exist. When the current reaches its maximum value, because of the armature being in the best position, the cam operates the interrupter and the points are separated, breaking the short circuit which has existed in the primary winding.

The secondary circuit has been open while the distributor arm has moved from one contact to another and there has been no flow of energy through this winding. While the electrical pressure will rise in this, even if the distributor arm contacted with one of the segments, there would be no spark at the plug until the contact points separated, because the current in the secondary winding would not be of sufficient strength. When the interrupter operates, however, the maximum primary current will be diverted from its short circuit and can flow to the ground only through the secondary winding and spark-plug circuit. The high pressure now existing in the secondary winding will be greatly increased by the sudden flow of primary current, and energy of high enough potential to successfully bridge the gap at the plug is thereby produced in the winding.

Dynamo Electric Machines. Two distinct types of mechanical generators are in common use, and while their principles of action are practically the same, they differ somewhat in construction and application. The forms first used to succeed the battery were modifications of the larger dynamo electric machines used for delivering current for power and lighting. Later developments resulted in the simplification of the dynamo, by which it was made lighter and more efficient, and the modern magneto igniter is the form usually furnished on conventional power plants. A dynamo uses electro-magnets to produce a magnetic field for the armature to revolve in, and is necessarily somewhat heavier and larger than a magneto of equal capacity because the field in the latter instrument is produced by permanent magnets. An important advantage in using the magneto form of construction is that the weight of the windings is saved because the permanent magnets retain their magnetism and do not require the continual energizing that an electro-magnet demands.

The dynamo construction is superior where a continual drain is made upon the apparatus, because if a magneto is used continu

ously the magnets are liable to lose some of their strength, and as the magnetic field existing between the pole pieces decreases in value the amount of current delivered by the apparatus diminishes in direct proportion. When electro-magnets are used the constant flow of electrical energy through the windings keeps them energized to the proper point, and as current is continuously supplied, the strength of the magneto field remains constant. The dynamo form of generator is utilized where currents of considerable value are needed, such as in electric lighting systems now so widely used on automobiles.

Where the device is depended upon only to furnish ignition current the magneto is preferred by most engineers because it is simpler and lighter than the dynamo, and also because it may be made in such form that it will comprise a complete ignition system in itself. When a dynamo is utilized the conditions are just the same, as far as necessary auxiliary apparatus is concerned, as though batteries were used, and one merely substitutes a mechanical generator in place of the chemical cells. The same auxiliary apparatus necessary in one case is employed in the other as well..

A dynamo or magneto produces electricity by an inductive action, which is a reversal of the phenomena by which a current of electricity flowing around a bar of iron or steel makes a magnet of it. If a wire through which a current of electricity is flowing will magnetize a bar of iron, a bar of steel which is already magnetized will generate a current of electricity by induction in a conductor surrounding it if either the magnet or the coil of wire. is moved in such a manner that the magnetic influence is traversed or traverses the wire. In a dynamo or magneto a coil of wire mounted on a suitable armature is revolved between the pole pieces of the field magnet and as the conductor cuts across the zone of magnetic influence a current of electricity is induced in the coil. The faster the coil is rotated the more rapidly the winding passes through the magnetic field. As an electrical impulse is produced. every time the magnetic field is traversed, it is patent that the greater number of electrical impulses will produce a current of higher value.

A sectional view of a typical governed dynamo electric machine

of simple design is shown at Fig. 14. . All parts are clearly indicated and there should be no difficulty in understanding the principles of operation. The three main portions of the dynamo are the field magnets, which produce the magnetic field, the armature, which carries the coils of wire and which is mounted between the extremities or pole pieces of the magnet, and the brushes, which bear against segments of a collecting device known as a commutator serving to convey the current to terminals which are joined to the outer circuit. In the form shown the field magnets are

composed of a number of iron stampings which are surrounded by a coil of wire, and two such magnets are provided, one above, the other below, the armature. The armature is supported on a shaft mounted in ball bearings so that it will turn with minimum friction. The whole mechanism is protected by an outer casing.

The device shown is a constant speed dynamo, i.e., it should be operated at a certain speed to obtain the best results. If run faster than the speed for which it is designed the excess current generated is liable to burn out the windings of the field magnet. For this reason a governor of the fly ball type is interposed between the

dynamo armature and the driving shaft coupled to the source o power. At all normal speeds the tension of the governor spring keeps the two plates of the clutch in contact and the armature is turned at the same speed as the driving shaft.

Should the driving shaft speed exceed a certain predetermdu limit the governor weights will fly out by centrifugal force and the

governor spring will be compressed so the driving and driven plates of the clutch are separated and the driving shaft revolves independently of the armature. As soon as the armature speed becomes reduced sufficiently to allow the governor spring to overcome the centrifugal force and draw back the governor weights, the clutch

plates are again brought into contact and the armature is again joised to the driving shaft.

A current of air is kept circulating through the casing by means of the fan action of the reënforcing webs of the clutch plate, the object being to absorb any heat which may be produced while

the dynamo is in action. An appliance of this nature may be driven from the engine by belt, chain, or gear connection (Fig. 15). It will deliver low voltage current which must be transformed by means of an inducction coil to current of higher value in order that it may be successfully utilized to produce the spark in the combustion chambers of the engine.