is created every time a loop of wire passes through the magnetic field.

Oscillating Armature Forms.-The armature of a magneto is not always a revolving member except when this device is used to produce a continuous flow of current. In some ignition systems a single electric impulse is all that is needed to produce the spark. A number of low tension magnetos are made on the oscillating armature principle. One of these is clearly shown at Fig. 70. Instead of the armature being a revolving member it is an oscillating member and does not make a complete revolution. In the device shown the armature is replaced by an inductor member which carries no wire and which oscillates freely between the pole pieces of the magnets. The form depicted is really a unit with the igniter plug and is adapted to be bolted to the side of the cylinder in such a way that while the igniter points project into the combustion chamber, the device is in a position close to the cylinder head where it may be readily operated by means of a push-rod and trip from the engine cam shaft.

The device is so arranged that when a spark is desired between the ignitor points, the inductor is given a quick movement which produces a flow of magnetic energy through the windings. At the same instant that the flow of electricity takes place through the coils, the ignitor points are separated and a spark is produced in the cylinder. The action of a device of this character is automatic, and each time that the magneto pushrod trips the inductor the points of the ignitor separate and a spark is obtained to ignite the compressed gas.

It is apparent that this method of current generation will take but very little power from the engine and that there can be no depreciation that will tend to

diminish the amount of current at a time that the engine needs a hot, fat spark. This device forms a complete ignition system and is suitable for practically all stationary engines of the single cylinder or two cylinder patterns. It would not be advisable to furnish this form of magneto for more than two cylinders because a separate device is needed for each cylinder and the mechanical complication incidental to the use of four of these devices with the trip rods, etc., would not be desirable.

FIG. 71.-Revolving Armature Magneto Delivering Low Tension Current. Used in Connection With Induction Coil, Timer and Spark Plug to Form Complete Ignition System. Type With Revolving Armature.-The revolving armature type of magneto is the form that is generally used for supplying current whenever a constant flow

is desired. When a device of this nature is intended to replace batteries it is of the low tension direct current form, as shown at Fig. 71. The magnetic field is produced by a pair of permanent magnets attached to pole pieces. between which the armature revolves. The electricity produced in the coils of wire in the armature is collected by means of two brushes which form the terminals corresponding to the positive and negative poles of a battery. As a device of this character is intended to be driven at a constant speed, an automatic governor is usually provided which becomes inoperative when the safe speed of magneto rotation is exceeded.

The form shown is intended to be driven by means of a friction drive pulley bearing against the flywheel. This pulley drives the armature by frictional contact between the governor arms and the outer tapered face of the drive pulley.. This member is faced with fiber or leather where it bears against the engine flywheel in order to secure greater frictional adhesion. The governor arms are carried by a fitting which is attached to the armature shaft, while the friction drive pulley is free to rotate on the armature shaft except at those times when the governor arms are in contact with its face. When the speed increases to a point where the governor arms will fly out because the centrifugal force is greater than the tension of the coil springs holding them in contact with the pulley, the drive is interrupted, and the friction drive pulley turns at one speed while the armature, which now revolves only by its own momentum, tends to slow down. The armature speed finally slackens to a point where centrifugal force is not strong enough to keep the weighted governor arms out of contact with the friction drive pulley. The tension springs then act and the armature is driven from

the friction pulley until the safe speed is again exceeded at which time the driving connection will be automatically broken again. The device shown is wired into the circuit just as the set of batteries would be and an induction coil and timer will be needed to complete the ignition system.

High Tension Device.-The high tension magneto is the form that is generally used in automobile ignition systems and its popularity is increasing among gas tractor manufacturers as well. The advantages of the true high tension magneto are that it comprises in one device all the elements of the current generating and intensifying devices and all that is needed in connection with a high tension magneto are the spark plugs and the wires by which they are connected to the instrument. A high tension magneto for a four cylinder engine is but very little more complicated than one used on a two cylinder power plant. The only difference is in the number of contacts in the distributor and the speed at which the device is driven.

A typical high tension magneto utilized in connection. with a four cylinder engine is outlined at Fig. 72 and all the parts are clearly shown. The armature is a two pole type having an approximately H section and it is wound with two coils of wire. One of these is a comparatively coarse one corresponding to the primary winding of an induction coil, while the other is a fine winding having many turns that correspond to the secondary coil. The armature shaft is mounted on ball bearings to insure easy rotation. The magnetic field is produced by means of three pairs of horseshoe magnets which are attached to pole pieces which form the armature tunnel. Mounted on and turning with the armature is a condenser which is placed in shunt con