is shown at Fig. 85. One system consists of a set of low tension igniter plates mechanically operated from a suitable camshaft, the other method, which is independent, has high tension ignition plugs operated through a timer of the usual form. At the present time where dual ignition systems are provided the usual practice is

to use two high tension systems, one of which will derive its current from a battery and coil, the other which will receive the energy of a high tension magneto. A typical double system adapted for six cylinder engine ignition is shown at Fig. 86. In this two spark plugs are carried in each cylinder, one over the intake, the other over the exhaust valve. A battery timer is mounted close to the dash from which six primary wires go to the

individual coil units of the coil box. High tension wires come from the bottom of the coil to one set of spark plugs. Another set of high tension wires extends from the magneto distributor to the remaining set of spark plugs.

It will be apparent that in both of the systems shown that considerable care is taken to have the wiring carried in an orderly manner and kept out of contact with the metal portions of the

Fig. 87.-Methods of Insulating and Supporting Secondary Cable As

sembly.

cylinder by suitable insulating blocks, usually made of fibre, as at Fig. 86 or Fig. 87, A, or in a fibre-lined metallic conduit, as shown at Fig. 87, D.

A typical double ignition system which has been used on some models of the Locomobile is clearly shown at Fig. 88. The method of running the wires for the primary circuit is very clearly outlined at A. The complete wiring diagram showing the high tension leads going from the magneto distributor to the spark plugs is shown at B. With a system of this kind the current may be derived from a battery which is timed by a primary circuit breaker attached to the magneto contact breaker box and sent through a single unit coil secured to the dash. The secondary current from the coil is led to the center of the magneto distributor, which serves the dual purpose of directing the high tension current from either the magneto armature or the induction coil to the spark plugs in the proper firing order. The usual method of housing the secondary cables in a conduit of insulating material so that there will be no liability of short circuiting due to oil accumulations or to contact with metal parts is so clearly shown at Fig. 87, C, that further description is unnecessary.

The repairman does not generally recognize the fact that the manner in which the high tension cables are led from a magneto or spark coil to the spark plugs is sometimes the cause of misfiring and ignition irregularities which are hard to locate. A spark may sometimes occur in a cylinder in which the piston is going down on its suction stroke which is not due to defective insulation of the wires or to short circuiting, but to an electrostatic action between one wire and a neighboring one through which no current is flowing. Endeavor should always be made to keep the secondary cables as short as possible, as in some cases if a conductor is too long the tendency is toward an unreliable spark. Some ignition experts condemn the practice of running the secondary wires close together in a fiber-lined conduit and recommend the use of fiber cleats secured to supports extending from the engine and provided with grooves that will hold the cables some distance apart.

When individual unit coils are used a condition that often puzzles those who have had no previous experience with it is what is

Fig. 88.-Wiring Diagram of Bosch Double Ignition System Used on Locomobiles Models 38 and 48.

known among old-time repairmen as "bucking," this usually being evidenced on engines of the four or six cylinder forms. The symptom is the same as a premature explosion in some one of the cylinders, this having a tendency to cause the engine to come to an abrupt stop. One is often led to believe that a short circuit ex