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ciation. A safety spark gap M is attached to the dust cover Z, the function of this device being to act as a safety valve for the grounding of any excess current, such as might be produced if the magneto was run without having the distributor connected with the spark plug.

The arrangement of the distributor and the contact breaker mechanism is very clearly outlined at Fig. 257. The distributor, which is the upper portion, is employed in commutating the secondary or high tension current only. The contact breaker or lower assembly breaks the primary circuit. One end of the coarse winding of the magneto armature is led to the fixed contact member E by means of the screw plug D, which serves to retain the contact breaker assembly to the armature shaft. The contact breaker casing, which is capable of oscillation but not of rotating, carries the fiber cam rollers W. As the portion F of the contact breaker revolves the toe portion of the bell crank lever G is pushed up and in by the cam roller, this separating the contact point II from the fixed contact point carried by member E. Every time that the contact points separate a current of high potential is sent through the connector member L and the conducting pencil N to the distributing brush N. When the contact points separate this distributing brush should be bearing against one of the segments carried by the distributor block O, this being in turn connected to sockets at the top of the block in which the plug terminals P fit. The bell crank lever G is kept pressed out by the flat spring I, which keeps the contact points together except at such times as they are separated by the cam rollers Y. It will be seen by reference to Fig. 256 that as the distributor brush N is driven by positive spur gearing that it must turn in an opposite direction to the armature and at just half the armature speed on account of the distributor gear R being twice the size of the pinion Q. The brush N is therefore revolved in definite timed relation with a contact breaker assembly F and the armature B. It will take two revolutions of the armature to produce one revolution of the brush N. This means that the contact points separate each revolution of the armature and that the brush N makes contact with two of the distributor segments for each turn of the armature.

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Fig. 257.-Front View of Bosch Magneto Showing Arrangement of Contact Breaker and Distributor Mechanism.

Another form of Bosch magneto which is practically the same in general principles as that just described, except for slight differences in the contact breaker and distributor, is shown at Fig. 258. This is a smaller device, using two single horseshoe magnets, and is intended for small engines up to 30 H.P. The bigger magneto, with its three compound magnets, is more powerful and will

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Fig. 258. Sectional View of Bosch D U-4 Magneto.

produce a hotter spark, such as is necessary to ignite the volume of gas in large cylinders. The Bosch DU-4 magneto contact breaker and distributor are clearly illustrated at A, Fig. 261.

All magnetos do not have rotating windings, three makes, the K-W, Splitdorf Dixie and the Remy utilize a fixed winding and rotary inductor, substantially as shown at top of Fig. 259. The inductor pieces are used to conduct the lines of magnetic energy through the winding and produce the current by cutting the turns

of wire. In the armature shown in the lower portion of Fig. 259 the windings revolve in the magnetic field and generate the current. Another form of magneto which is used on but one make of car, the Ford, but which enjoys a wide distribution, is shown at Fig. 260 in connection with the complete ignition system of the car. Sixteen coils of coarse conductor are carried by a fixed plate,

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Fig. 259.-Remy Rotary Inductor Using Fixed Windings at Top, Conventional Double Wound Armature with Rotating Windings at Bottom.

which is bolted to the engine crank case. A number of horseshoe magnets, not shown in the illustration, are carried by the ends of the flywheel and revolve in front of the fixed coils, the space between the magnet poles and the cores of the windings being just enough to provide clearance without danger of hitting the magnets. Owing to the large number of magnets and coils employed, a very strong current is obtained, which, while alternating in character, is used in the same way as battery current would be

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Fig. 260.-Diagram Showing Ford Magneto Ignition System.

through four individual vibrator coils, which are brought into circuit progressively by the rotary contact timer.

The most popular form of magneto, if one can judge by the numbers of manufacturers using it, is the true high tension type with the revolving winding, though the low tension type using transformer coils have also been used to a large extent.

In case of trouble with a magneto the point to be determined, first of all, is whether the fault is with the current generator, if it is a true high tension form or in the plugs, or in the event of a transformer coil being employed, if that member is at fault. cases where only one cylinder is firing irregularly the fault is very

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