The third form of ignition we have to deal with is the electric.

There are a great number of different types made and used, but for gas-engine use perhaps that known as the magneto ignition is the most satisfactory. With this form, neither accumulators, dry batteries, or spark coils are required, and consequently a greater simplicity is arrived at than would otherwise be the case.

In fig. 19 we show diagrammatically the ordinary form of magneto machine. Virtually it is a small dynamo which is fixed to the side of cylinder casting, and is operated in the manner shortly to be described. As we do not propose to enter into more than a brief explanation of why and how this apparatus generates current to produce the required spark, perhaps a simple analogy will make matters most intelligible to any reader not well acquainted with electrical phenomena. We know that when a current of electricity is flowing in a wire, and the wire be suddenly broken, a spark will occur at the point of breakage. This fact may be observed in an ordinary electric bell when ringing; at the tip of the contact breaker a number of tiny sparks may be seen to occur, due to the rapid make and break of the current flowing in the circuit. Precisely the same action takes place in our magneto-igniter, but, instead of a multitude of tiny sparks, we produce one at a time, at definite intervals, viz., at the commencement of each explosion stroke.

In the later form of magneto machines there is a soft iron sleeve between the magnet poles and the armature. The former is connected to a system of levers by which a reciprocating motion is imparted to it by means of a suitably arranged cam on the side shaft. It has been found that better results are obtained by causing the magnetic field to move relative to the armature winding than to move the latter through a stationary field. Reference to the diagrams, figs. 20 and 21, will make this clear.

In fig. 19 the cam C is shown just on the point of allowing the lever L to fly back into its normal position, due to the action of the springs comprising a dashpot S. As the cam rotates, it pushes the lever L to the left, the sleeve (or virtually the armature A) is also rotated through a portion of a revolution comparatively slowly; but as soon as L is released, the sleeve (or armature) flies back again almost instantaneously and for the moment is generating a current in the same manner as would any ordinary continuous current dynamo.

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.

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.