All petrol-cars now use electrical ignition. There are two main systems--(1) by an accumulator and induction coil; (2) _magneto ignition_, by means of a small dynamo driven by the engine. A general arrangement of the first is shown in Fig. 45. A disc, D, of some insulating material--fibre or vulcanite--is mounted on the cam, or half-speed, shaft. Into the circumference is let a piece of brass, called the contact-piece, through which a screw passes to the cam shaft. A movable plate, M P, which can be rotated concentrically with D through part of a circle, carries a "wipe" block at the end of a spring, which presses it against D. The spring itself is attached to an insulated plate. When the revolution of D brings the wipe and contact together, current flows from the accumulator through switch S to the wipe; through the contact-piece to C; from C to M P and the induction coil; and back to the accumulator. This is the _primary, or low-tension, circuit_. A _high-tension_ current is induced by the coil in the _secondary_ circuit, indicated by dotted lines.[10] In this circuit is the sparking-plug (see Fig. 46), having a central insulated rod in connection with one terminal of the secondary coil. Between it and a bent wire projecting from the iron casing of the plug (in contact with the other terminal of the secondary coil through the metal of the engine, to which one wire of the circuit is attached) is a small gap, across which the secondary current leaps when the primary current is broken by the wipe and contact parting company. The spark is intensely hot, and suffices to ignite the compressed charge in the cylinder. [Illustration: FIG. 46.--Section of a sparking-plug.] ADVANCING THE SPARK. We will assume that the position of W (in Fig. 45) is such that the contact touches W at the moment when the piston has just completed the compression stroke. Now, the actual combustion of the charge occupies an appreciable time, and with the engine running at high speed the piston would have travelled some way down the cylinder before the full force of the explosion was developed. But by raising lever L, the position of W may be so altered that contact is made slightly _before_ the compression stroke is complete, so that the charge is fairly alight by the time the piston has altered its direction. This is called _advancing_ the spark. GOVERNING THE ENGINE. There are several methods of controlling the speed of internal-combustion engines. The operating mechanism in most cases is a centrifugal ball-governor. When the speed has reached the fixed limit it either (1) raises the exhaust valve, so that no fresh charges are drawn in; (2) prevents the opening of the inlet valve; or (3) throttles the gas supply. The last is now most commonly used on motor cars, in conjunction with some device for putting it out of action when the driver wishes to exceed the highest speed that it normally permits. [Illustration: FIG. 47.--One form of governor used on motor cars.]