The most popular type is the tube coil with switch, which can be mounted on the dash under the hood with switch on the outside, within reach of the operator. In some cases the coil is made separately and mounted under the hood, while the switch has the usual position on the dash. Fig. 53 illustrates the wound core of a transformer coil, complete with condenser casing. These coils are mounted in a tubular case provided with front and rear end plates. The front plate carries the switch handle and push button, while the rear end carries the terminals and is enclosed by a cover, so that the coil and connections are thoroughly protected.

Wiring Diagram for Low Tension Magneto with External Transformer Coil.

Fig. 52. Wiring Diagram for Low-Tension Magneto with External Transformer Coil.

The push button is used for producing a spark in the cylinder by interrupting the primary circuit leading from the battery, taking the place momentarily of the breaker on the magneto. Some coils are fitted with a ratchet mechanism giving a series of sparks in the cylinder. Some are also provided with a lock and key, so that the switch may be locked in the "off" position, preventing the unauthorized use of the truck.

The action of the switch was explained previously in connection with the dual system, and requires no further discussion.

The battery system, providing a jump spark, was extensively used before the magneto became so popular. This system requires a series of dry cells or a storage battery, affording a low-tension current and a tinier, the current being stepped-up to a high-tension by induction. The principle of self-induction is as follows:

A current flowing through a coil of wire will set up a magnetic field in the surrounding space, but when the current is stopped the magnetic field will stop. The effect of the stoppage of the flow of current in this coil is the same as that due to the change of position of the wire coil in a magnetic field, so that when the current is stopped there will be a current induced in the coil. The result is that when the circuit is broken to stop the current, the decrease in the current adds momentarily to the electromotive action in the circuit and a visible spark, or even an arc, is formed at the break.

Fig. 54 makes clear the principles of an induction coil, showing the primary and secondary circuits and the other components of the system.

The primary and secondary windings are identical with those of the compound armature, or high-tension magneto, the former consisting of a small number of turns of coarse insulated wire, while the latter is a very fine silk insulated wire, and the numl>er of turns greatly exceeds those of the primary winding. In this system no mechanically operated interrupter is used for breaking the primary circuit, this being accomplished by a device for automatically and rapidly making and breaking the circuit.

The device is known as a vibrator or trembler, as it is sometimes termed, and consists of an iron disc of approximately the same diameter as the core of the coil, attached to flat spring and a platinum point located above the disc. An adjusting screw mounted above the disc carries another platinum point. These points are so situated on the coil that they make contact with each other. When these two points are in contact, the primary circuit is closed.

The automatic action of this vibrator is as follows: The current flowing through the primary circuit of the coil makes an electromagnet of the core, which attracts the iron disc attached to the vibrator spring, causing the points to separate. As the current is interrupted by the separation of these points, the core ceases to be an electromagnet, since no current is flowing through it and the disc is no longer attracted permitting the points to make contact and again complete the primary circuit. This action continues as long as the current flows and is interrupted in the primary circuit. By varying the adjustment of the adjusting screws the number of sparks in a given time are varied, as is also the strength of the individual sparks.

To prevent prolonging the magnetization of the core beyond the desired limit, a condenser is connected with the contact points to absorb the surplus current induced in the primary circuit, due to the breaking of the circuit. In other words, when the points are in contact, the condenser is short circuited, but when the circuit is broken the induced current, instead of jumping across the gap, passes into the condenser, and as the circuit is again completed it passes out again and into the circuit. A commutator, or timer, is used to determine the exact time in the cycle of the engine at which ignition occurs.

The diagram shown herewith represents a single-cylinder system, while multi-cylinder engines require a coil for each cylinder, but the battery current passing into the primary winding of the coils is controlled by a single switch. The coil units are incorporated in a box or housing which is mounted on the dash board, with a removable cover, so that any coil may be adjusted. As each coil is a separate unit, they may be so constructed that they may easily be replaced should they become defective, without disturbing any connections.

In every high-tension battery system a device is required for opening and closing the primary circuit at the proper instant, with respect to the cycle of the engine, and the position of the piston in the cylinder. This device is known as the tinier, because it determines the exact time in the cycle of the engine at which ignition occurs and permits of varying this point at will while the engine is in operation. It is positively driven by gears from the motor either direct or through an auxiliary shaft from the cam shaft. This timer (Fig. 55) consists of a housing containing a roller and arm members so mounted upon the driving shaft that the roller and arm members may rotate, while the housing is held stationary by means of a rod or lever, which may be moved in either direction to advance or retard the spark. Within the housing is a fiber ring in which are mounted metal contact segments, the surfaces of which are flush with the fiber.