The petrol motor, which to-day is busily engaged all over the world in driving thousands upon thousands of self-propelled vehicles or automobiles, belongs to the important class of internal-combustion engines. Combustion means the operation of burning, and an internal-combustion engine is one in which the motive power is produced by the combustion of a highly explosive mixture of gases. In the ordinary petrol motor this mixture consists of petrol and air, and it is made by means of a device called a “carburetter.” By suction, a quantity of petrol is forced through a jet with a very fine nozzle, so that it is reduced to an extremely fine spray. A certain proportion of air is allowed to enter, and the mixture passes into the cylinder. Here it is compressed by the rising piston so that it becomes more and more heated, and at the right point it is ignited. Combustion takes place with such rapidity that it takes the form of an explosion, and the energy produced in this way drives forward the piston, which turns the crank-shaft and so communicates motion to the driving-wheels. The part played by electricity in this process is confined to the ignition of the compressed charge of petrol and air. This may be done in two ways; by means of an accumulator and a small induction coil, or by means of a dynamo driven by the engine. At one time the first method was employed exclusively, but to-day it is used as a rule only for starting the car engine, the second or magneto method being used when the engine has started up.