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on the Westinghouse ignition and lighting generators, but no automatic spark advance feature. It can be used equally efficiently for either direction of rotation without change. The interrupter is enclosed by a spring collar which can be readily removed for

To Spark Plugs inspection or adjustment of the contacts.

FDistributer Brushes The collar makes tight joint and is

Induction Coil clamped by a screw which prevents it from

Interrupter

Contacts slipping. The spark coil is embedded in

Condenser heat - proof insulating material, and the condenser is well insulated. Both are con

Ignition tained in a tube of

Swilch Bakelized Micarta which forms the body of the unit. The distributor is of very simple construction with a wiping brush contact of the same

Bottery type as that used on the ignition generators. It clamps to the upper end of the set. The wiring diagram

Fig. 48.—Showing Internal Wiring of West

inghouse Timer-Distributor and Coil Igniof this system is shown

tion Unit. at Fig. 48. The device is sometimes mounted in connection with a generator when that member is driven by direct gear connection from cam shaft which provides a properly timed drive for the ignition unit. This method of application is clearly shown at Fig. 49.

Ballast

Spark Plug Design and Application.—With the high-tension system of ignition the spark is produced by a current of high voltage jumping between two points which break the complete circuit, which would exist otherwise in the secondary coil and its external connections. The spark plug is a simple device which consists of two terminal electrodes carried in a suitable shell member, which is screwed into the cylinder. Typical spark plugs are shown in section at Figs. 50 and 51, and the construction can be

easily understood. The DISTRIBUTOR COVER secondary wire from

the coil is attached to COMBINED TIMER ZAND DISTRIBUTOR

a terminal at the top

of a central electrode GENERATOR

member, which is supported in a bushing of some form of insulating material. The type shown at A employs a molded porcelain as an

insulator, while that DRIVE

depicted at D uses a SHAFT

bushing of mica. The insulating bushing and

electrode are housed Fig. 49.-Westinghouse Generator with At

in a steel body, which tached Timer-Distributor Coil Unit.

is provided with screw thread at the bottom, by which it is screwed into the combustion chamber.

When porcelain is used as an insulating material it is kept from direct contact with the metal portion by some form of yielding packing, usually asbestos. This is necessary because the steel and porcelain have different coefficients of expansion and some flexibility must be provided at the joints to permit the materials to expand differently when heated. The steel body of the plug which is screwed into the cylinder is in metallic contact with it and carries sparking points which form one of the terminals of the air gap over which the spark occurs. The current entering

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Fig. 50.-Sectional Views Showing Construction of Typical Spark Plugs.

at the top of the plug cannot reach the ground, which is represented by the metal portion of the engine, until it has traversed the full length of the central electrode and overcome the resistance of the gap between it and the terminal point on the shell. The porcelain bushing is firmly seated against the asbestos packing by means of a brass screw gland which sets against a flange formed on the porcelain, and which screws into a thread at the upper portion of the plug body.

The mica plug shown at D is somewhat simpler in construction than that shown at A. The mica core which keeps the central electrode separated from the steel body is composed of several layers of pure sheet mica wound around the steel rod longitudinally, and hundreds of stamped mica washers which are forced over this member and compacted under high pressure with some form of a binding material between them. Porcelain insulators are usually molded from high grade clay and are approximately of the shapes desired by the designers of the plug. The central electrode may be held in place by mechanical means such as nuts, packings, and a shoulder on the rod, as shown at A. Another method sometimes used is to cement the electrode in place by means of some form of fire-clay cement. Whatever method of fastening is used, it is imperative that the joints be absolutely tight so that no gas can escape at the time of explosion. With a mica plug the electrode and the insulating bushing are really a unit construction and are assembled in permanent assembly at the time the plug is made.

Other insulating materials sometimes used are glass, steatite (which is a form of soapstone), and lava. Mica and porcelain are the two common materials used because they give the best results. Glass is liable to crack while lava or the soapstone insulating bushings absorb oil. The spark gap of the average plug is equal to about 1e of an inch for coil ignition and from 164 to 132 of an inch when used in magneto circuits. A simple gauge for determining the gap setting is the thickness of an ordinary visiting card for magneto plugs, or a space equal to the thickness of a worn dime for a coil plug. The insulating bushings are made in a number of different ways, and while details of construction vary,

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Priming
Cup

Steel Body

Stool
Shell

Electrodas

Mica
Insulation

Electrode

Porcelain

Fibre Washer

Wire

Mica

Spring to Allow

for Expansion
and Contraction

Beat French
Porcelain

Spark Plug

Brass Screw

Gland

Gland

Brass Screw

Gland

8park Pointa

Clip

Terminal for -High Tension Wire

Terminal

High Tension

Cable

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Fig. 51.—Group Showing Wide Diversity in Spark Plug Design.

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