Page:Motors and motor-driving (1902).djvu/181

Rh by which the circuit between the commutator and the coil is completed are technically known by the misleading term 'earth,' they are not used to convey the current to the ground but back to the coil.

Thus we have two complete electrical circuits acting in unison with one another. It is obvious that if there is any fault in the primary circuit no spark will be produced in the engine. Faults may arise from many causes.

Insulation. Around wires intended to convey electricity are laid and woven many layers of rubber, cotton, &c. This lapping is to prevent the electricity which is being conducted by the wire from escaping, and is known as 'insulation.' On this 'insulation' the success of electrical ignition to a large extent depends, and the importance of keeping it perfect cannot be too greatly impressed on the novice.

The wires which convey the current from the coil to the commutator and from the commutator to the sparking plug have to be specially insulated, as the current, being at such a high pressure, will take every opportunity of leaving its legitimate path if allowed to.

All wires used for connecting the various parts of the systems should be very flexible, and composed of many strands of fine copper wire. Too much stress cannot be laid on keeping the insulation perfect.

If it be imperfect, the current will leave the wire and jump to the frame and thence back to the battery without performing the work required of it. When this occurs it is known as a 'short circuit.' Electricity always travels by the easiest path, and, if it can avoid doing any work, it will do so.

.—Imperfect Insulation.—If a buyer has any doubts as to whether the insulation of the electric system of his car is sufficient under all conditions of weather and to withstand water splashed during washing the car, &c., he would save much trouble in the future by having all the important wires sheathed in fibre or indiarubber piping.