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. [Illustration: FIG. 58.—VIBRATOR COIL IGNITION SYSTEM] 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. [Illustration: FIG. 59.—SPARK PLUG] 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. CHAPTER VIII TRANSMISSION The parts of a tractor by which the power of the engine is applied to the driving wheels are called the _transmission_, and include the _clutch_, _the change speed gear_, the _differential_ and the _drive_. It has been shown that a gas engine delivers power only when it is running at speed; it cannot run until some outside power drives it through the inlet and compression strokes.