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by referring to Fig. 77A. The revolving brush of the timer is turned by the cam shaft which revolves at half the engine speed. If the switch lever is placed on the storage battery contact, the current from the storage battery will flow through the primary winding of the induction coil only at such times that the circuit is closed by the revolving brush member of the timer coming in contact with the insulated segment on the timer case, and thus closing the circuit so the electrical energy has nothing to impede its passage from the battery, through the timer and primary winding back through the switch and return to where it started from. When the low tension current flows through the primary winding of the induction coil, a secondary current of high pressure is induced in the secondary winding of the transformer coil, and is led to the spark plug where it must leap the air gap between the electrodes and produce a spark before it can return to the winding again through the medium of the metal of which the engine is composed, which thus acts as a common ground for both primary and secondary currents.

Q. What is the function of the switch?

A. The switch is an easily controlled member that permits one to have an open or closed circuit at will. In most ignition systems, two switches are provided, one under the direct control of the operator serves as a master switch, while an auxiliary member commonly termed a "timer" and operated by the engine acts in conjunction with it to produce a closed circuit only at such times that a spark is needed in the cylinder. It will be evident that a closed circuit is obtained only when the members of both switches are in metallic contact. The master switch in most cases serves to couple either source of current to the other appliances of the ignition system. When the engine is once in motion and the timer is closing the circuit automatically, opening the master switch will prevent the electrical energy from flowing, interrupt the spark in the cylinder and stop the engine. The timer which is outlined at Fig. 81 is thus a mechanically actuated automatic switch that closes the circuit only at such times that a spark is needed in the cylinder to explode the gas.

Q. What is the spark plug?
A. As previously outlined, the spark occurs in the cylinders

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Fig. 80.—Diagram Showing Parts of Typical Four-Cylinder High Tension Ignition System Utilizing

Batteries for Current Supply.

(when a high tension system is used) between the electrodes or points of a device which carries them and which may be easily screwed into the combustion chamber so that when the spark points are introduced into the interior of the motor there is no opportunity for the escape of gas.

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Fig. 81.-Roller Contact Timer Used for Distributing Current D

rived from Ford Magneto or Battery to Individual Units of Spark Coil.

Q. How many types of spark plugs are used?

A. Spark plugs are divided into two main types, the simple form as shown at A and C, Fig. 82, and the priming type as outlined at B, Fig. 82, and in illustrations at Fig. 83. The priming type is a combination form which has a small petcock, or valve, through which gasoline may be introduced into the combustion chamber to facilitate starting in cold weather when it is difficult to draw in a proper charge of gas from the carburetor.

Q. Describe important parts of spark plug.
A. Conventional spark plugs are composed of a steel or brass

body having an external thread at its lower end which screws into the cylinder and an internal thread at its upper end in which the gland or packing nut that holds the insulator and insulated electrode assembly in place, is screwed. The grounded electrode usually forms part of the plug body, while the central electrode is invariably kept from metallic contact by means of an insulator of porcelain, as shown at Fig. 82-A, or combination mica and porcelain, as shown at Fig. 82-C. The electrodes are separated by an air gap.

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Fig. 82.–Views Detailing Construction of Conventional Spark Plug.

A-Member With Porcelain Insulator. B_Heinze Priming Cup
Type Utilizes Mica. Insulation. C-Plug Using Combination

Porcelain and Mica Insulation.
Q. What is the proper amount of air gap between electrodes?

A. The electrodes of plugs used with magneto ignition system are usually separated about 1-32 of an inch, while those of a plug intended for battery and coil current may be separated by an air gap of 1-16 inch.

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