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in the make-and-break devices become separated, a current of high potential passes through the wires attached to the spark plug and produces a spark between the points. The magneto is the simplest and most practical form of ignition appliance as it is self-contained and includes the current generator and the timing device in one unit. In the one-cylinder form shown all connections are made inside of the device and but one wire leading to the spark plug is necessary to form the external circuit.
Fig. 92.—Simple High Tension Magneto for One Cylinder Ignition, a
Complete Apparatus Comprising Source of Current and Timing Device as well.
A magneto employed for multiple-cylinder ignition is not much more complicated than that used for single-cylinder service, the only difference being that a different form of cam is provided in the breaker box and that a secondary distributor is added to commutate the current to the plugs in the various cylinders. The distributor consists of a block of insulating material fixed to the magnets which carries as many segments as there are cylinders to be fired. A central distributing arm or segment is driven from the armature shaft by means of gearing, and is employed to distribute the high-tension current to the spark plugs. The spacing of the distributor segments does not differ materially from that of the battery timers previously described.
Various distributor forms used on magnetos are shown at Fig. 93. That at A is employed for a double opposed cylinder motor and the contacts are separated by a space of 180 degrees. When
Fig. 93.—How Distributor Contacts are Spaced on Two, Three, Four and
Six Cylinder Magneto.
a three-cylinder engine is used, as is sometimes the case in the two-cycle forms, the distributor segments are separated by distances of 120 degrees. If the distributor is used on a four-cylinder motor the segments are spaced 90 degrees apart, as shown at C. To fire a six-cylinder motor, six segments must be used and they
Fig. 94.—Partially Dismantled Four Cylinder Magneto Showing Important parts of Current Producing and
are placed 60 degrees apart, as indicated at D. The speed at which the armature of the magneto turns also varies with the number of cylinders. One- and two-cylinder forms turn at cam-shaft speed. The three-cylinder types when applied to a four-cycle engine turn at three-quarters the crank-shaft speed. The four-cylinder magneto armature is driven at crank-shaft speed, while that of the sixcylinder forms turn at one and one-half times crank-shaft speed. When used on two-cycle motors, the speeds given for four-cycle engines of the same number of cylinders should be doubled.
The important parts of a four-cylinder form of high-tension magneto are shown at Fig. 94, which is a view of a partially dismantled device. The armature assembly and one of the end plates by which it is supported are shown at the extreme left. Attached to the end of the armature shaft are the distributor drive pinion and the ebonite spool which carries the collector ring. The timer case and interrupter assembly are shown at the extreme right. Above it the distributor case is clearly depicted. When the device is assembled the end of the armature shaft protrudes through the housing at the lower part of the magnet assembly which is shown in the center of the group, with the end plate which carries the distributor gear and disk and one end of the armature in place. The distributor gear serves to drive a hard rubber plate in which the distributor segment is imbedded. When the distributor case is screwed in place, the carbon brushes, which are spaced around the interior of the distributor case, collect current from the revolving distributor segment and lead it to the spark plugs by suitable cables which run from the terminals at the top of the distributor casing.
Two systems of high-tension magneto ignition are used, one termed the true high-tension system, in which a current of high potential is delivered directly from the armature; the other is the transformer coil system, so termed because the current produced by the armature winding is of low tension and must be stepped up or increased in value before it is delivered to the spark plug by an induction coil similar in construction to that needed in battery. ignition systems. In the former apparatus the high-tension current is produced by means of a secondary winding on the armature itself, and as the whole apparatus is self-contained it is much more