This high pressure is obtained through the use of a secondary induction coil, which consists of a core and primary winding similar to those of the primary induction coil already described, and in addition has a secondary winding, formed by a great length of insulated copper wire, which to save bulk and weight is very fine, wound over the primary winding. The change in the strength of the magnetic field produced by the core affects this secondary winding, just as it affects the primary winding; that is, a current is set up in it at every change in the strength of the magnetic field, the pressure of the current depending on the greater or less extent of the change, and the rapidity with which it occurs. The magnetization of the core takes place but slowly, for the particles of iron must absorb it one from the other, but the demagnetization occurs instantly; the current that is induced during the demagnetization is therefore the greater, and is the current that has sufficient pressure to jump across the gap.
The ignition system employing this coil consists of two distinct parts: the primary circuit, which magnetizes the core, and the secondary circuit, which leads to the combustion space the current that is induced in the secondary winding.
The primary circuit includes the battery or generator that supplies the current, the primary winding of the coil, a timer, and a vibrator.
THE TIMER
The spark must occur in the combustion space only at the instant when the mixture is in a condition to be ignited, and it is obvious that only then is the secondary current required. As the secondary current results from the flow of the primary current through the primary winding, a device must be included in the primary circuit to permit the current to flow and magnetize the core at this instant. This device is called a timer, or commutator, and is nothing more than a revolving switch operated by the engine, that completes the primary circuit at such times as the secondary current is required, and breaks the circuit when the secondary current has done its work in the production of the spark. Because ignition occurs but once in each cylinder during two revolutions of the crank shaft, the timer completes the circuit but once in that interval, and is placed on the half-time shaft that operates the exhaust valves. In its simplest form, a timer consists of a disk of hard rubber, wood fiber, or other insulating material in which is set a metal plate, mounted on the half-time shaft and revolving with it. A flat spring carried on a plate of hard insulating material bears against the edge of the disk, and during one revolution of the latter the metal plate set in it will be brought into contact with the spring (Fig. 26, first diagram). If one wire of the primary circuit is connected to the metal plate of the disk and the other wire to the spring, it will be seen that the circuit will be complete when the revolution of the disk brings the two together, and broken when the further revolution of the disk separates them.
Fig. 26.—Types of Timers.
There is a great variety in the methods by which this result is secured, the chief essential being that the contact must be made as positively as possible in order that the resistance to the current in passing from one part of the timer to the other may be as low as possible, and broken equally positively. The number of contacts that the timer makes during one revolution depends on the number of cylinders of the engine, the revolving part touching two, three, four, or more springs or contact points on the stationary part as it turns. A timer for a two-cylinder engine is shown in the second diagram on Fig. 26, the third diagram being of a form of timer in which the revolving part is not connected into the circuit, but serves to bring together a spring and a contact point, the circuit being completed when they touch. A modern form of timer consists of a ring of hard rubber on the inside face of which are set the metal plates, while the moving part revolves within it, and may be a flat spring, or a plunger, or a roller, contact being maintained by a coil spring. The difficulty of attaching a wire to the moving part of the timer is overcome by having its contact in connection with the half-time shaft, the current flowing from the contact on the stationary part to that on the moving part, and from there flowing by the half-time shaft to the ground connection of the battery, for, as all parts are of metal, the current can pass from the shaft to the ground connection through the bearings and the engine.
If the spark were always required to pass in the combustion space at the same point in the stroke of the piston, this could be secured by setting the stationary part of the timer in such relation to the moving part that contact would be made when the half-time shaft reached any desired point in its revolution. As it is necessary to change the point at which ignition occurs, however, the timer must be so arranged that it closes the circuit earlier or later, according to conditions. To effect this, the stationary part of the timer is so made that it may rotate around the shaft for about a half turn, and it is controlled by a lever on the steering column, which holds it in any desired position. If the stationary part is rotated in the direction opposite to that in which the half-time shaft revolves, contact will be made and the circuit closed at an earlier point in its revolution than if it is rotated in the same direction. It must be remembered that the revolution of the half-time shaft, in operating the exhaust valve, revolves exactly in accordance with the crank shaft and the movement of the piston, and that as the moving part of the timer is attached to it, contact can be made at any desired point in the stroke of the piston by the position in which the stationary part is placed.