Motor-car principles; the gasoline automobile

by Roger B. Whitman

GENERAL PRINCIPLES

The red-painted toy magnet that is one of the properties of childhood and with which everyone is familiar, may well be used as the beginning of a study of the magneto, for with it the characteristics of magnetism may be observed. A little experimenting will show that the magnet will attract, or “pick up”, iron and steel objects only, having no effect on copper, brass, lead, wood, or, for practical purposes, any other substance. Furthermore, it illustrates the fact that when iron and steel are in contact with it, they in turn become magnetic, able also to attract tacks and other bits of the same metals. Iron, however, is shown by experiment to be magnetic only when in actual contact with the magnet, losing its magnetism as soon as the contact is broken, while when steel is magnetized by touching it to a magnet it remains magnetic. This fact is illustrated by the magnet itself, which is of steel and therefore capable of retaining its power for a greater or less time, depending on its quality and hardness.

Fig. 1.
Magnetic
Lines of
Force.

If iron filings are scattered on a piece of paper laid over a magnet they will not fall evenly and regularly, but will collect most thickly at the ends, or poles, of the magnet, showing that there the magnetic attraction is stronger than at any other points. If the filings are examined closely it will be seen that they have taken up definite positions, forming lines and curves extending between the two poles (Fig. 1). This is the simplest method by which the magnetism may be made visible, and it illustrates the fact that the power of a magnet acts in a series of lines passing from one pole to the other. If a piece of iron or steel is placed across the poles of a magnet these lines, or as many of them as possible, will use it as a bridge or conductor, because they can pass through it more easily than through air. Such a piece of iron or steel is called a keeper, and by its use the magnet will retain its strength for a much greater time than if the lines are obliged to make their way through the far greater resistance that the air presents to their passage.

These lines, which are known as magnetic lines of force, always move in the same direction, passing through the air or the keeper from the north pole of the magnet to the south pole, and passing through the metal of the magnet itself from the south pole to the north pole.

The strength of a magnet depends on the number of these lines of magnetic force that it possesses. If two magnets, one strong and the other weak, are placed under sheets of paper on which iron filings are scattered, their comparative strengths are clearly shown by the difference in the number of lines of force that the filings show them to possess.

The space through which a magnet makes itself felt is known as the magnetic field, and this is large or small, according to the number of lines of force. The stronger the magnet, the larger will be the sweep of the curves of its lines of force, and the greater will be the field that they form.

When a piece of iron or steel is placed in contact with a magnet, the lines of force flow into it, and it becomes magnetized, throwing out lines of force and forming its own magnetic field, which is quite distinct from the magnetic field of the original magnet. If the piece is of iron, its lines of force and its field die away as soon as it is separated from the magnet, but a piece of steel once magnetized will retain its magnetism and, of course, its lines of force.

A wire of nonmagnetic metal, such as copper, for instance, will not have the slightest attraction for iron filings, but when an electric current is passed through it the filings will act as if the copper were a magnet, clinging to it as long as the current passes, and dropping as soon as the circuit is broken. As a matter of fact, an electric current sets up lines of magnetic force exactly similar to those of a permanent magnet, their number being in proportion to the strength of the current.

As has been stated, iron becomes magnetized when magnetic lines of force flow into it. If, therefore, a wire through which an electric current passes is wound around an iron rod, the lines of force set up by the current will pass into the rod and magnetize it, so that it sets up its own magnetic field. This magnetic field is created when the electric current starts flowing in the wire and dies out when the flow of the current is stopped, for then the lines of force due to the current die out, and the iron, which depends on them for its magnetism and which has not the ability to retain its lines of force, returns to its original nonmagnetic condition. An arrangement of this sort, consisting of a soft iron core, around which is wound a number of layers of insulated wire, forms an electro-magnet, and will produce a magnetic field whenever an electric current passes through the wire. The action is practically instantaneous, the magnetic field appearing and dying out on the making and breaking of the electric circuit.

A magnetic field may thus be produced by the action of an electric current, and an electric current in turn may be produced by the action of a magnetic field. To generate a current by this method it is only necessary to place a conductor forming a closed circuit in a magnetic field, and to change the strength of the field, making it stronger or weaker. For an example, a length of insulated wire may be wound on an iron bar, and the bar then touched with a magnet. As soon as the lines of force flow into the bar it becomes magnetized and sets up a magnetic field in which the lines of force follow the law and flow from one pole to the other through the air. The formation of this field is exceedingly rapid, but during the time that the field is forming and increasing to its full strength, an electric current will flow in the wire winding. When the field has reached its full strength, the flow of the current ceases. On separating the bar from the magnet it loses its magnetism, and the field set up by it dies away; or in other words, its strength undergoes another change, now growing weaker as the bar returns to a nonmagnetic condition. This dying out of the field generates another momentary flow of current in the wire, which moves in the direction opposite to the flow of the current generated while the strength of the field was increasing.

The current generated is called an induced current, and the method of producing it is called induction.

The intensity of the induced current in any given winding depends on the extent of the change in the strength of the field and upon the rapidity with which it occurs. A bar of iron is limited as to the strength to which it can be magnetized; or in other words, it can only set up a limited number of lines of force. Increasing the strength of the field from nothing to this point, or reducing its strength from this point to nothing, gives the greatest change in strength possible to obtain, and if this change occurs in the shortest possible time, then the current induced will be of the greatest intensity that can be obtained from a conductor of the size and length used.

For an understanding of the action of a magneto it is necessary to bear in mind the following points:

First. That a magnetic field is composed of lines of magnetic force that flow from one pole of the magnet to the other.

Second. That the lines of force will take the path that presents the least resistance.

Third. That an electric current will be generated, or induced, in a conductor placed in a magnetic field whenever the strength of the field changes.

Fourth. That the current flows only while the change in strength is taking place, ceasing to exist when the field becomes uniformly strong or weak.

To make a practical application of the laws governing the production of an induced current, a conductor forming a closed circuit is placed in a magnetic field, and the strength of the field caused to change, alternately becoming strong and weak. In magnetos, the magnetic field is due to two or more powerful steel magnets, and the conductor, a length of insulated copper wire, is wound on a soft iron core and revolved between the poles of the magnets where the field is strongest. The magnets are known as the field of the magneto, and the wire on its core the armature. The shape of the iron armature core is shown in Fig. 2, the winding being indicated by heavy black lines. On the inside of the poles of the field are pole pieces, which are blocks of soft iron hollowed out to receive the armature. As the successful operation of the magneto requires the lines of force to have as easy a path as possible, the air space between the armature heads and pole pieces is very small, being in the neighborhood of 1/100 of an inch.

Fig. 2.—Armature.

Fig. 3.

When the armature is not in position, the lines of force will be required to pass from one pole piece to the other through the air, and as they will seek the path of lowest resistance, most of them will pass between the lower points of the pole pieces, where the air gap is short, and where the lines of force are present in the greatest number (Fig. 3). When the armature is placed between the pole pieces, however, it gives the lines of force a path of still lower resistance, and they will therefore follow it, whatever its position may be. If the armature is free to turn, it will take a horizontal position, as shown in Fig. 4, for then the heads are entirely in contact with the pole pieces, and the greatest possible number of lines of force can take the path offered by the neck of the armature.

Fig. 4.

Fig. 5.

When the armature is in this position, the neck is magnetized by the flow of lines of force through it and sets up a powerful magnetic field which is quite distinct from the field thrown out by the field magnets. If the armature is revolved, it takes the lines of force, or most of them, with it, and they continue to flow through the neck as long as one head of the armature is in contact with the north pole piece and the other head in contact with the south pole piece, for even this long and distorted path, as shown in Fig. 5, is of less resistance than the air gap. The lines of force, however, resist this lengthening of their path, and tend to hold the armature with the neck horizontal, when their passage is easiest. If the armature is turned by hand, it will be noticed that it becomes harder to turn as the neck approaches the vertical position, and if the magnets are sufficiently powerful, a great effort will be necessary. Once vertical, however, the armature hangs, and a still greater effort is required to continue the revolution, for the lines of force have found new paths of low resistance (Fig. 6). Each armature head now forms a bridge between the pole pieces, and the lines of force divide, some going through the upper head and some through the lower. The lines entirely abandon the neck, and in consequence its magnetic field dies out. When these paths are broken by continuing the revolution of the armature, the lines of force again flow through the neck, and its magnetic field is again established (Fig. 7). This action occurs twice during each complete revolution of the armature, and if the armature is revolved by hand, the two points when the neck is horizontal and the movement easy will be very distinct from the hard points when the neck is going over the vertical position.

Fig. 6.

Fig. 7.

In one revolution the neck is twice magnetized and demagnetized; or in other words, the magnetic field set up by the neck as the lines of force pass through it twice changes its strength, being strong when the neck is horizontal and weak when it is vertical. The winding on the armature is affected by this magnetic field, and electric currents are induced in it by these changes in its strength.

The greatest change in the strength of the field occurs as the armature moves into the vertical position, when its heads form bridges between the two pole pieces (Fig. 6). Up to this point the armature neck is strongly magnetized, but its magnetic field dies out as the neck becomes vertical. It is at this point then that the induced current is at its greatest intensity and becomes sufficient for ignition purposes.

In making an armature the channels are wound full of wire, and this is retained against the action of centrifugal force by two or three short lengths of wire bound around the armature and lying in grooves cut in the heads for that purpose. Disks of brass are also screwed to the ends of the armature core, and assist in making it dust and water proof. The magneto must have a base, and this must be of some nonmagnetic metal, like brass, for if it were made of iron it would provide a convenient path for the lines of force, and they would have no interest in passing through the armature. There must also be bearings for the armature shaft, and these are either plain or ball, set in brass plates screwed to the ends of the pole pieces. A zinc or aluminum plate covers the space over the armature and between the upper edges of the pole pieces, so that the armature revolves in a tunnel that is proof against the entrance of dust and water.

There are, of course, two ends to the wire wound on the armature, but the simplicity of a magneto is increased by grounding one end on the metal of the armature, the other end being brought to the single terminal (Fig. 2). The circuit is therefore complete when this terminal is connected to any metal part of the magneto; or, as the magneto is mounted directly on the metal of the engine, to any metal part of the engine or frame of the car. The live end of the armature winding is brought out by means of a metal rod passing lengthways through the shaft of the armature, the rod being insulated from the shaft by means of a hard rubber bushing or tube. The terminal of the winding is therefore found at one end of the armature shaft, and the current flows from this revolving part to the stationary binding post by means of a carbon or steel spring that is kept pressed against the end of this rod.

Magnetos are classified as L. T. (low tension) and H. T. (high tension) according to the current that they deliver, the word tension being used to indicate the pressure or voltage of the current, but more accurate expressions would be primary and secondary magnetos. The magneto already described is of the low-tension type, and is used for the make-and-break ignition system, its winding being so proportioned that at maximum speed it delivers a current of from 100 to 150 volts. What is often spoken of as a high-tension magneto is employed for the jump spark system, a magneto of the type described delivering a current that flows through the primary winding of a secondary induction coil; but this use of the term is erroneous, for while the system delivers a high-tension spark, this is from the coil. The magneto itself is not only of the low-tension type, but its current must be so feeble that the danger of burning out the coil is obviated. A true high-tension magneto has two windings on the armature; one, the primary, consisting of a few layers of coarse wire, over which the very great number of layers of fine wire forming the secondary is wound. This may give a current of from 10,000 to 20,000 volts, and is used for the jump spark ignition system.

These types and their applications will be discussed in the succeeding chapters.