At the instant the maximum current is being generated, the circuit is broken by means of the contact breaker D, fig. 19, which we show in detail in fig. 22. The latter is mounted on the end of the combustion chamber, and consists of two parts, D and P.
Fig. 22.
D is an easy fit in the hole bored to receive it, and has a mushroom valve head and seating, as shown, so that it moves readily when struck by the projection E on the rod R (fig. 19); but yet, acting in the manner of a non-return valve, it allows no gas to escape when the explosion takes place in the cylinder. D is therefore in direct metallic communication with the engine frame and earth.
P is a fixed metal pin, carefully insulated from all contact with the engine frame and earth. To this pin one end of the armature winding is connected, whilst the other end is connected to the engine frame.
Thus a closed circuit is formed, and when the current is generated it flows from one terminal of magneto through wire to pin P, on to D, through D to earth (i.e., engine frame), and so back to other terminal on magneto.
And as the circuit is broken between D and P, we obtain a spark, as previously explained, which may be timed to take place by adjusting the position of cam C on side shaft relatively to the position of piston.
It may be said that the position of the magneto-igniter is immaterial; it will be fixed in different positions on different types of engines, and so long as the operating mechanism is simple and effective, i.e., as direct as is practicable, it works well, and requires little attention. The timing of the spark will be dealt with in the chapter on Cams and Valve Settings.