Page:Motors and motor-driving (1902).djvu/180

148 the De Dion coils the brass rings on the outer case should be connected to the framework of the car, called 'earth.'

The Function of the Commutator.—Following the path of the current from the primary circuit of the coil, a wire is taken to the device known as the commutator. This takes many forms, which will be found under the description of the various systems.

The function of the commutator is to automatically make a break in the circuit, with the result that when the moment for firing arrives a flood of electricity at great pressure is induced in the secondary circuit.

The Sparking Plug.—In order to create a spark in the cylinder the wire from the coil is attached to a device known as the 'sparking plug.' This 'sparking plug' may be one of many forms, but all consist of a small central rod or wire, to one end of which is fixed a terminal to which the wire from the coil is attached. The other end takes the form of a knob or is bent at right angles. This conducting core passes through a tube of porcelain, mica, asbestos, or other non-conducting material, capable of resisting the great heat from the combustion chamber. The tube is fastened into a socket, easily screwed into the combustion chamber. The current flowing down the centre conductor finds itself compelled to jump a small gap to a piece of wire or other conductor let into the metal of the sparking plug. This jump gives rise to the spark which ignites the charge.

The Return of the Current to the Coil.—The metal of the sparking plug being in contact with the metal of the engine, the current is conducted from it to the coil.

This is usually done through the metal frame or pipes, which, of course, are good conductors of electricity. These connections are, however, a frequent source of annoyance. The wires are often attached to the frame by small screws, which shake loose owing to the vibration from the engine and uneven surface of the road.

It must be clearly understood that, although the wires, &c.