Motor-car principles; the gasoline automobile

by Roger B. Whitman

Because of the desire of the current to flow from the positive to the negative pole, the circuit must be so guarded that there may be no leakage, for if it can return to the generator without doing its work it will do so, taking any path that offers less resistance to its passage. Leakage is prevented by insulating the wires, which may be done by wrapping them with silk or cotton thread, or by coating them with rubber. The wire by which the current flows from the generator to its work is called the lead (pronounced leed), and the conductor by which it flows back, the return. The greatest care must be taken that there is no leakage from the lead wire in order that there may be enough current to perform the work; but when the current has done what was expected of it, any conductor that does not present too much resistance will serve to return it to its source.

While wire is sometimes used on ignition circuits for the return as well as the lead, the most usual method is to utilize the metal of the engine to return the current to its source. The engine being made of iron, it is a conductor, and while this metal is not so good a conductor as copper, yet there is so much of it in an engine that its resistance need not be considered. This method is called grounding the return, the term coming from telegraphy, in which the dampness of the earth forms one of the conductors.

A grounded circuit requires careful insulation of the lead wire, for as it is very likely to come into contact with the metal of the engine, a break in the insulation would permit the current to leak and to return to the generator without doing its work, this condition being called a short circuit. Grounding the circuit saves wire and reduces complication; simplicity is important in automobile work, and a reduction of the parts much to be desired.

In the foregoing the word flow has been used in describing the action of an electric current in a wire, and there are so many points of similarity with the flow of water in a pipe that the term conveys a better idea than would technical expressions. Electricity, however, has neither substance nor weight, and cannot be said to flow or to move in the strict sense of the words; but as flow is in general use, and is the most descriptive of the commonly understood words that are applied to it, it is made use of here.

A current of electricity may be generated either chemically or mechanically, and always at the expense of something else. A chemical generator produces a current at the expense of a metal, usually zinc, which is eaten by the chemicals in proportion to the amount of current delivered; a mechanical generator produces a current at the expense of the power that drives it.

Chemical generators are of two kinds, primary and secondary cells. While primary cells are of many forms, that called the dry cell is universally used for the ignition of automobile engines, being adapted to the purpose because there is nothing to spill, and may be used in any position. It consists of a zinc cup lined with some material of the nature of blotting paper, in which, but not touching it, is a stick of carbon, the space between being filled with absorbent material and broken bits of carbon moistened with the proper chemical solution. The top of the cell is sealed to prevent evaporation, and thumb nuts on the projecting end of the carbon stick and the zinc cup form the terminals to which the wires are attached. The carbon being the positive pole and the zinc the negative, the current flows in that direction whenever a circuit is provided.

The dry cell gives a current at a pressure of about one and a half volts when new, the voltage dropping as the cell is used until it is exhausted, when the trifling cost of a new cell should not be considered in replacing it.

A better source of current than the dry cell is the secondary or storage cell, which is a reservoir rather than a generator. When a current of electricity is passed to it, usually from a lighting circuit, a chemical change takes place, the action being called the charging of the cell. When the change is complete, the cell is disconnected, and will then deliver a current because the parts of the cell that underwent a chemical change tend to return to their previous condition. The current that it gives off is practically equal to that by which it was charged, and may be used steadily or intermittently until the cell is exhausted. It is made of prepared lead plates, which are placed in a hard-rubber or celluloid jar, with a cover to prevent the spilling of the solution with which the plates are covered.

The charging of storage cells must be done by an expert, for mishandling will ruin them, and it is far better to put this matter, as well as repairs, in the hands of a man who knows his business than to attempt it without the proper knowledge and appliances.

One point that the automobilist must watch, however, is the position and quality of the solution in the cells, called the electrolyte. The cells should always be full enough to have the plates well covered, and any loss by leakage or splashing should be made up, for the plates must not be exposed to the air.