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where the current is to be used, and to accomplish this purpose small pieces of carbon or other substance known as brushes, D, are used which rub on small segments of copper known as a commutator, E, at
tached to the end of the armature and to which are fastened the ends of the wires used in winding the armature. The brushes are so arranged that they touch the copper pieces connected with the proper wires so that a direct current will flow through the outside circuit.
In Fig. 78 is a diagrammatic section of a magneto. Here A and B are the magnets, but in this case they are
permanent, or true, magnets and no shunt current is required; it is mainly in this detail that dynamos differ from magnetos. An electrical field exists between the poles or ends of these permanent magnets, and a
revolving armature C with its windings cuts through the lines of force, producing currents in the wires exactly as in the dynamo. Brushes D and a commutator E are again used in this case to gather the flow of current in one direction. The strength of current generated by the above machines is dependent upon the size of wire, the strength of the magnetic field, the number of turns of
wire, and the speed at which the armature is rotated. For this reason the shunt current of the dynamo increases rapidly as the speed of the machine increases, thereby increasing the magnetism of the electro-magnets and the field so that the strength of current will increase more rapidly in this type with the speed at which it is operated than it will in a magneto with permanent magnets. To obviate this difficulty with the dynamo a governor is supplied which regulates the speed of the armature regardless of the speed of the engine to which the mechanism is attached. This governor is essential where a magneto or dynamo is to be used for ignition in order to prevent burning out the wires at high speed and yet produce a good spark at low engine speed. As a source of current either a dynamo or magneto may be used exactly as if it were a storage or dry battery, or it may be attached to a switchboard and used to accumulate electricity in a storage cell which can then be utilized for ignition and lighting.
Magnetos are divided into two general classes, hightension and low-tension. The low-tension magneto consists of magnets as usual, but with a primary winding only, as in primary coils, and the current is broken at the instant of its greatest intensity. Other low-tension magnetos are fitted with timing devices by which the current can be broken and a spark produced when desired, thus allowing the spark to be retarded or advanced and thus regulating the time when ignition of the charge in the engine takes place.
High-tension magnetos have an armature provided with two windings like the jump-spark coils. These
machines are fitted with a mechanical breaker which produces an induced effect on the secondary winding and therefore no spark coil is required. The Bosch, , Simms, and various other magnetos are of this type. Another high-tension system consists in using a sort of low-tension magneto in which the current is passed through an induction coil fitted with a mechanical breaker. The Splitdorf, Holley, and Eiseman magnetos are of this type, while the Remy magneto has a stationary winding with two revolving inductors, thus eliminating revolving wires, brushes, and moving contacts. The great majority of magnetos in use, however, are of the low-tension type, and these possess the great advantage that their current can be passed through the regular spark coil and thus be switched off or on from the battery circuit at will. Moreover, these machines may be used with advantage for lighting purposes. The Eureka magneto manufactured by the Henricks Novelty Co. is of this type, and this wonderful little machine will ignite the motor charge and operate a total of 36 candle-power electric lights at the same time. This firm also manufactures a number of other magnetos, among them the "Comet,” which is probably the smallest and most compact magneto made that is really practical. Many magnetos are inaccessible and are so complicated in construction that if out of order an
ectrical expert must be called on to adjust them. In the Comet the parts are very few and any one can take down, adjust, and repair one of these machines. The illustration (Fig. 79) shows the few parts and simple construction, while the brush holders can be removed
without wrench or screw-driver, thus permitting examination or cleaning at any time.
A unique variety of magneto which has recently been
Fig. 79.–Parts of “Comet " Magneto showing Accessibility perfected is the “Wico Igniter” made by the Witherbee Ignition Co. In this machine there is no rotary motion, the electrical current being generated by reciprocating,