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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

flies back again, thus re-establishing electrical contact. It will be apparent that the circuit can be broken and re-established many times per second, and as each interruption in current flow induces an impulse in the secondary winding, the greater the number of vibrations of the armature the higher the value of the induced secondary current.

Q. How is the secondary winding made?

A. The secondary winding is composed of a large number of coils or layers of very fine, thread-like insulated wire carefully wound and thoroughly insulated from each other. The greater the number of turns of secondary wire in proportion to the number of layers of primary wire, and the greater the relative difference between the size of primary and secondary wires, the higher the value of the induced secondary current in proportion to the pressure of the primary current. The voltage of the primary current may be increased several thousand times by properly proportioning the primary and secondary windings, and at the same time it will have sufficient heating value to explode the gas as it overcomes the resistance of the air gap between the spark plug electrodes.

Q. What is the condenser and how is it made?

A. The condenser is a simple assembly introduced between the adjusting screw and armature spring in such a way that it is in shunt or parallel connection with the contact points. It is composed of two sets or layers of tin foil insulated from each other by layers of wax paper. Alternate sheets are joined together so there is no electrical connection between one set of tin foil sheets and the other. The function of the condenser is to absorb any excess current present between the platinum points that would tend to produce a spark as they separate. This excess current is the result of selfinduction of one coil of the primary winding upon the neighboring one, and would tend to oppose the passage of the current from the battery if it was not absorbed and stored in the tin foil layers of the condenser. If no condenser is provided, the spark between the points would pit them and burn them to such a degree that prompt vibrator action would be prevented because of the uncertainty of metallic contact when the platinum points came together. This

would result in sluggish vibrator action and diminish strength of secondary current.

Q. Describe induction coil action.

A. The diagram at Fig. 85 will assist the reader in obtaining knowledge of induction coil action. Here a solenoid or hollow coil of wire is shown attached to a sensitive form of current indicator. A primary coil, which contains a core of soft iron wire, is depicted as just entering the solenoid. The ends of the primary winding are joined to a simple battery composed of two dry cells. A current of electricity is thus flowing through the primary coil all the time, and the core member is a magnet from which magnetic influence is radiated. When the primary coil assembly is first introduced into the solenoid, the needle of the current indicator will move, thus showing that another current of electricity, other than that flowing through the primary coil from the battery, has been induced in the secondary winding or solenoid. If the primary coil and core assembly is allowed to remain inside of the solenoid, the current indicator needle comes to rest, showing that there is no current induced in the solenoid. If the primary coil be withdrawn from the interior of the secondary winding, another movement of the current indicator needle in the opposite direction to that which it moves when the primary coil was introduced will show that another impulse or induced current has been produced by withdrawing the primary coil as embly. If the primary coil could be thrust in and out of the secondary coil sufficiently fast, practically a continuous flow of alternating current would be induced in the solenoid.

The induced current is produced by magnetic influence from the core, and obviously if the primary coil was allowed to remain in the secondary coil and the electric current from the battery interrupted and the flow re-established often enough, we would have practically the same effect as though the primary coil was thrust in and out of the secondary coil a number of times corresponding to the making and breaking of the primary circuit at the batteries. It will be apparent that the induction coil assembly outlined at Fig. 84 is practically the same as that shown at Fig. 85, except that the primary coil is a fixed member inside of the secondary winding and the number of current interruptions through the primary coil is obtained

by means of a very rapid automatic magnetic circuit breaker or vibrator, the action of which has been previously considered.

Q. Describe timer construction.

A. The usual form of timer as outlined at Fig. 81 is composed of a fiber insulating ring which carries a number of metal contacts which are out of electrical connection with each other and with the metal body of the timer. The rotary contact member revolves in the interior of the insulating fiber ring and makes contact with the segments as it revolves. Each segment communicates with one of

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Fig. 85.-How Current is Induced in Secondary Coil by Varying Flow of Energy Through Primary Coil.

the units of an induction coil and the number of segments and terminals used is determined by the number of cylinders to be fired. The device shown is adapted for a four cylinder motor.

Q. What is the difference between a single cylinder engine timer and one for multiple cylinder ignition?

A. The difference is only in the number of insulated contacts. The timer intended for the multiple cylinder engine having the greater number. A one cylinder timer has but one insulated contact segment and one external primary terminal. As the number of

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