The perfection of the magneto and its application to cars of all classes and sizes has marked the most important step in gasoline motor ignition since the introduction of the electric spark. The magneto is now considered one of the most vital parts of the car, and while it is possible for the motor to be run for many miles on the batteries that form the auxiliary ignition sources, the mechanical current generator has left the field of the desirable accessories and has become an actual, physical portion of the engine.

The operation of the magneto is simple, its whys and wherefores are logical, and if one investigates the subject, even superficially, he will discover that the much-maligned machine seldom gives trouble, and that when it does, such action, or failure to act, is due to neglect, abuse, or some other perfectly legitimate reason, rather than "pure cussedness" on the part of the instrument itself. If the mere mechanical aspect is considered; if it is realized that the magneto consists mainly of a bundle of wires which, when revolved near the ends of a magnet, collects that magnetism and sends it through the circuit in the form of the electric current, and that consequently the magneto is a converter that changes part of the mechanical energy of the motor into the spark-forming fluid, the chief idea of magneto principles may be more easily grasped.

To be sure, the magneto is delicate, and for that reason it should never be dissected by the amateur, but inasmuch as what few adjustments it has are readily accessible, it is seldom that the machine need to be taken apart. The platinum points of the contact breaker, usually located in the small box on the end of the armature shaft, may need to be smoothed with emery paper occasionally if they have become pitted from excessive sparking, but this is a simple operation and is not greatly different from the care given to the vibrator of the dashboard spark coil, as described in the preceding chapter.

A few drops of oil should be fed to the lubricating cups or holes of the armature shaft as often as the directions call for—usually about once every five hundred miles—but aside from this, the owner can generally forget that he has a magneto, and will only be reminded of the fact by the pleasing absence of ignition trouble. If ignition trouble does occur, it is more than probable that the fault lies with the plugs, timer, or wires, rather than with the magneto.

The man who drives a magneto-equipped car knows that the current producer is run by a gear connected, either directly or through the medium of other gears with the crank shaft of the motor. He knows, then, that the magneto is driven positively and that there is a constant relation between its speed and the number of revolutions of the motor.

But does he know that it is absolutely necessary that a certain position of the armature shall always correspond with a similar position of the crank shaft of the motor, and that consequently the same teeth of the driving gears must always mesh? He will most assuredly be made aware of this if he disconnects his magneto and then fails to replace the gears so that exactly the same teeth are in mesh, for even the difference of a single tooth between the normal positions of the armature and crank shaft will prevent the magneto from delivering a sufficient spark to enable the motor to run.The reason for this is simple. All of these direct-driven magnetos are of the alternating current type, as this form allows of the simplest construction of armature and windings. The alternating current generator obtains its name from the fact that there are no regularly-defined north and south poles at any part of the circuit, as these keep changing continuously, or alternating.

During each revolution of the armature of the alternating current magneto, there are but two positions at which a current will be formed. Now the spark in any cylinder of a motor is required at about the top of the compression stroke of the piston in that cylinder. Consequently when the piston is at the top of its compression stroke, ready for the spark that will ignite the charge, the armature of the magneto must be in one of its two current-generating positions, and there must therefore be a constant relation between the position of the crank shaft, to which each piston is connected, and that of the revolving part of the magneto.

If, now, the driving gear of the magneto is returned to its place without regard to the teeth of the next gear with which it meshes, it will be seen that the proper relation between the position of the armature and that of the crank shaft will not be maintained. Under these conditions, when the piston is at the top of the compression stroke, ready for the spark, the armature will not be in a position at which a current can be generated, and there can consequently be no spark formed at the plug. Conversely, when the armature has been revolved to the position at which a current will be formed, none of the pistons will be requiring the spark, and this consequent lack of "team work" will prevent the operation of the motor.

In order to maintain this team work between the armature of the magneto and the crank shaft of the motor, the intermeshing teeth of the gears should be marked with a prick punch before they are removed, so that they may be returned to their proper place without trouble. Only in this manner can accurate results be obtained, if it is at any time necessary to remove all or part of the magneto driving gear.

The magnets forming the "fields" of the magneto in which the armature revolves are of the permanent kind; that is, they do not depend upon windings and a separate electric current for their excitation, as is the case with some of the larger generators. These magnets may be considered to be the most faithful part of the machine, for they generally retain their strength under all conditions of rest or work, and it is upon them that the proper operation of the magneto largely depends.

A magneto in which the magnets have become weakened is useless for ignition purposes until the fields can be remagnetized, and as this can only be done at the factory, the machine in its entirety must be removed from the motor. It is a comparatively easy matter to determine whether or not the fields have lost their magnetism by placing a piece of iron or steel within close range of the base or sides of the magneto. An appreciable pull will be exerted by the magnets if they still retain their strength, although it is not to be supposed that the force thus exhibited will be very vigorous from such a small machine.

If the magneto has been disconnected from its driving gear for any reason, the amount of magnetism remaining in the fields will be best determined by turning the armature shaft with the hand. A resistance should be offered to the turning at first until a certain point is reached, after which the armature should exhibit a strong tendency to fly forward to a new position, one hundred and eighty degrees beyond its former normal position of rest. This activity of the armature is one of the best guides to the amount of magnetism remaining in the fields.

Many magnetos that have been installed on old motor cars not previously so equipped are of the friction-driven, direct-current type that produces a uniform spark at any point throughout the armature revolution. Current from these may be used to charge a storage battery for the operation of electric lights or to supply auxiliary ignition current for starting. The positively-driven, alternating-current magneto may also be used to operate electric lights on the car, but this type of current cannot be stored in a battery, and consequently the lights are available only when the motor is running. The magneto, however, is not primarily an electric-lighting outfit, and unless it is especially designed for the double purpose, a separate machine should generally be used for supplying illuminating current.