MAKE-AND-BREAK SYSTEM
In the make-and-break system, the igniter, which is the device in the combustion space at which the spark occurs, is made with two metal points, one of which is stationary and the other movable, the latter being acted on by a cam, through a tappet. As the cam revolves, the tappet is lifted and the movable point brought into contact with the stationary; when the nose of the cam passes from under the tappet, the movable point is snapped away from the stationary through the action of a spring (Fig. 25). These points are so connected into the circuit that when they are in contact the current flows, the circuit being broken when they separate. Shortly before the spark is desired the movable point is brought into contact with the other so that the circuit is completed and the current flows, and they are separated at the instant when the spark is required to ignite the mixture, this occurring as they separate and break the circuit.
Fig. 25.—Make-and-Break Ignition.
The current that is supplied by the battery is not capable of producing this spark, for it flows with large volume and small pressure—i.e., high amperage and low voltage—and it is necessary to transform it to a current of higher voltage and lower amperage, which is done by means of a primary induction coil. A coil of this description consists of a core made of a bundle of soft iron wires, around which is the primary winding, formed by winding several layers of insulated copper wire on it. The core becomes a magnet when a current of electricity flows through the winding, and ceases to be a magnet when the flow ceases, the magnetization occurring slowly on the completion of the circuit, and demagnetization instantly on the breaking of the circuit.
The influence of a magnet, as, for instance, its attraction for a piece of iron, is felt throughout a field the extent of which depends on the strength of the magnetization. When a loop of wire forming a closed circuit is placed in the magnetic field, a current of electricity will be set up in it whenever the strength of the field changes. If the strength of the field does not change, no current will be set up; but the greater the change in strength, whether from strong to weak or from weak to strong, the greater will be the strength of the current. This principle is applied in the case of a coil, for the wire that forms the winding is in the magnetic field of the core, and a current is set up in it whenever the strength of the field changes. The strongest current will be set up, or induced, when the strength of the field changes from no magnetism to a degree when the core is magnetized to its fullest capacity or the reverse, and when the change occurs in the least possible time. The current that is set up will last only during the time when the change in strength occurs, ceasing to exist when the strength of the field is uniformly strong or weak. The more rapid the change, the greater will be the pressure of the current, but the shorter the period of its existence.
It is this induced current that forms the spark in the combustion space, the battery current serving only to establish the condition that will produce it. When the two points of the igniter come into contact and complete the circuit, the flow of the battery current through the winding of the coil will gradually magnetize the core, and on the igniter points being separated, this flow through the winding will cease, the strength of the magnetic field dropping instantly from full intensity to practically nothing. This change induces a powerful current in the winding, which current in passing over the circuit causes a spark between the points of the igniter as they separate. The induced current is set up so instantaneously on the breaking of the battery circuit that the spark is usually considered as being that of the battery current, but, as stated, the battery current serves only to magnetize the core, the spark resulting from the current induced by its demagnetization on the breaking of the circuit.
A magneto is a machine that when driven generates an electric current, and is in almost universal use for this system of ignition. In many cars the system is so arranged that the driver has his choice between ignition by battery and coil, as described, and ignition by magneto, but the mechanical production of a current is so reliable that many automobile builders rely on the magneto alone. Magnetos of the type used for ignition are described in the appendix.