If a piece of iron is wound with insulated wire through which an electric current passes, the iron will become a magnet, and will remain so as long as the current flows. This is called an electro-magnet.
The two ends of a magnet are the positive and negative poles, and the positive pole of one magnet will repel the positive pole and attract the negative pole of another magnet. Between the two poles of a magnet there is a constant flow of what are known as magnetic lines of force, which may be made visible by laying a piece of paper over the two poles and scattering iron filings on it. The filings will form into curves from one pole to the other, showing the presence of these lines.
If a wire is moved across these lines of force, so that it cuts them, a current of electricity will be set up in it, and if the wire is wound into a coil and moved across the lines, the current will be as much stronger as there are turns of wire cutting them. A mechanical generator consists of a coil of insulated wire, arranged so that as it revolves in the magnetic field each turn of wire cuts across the lines of magnetic force.
The magnet producing the lines of force that compose the magnetic field is called the field, and the revolving coil of wire the armature.
Mechanical generators for producing the current for an ignition circuit are of two types, magnetos and dynamos, the former being in much more general use than the latter. In a magneto the field is a permanent magnet, and in a dynamo it is an electro-magnet. The magnets forming the field of a magneto are bent into horseshoe shape, so that the poles are close together, and the armature revolves between them. The field of a dynamo is of similar shape, wound with insulated wire, the current that flows through it being generated by the dynamo itself; the field is strongly magnetized only when the dynamo is running.
The armature of either type is a piece of soft iron, with grooves cut in it lengthways, in which the wire is wound; the winding thus being contrary to that of thread on a spool. The current generated in the wire of the armature is taken off by means of brushes, and connection is made through binding posts set in the base.
As the care of a mechanical generator requires a knowledge of electrical engineering, it is better to return it to the makers in case of injury, rather than to attempt to repair it without the necessary experience. (See Appendix.)
Practically all ignition circuits are provided with two sources of current, so that in case of the failure of one the other will be available, and either may be connected into the circuit by means of a switch. The simplest form of switch is a piece of flat spring brass, pivoted at one end so that the other may be swung from side to side to make contact with one or the other of two brass knobs, or switch points. One pole of each generator, usually the negative, is grounded, and the other connected to one of the points. The flat spring, or switch blade, is connected to the ignition circuit, so that when it is in contact with one of the points the corresponding source is called on to supply the current. The blade may rest between the points, not touching either, in which case the circuit is broken, and no current flows.
CHAPTER VII
IGNITION—(Continued)
While there are various methods by which an electric spark may be produced, but two are in use for automobile work, one being the high-tension or jump-spark system, and the other the low-tension or make-and-break system. The difference between these is in the pressure of the current, which in the first is great enough to enable it to jump from one terminal to another a short distance away, forming a spark as it passes. The pressure of the current of the low-tension system is not sufficient to permit it to do this, but when two terminals of the system are brought into contact so that the current may flow, a spark will form as they are separated.