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Fig. 80.-Diagram Showing Parts of Typical Four-Cylinder High Tension Ignition System Utilizing Batteries for Current Supply.

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(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 Derived 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.

Q. What is the induction coil?

A. The induction coil is a transformer of the "step-up" type which changes the direct current from the batteries, which does not have sufficient pressure to overcome the resistance of the air gap at the spark plug, to an alternating current having higher pressure to jump the air gap and produce a spark.

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Fig. 83.-Types of Spark Plugs That Permit Priming the Cylinder With Gasoline or Acetylene Gas When Combustion Chamber is Not Provided With Compression Pet Cocks.

Q. Name the principal parts of an induction coil.

A. The components entering into the construction of a typical induction coil are clearly indicated in diagram form at Fig. 84. An induction coil consists of a core, primary and secondary windings, vibrator, and condenser.

Q. What is the core made of?

A. The core is composed of a bundle of short pieces of soft iron wire closely bound together.

Q. What is the primary winding composed of?

A. The primary winding is composed of from two to four coils or layers of silk insulated wire about 16-18 gauge such as used to wind around electro-magnets.

Q. What is the vibrator?

A. The vibrator is composed of a hammer, or armature member carrying one platinum contact point and a movable adjusting screw

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Fig. 84.-Diagram Showing Relation of Parts of Induction Coil.

carrying another platinum contact point, the assembly forming an automatic circuit breaker. When the current of electricity from the battery passes through the primary coil it energizes the core which becomes a magnet. The core attracts the hammer of the armature and breaks the contact between the platinum point on the armature spring and that on the adjusting screw. As the battery current must pass through these points before it reaches the primary winding, as soon as the hammer is drawn toward the core the circuit is broken, the core ceases to become a magnet and the armature

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