds of vibrations a second.

A spark plug is illustrated in Figure 59. It consists of a metal shell screwed into the cylinder, enclosing an insulator of porcelain, mica, or some similar material. Through the insulator passes the center electrode, which is a rod of metal, with its lower end separated by a short distance from the shell or from a wire attached to the shell. This separation is the gap across which the sparking current passes, and at which the spark occurs.

Spark plugs receive the pressure of the power stroke, and must be strongly made in order to withstand it. A leaky spark plug will cut down the power of the engine, just as a leaky valve will.