Page:Motors and motor-driving (1902).djvu/175

Rh the fresh gas to mingle partially with this, but not sufficiently to ignite it until the greatest amount of compression is obtained. It is obvious, therefore, that with variable speed of engine, the moment of ignition is not always theoretically correct. In theory, that moment varies with the speed of the engine: many methods of timing 'tube ignition' have been suggested, but up to the present no satisfactory solution appears to have been discovered.

To get over this difficulty, and thereby greatly increase the efficiency of the motor, ignition by means of the electric spark has been devised. By the contrivance known as the shifting 'commutator'—afterwards described—it is possible to alter the moment at which the spark is caused to fire the charge in the combustion chamber, so that whatever the speed of the engine the moment of firing the charge is theoretically accurate.

Two Systems.—There are numerous types of electric ignition on the market, which may be divided up into the following classes:—

(1) With a battery and induction coil.

(2) The magneto system.

(1) . The essential of this system is an electric battery. The function of a battery is to supply the necessary quantity of electricity to create the spark in the combustion chamber. Broadly speaking, there are two kinds of battery used for this purpose on automobiles viz., the 'dry battery' and the 'accumulator' or storage battery.

The Dry Battery. The dry battery, so called because of the absence of any visible fluid, has become exceedingly popular, because it requires little attention. Each battery is usually composed of four separate cells, coupled together by means of small pieces of wire. Each cell consists of a plate of carbon surrounded by a chemical compound, the whole being contained in a thin zinc case. The carbon plate forms one