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its way out of the cylinder tends to dislodge any particle of foreign matter which may be present near the spark gap. The same objections apply to this method of mounting as to that illustrated at A.

Some spark plugs have been designed with a view of permitting one to see if the charge is being exploded regularly in the cylinder by some form of transparent material for insulation, so that the light produced by the explosion could be seen from the outside of the cylinder. The simplest method of determining if a spark is occurring regularly between the points is to use some form of spark gap which is interposed between the source of current and the plug terminal. A device of this nature is shown at Fig. 51, G. It consists of a body of insulating material which carries in a glass tube two points, which are separated by a slight air space. The eye or hook end is attached to the plug terminal, while the other end is attached to the secondary wire. If the current is passing between the points of the plug, a spark will take place between the points of the auxiliary spark gap every time one occurs between the points of the plug in the cylinder.

It is claimed that there are certain advantages obtained when a spark gap is used in the circuit, in that the spark in the cylinder is more effective and less liable to be short circuited by particles of foreign matter. At the other hand, others contend that the current must be stronger to jump two gaps than would be required if only the resistance of one was to be overcome. While very popular at one time, the spark gap is of rather doubtful utility and is seldom used at the present time, except as a means of indicating if spark has taken place between the points of the spark plug. It is apt to be somewhat misleading, however, because even if the points of the plug are short circuited and no spark is taking place between the plug points, and yet current is passing to the ground, a spark will continue to take place at the auxiliary spark gap. The device is useful in showing when there is a break or derangement of the wiring or coils.

A form of spark plug having glass bull's-eyes set into the plug shell or body is shown at Fig. 51, H. These simple lenses are made of specially compounded glass, which has a high resistance to heat

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and every time an explosion takes place in the cylinder the light resulting causes a flash which is readily seen through the lens. If the flashing is regular it is safe to assume that the cylinder is functioning properly, but should the flashes be intermittent or separated by unequal intervals of time the cylinder is missing explosions.

It is often desirable to have a water-tight joint between the high-tension cable and the terminal screw on top of the insulating bushing of the spark plug, especially in marine applications. The plug shown at Fig. 51, F, is provided with an insulating member or hood of porcelain, which is secured by a clip in such a manner that it makes a water-tight connection. Should the porcelain of a conventional form of plug become covered with water or dirty oil, the high-tension current is apt to run down this conducting material on the porcelain and reach the ground without having to complete its circuit by jumping the air gap and producing a spark. It will be evident that wherever a plug is exposed to the elements, which is often the case in motor-cycle or motor-boat service, that it should be protected by an insulating hood which will keep the insulator dry and prevent short circuiting of the spark.

Spark plugs are made in infinite variety, more simple forms being shown at Fig. 50. Those in section at A, B and C utilize a porcelain insulator through which a central rod or electrode passes. This terminates at the top in a threaded member, to which the thumb nut is screwed. In most plugs using porcelain insulators a cap is cemented to the top of the porcelain in order to form a seating for the thumb nuts. The form outlined at A is the type of plug most generally used, as it is a simple and effective design. It is easier to clean the points or the interior of the body than in the form shown at B, which has a closed end and which must be dismembered in order to remove the sooty deposit from the insulator surface. The type of plug at C has a very fine wire imbedded in the lower portion of the porcelain, which is in connection with a conductor of heavier material used to transmit the current from the terminal nuts to the fine wire. The theory of action of a plug of this nature is that the fine wire

is not so apt to be short circuited by soot as the projecting electrode forms are, and that the spark tends to clear away material that might short circuit the current by burning it.

The plugs shown at D and E have mica insulators instead of porcelain. When mica is used a sheet of that laterial is wrapped around the central electrode several times, after which a series


Fig. 53.-Conventional Type of Spark Plug at A, Showing Air Gap Be

tween the Points. B—Priming Plugs. C—Two-Point Spark Plug.

of mica washers is clamped tightly together and turned down to form a smooth insulator. The plug at F is the only one marketed using glass insulation. Other plug forms made on the same general principles as that at A use lava or steatite as an insulator instead of the porcelain or mica. For all-around service the porcelain insulator gives the best results, as the mica and lava insulators are apt to become oil soaked and permit the current to short circuit through the insulator and the plug body instead

of jumping the air gap. Another representative form of spark plug showing the proper space between the spark points is shown at Fig. 53, A.

The plug at Fig. 53, B, is one that combines a priming feature and is intended for use in engines of the Ford type in which no provision is made for using priming cups or compression relief cocks. The plug body is formed in such a way that a needle valve fitting may be screwed into it, this being intended to close a passageway communicating from a channel around the top of the plug body to the interior of the plug body. It is said that if this needle valve is opened for a minute or so while the engine is running that there will be a tendency to clear the plug points of any loose oil or carbon. The compression may be relieved by opening the needle valve, and if it is desired to inject gasoline into the cylinder to promote easy starting this may be easily done by filling the channel or groove on top of the plug body with the fuel, then opening the needle valve to allow it to pass to the plug interior. The gasoline will run down the walls and collect around the spark points, where it will be readily ignited by the spark.

Plugs for Two-Spark Ignition.-On some forms of engines, especially those having large cylinders, it is sometimes difficult to secure complete combustion by using a single-spark plug. If the combustion is not rapid the efficiency of the engine will be reduced proportionately. The compressed charge in the cylinder does not ignite all at once or instantaneously, as many assume, but it is the strata of gas nearest the plug which is ignited first. This in turn sets fire to consecutive layers of the charge until the entire mass is aflame. One may compare the combustion of gas in the gas-engine cylinder to the phenomena which obtains when a heavy object is thrown into a pool of still water. First a small circle is seen at the point where the object has passed into the water, this circle in turn inducing other and larger circles until the whole surface of the pool has been agitated from the one central point. The method of igniting the gas is very similar as the spark ignites the circle of gas immediately adjacent to the sparking point, and this circle in turn ignites a little larger one concentric with it. The second circle of flame sets fire to more

of the gas, and finally the entire contents of the combustion chamber are burning.

While ordinarily combustion is sufficiently rapid with a single plug so that the proper explosion is obtained at moderate engine speeds, if the engine is working fast and the cylinders are of

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Fig. 54.—Double Pole Spark Plug and Method of Applying It to Obtain

Two Sparks in Cylinder.

This may

large capacity, more power may be obtained by setting fire to the mixture at two different points instead of but one. be accomplished by using two sparking plugs in the cylinder instead of one, and experiments have shown that it is possible to gain from twenty-five to thirty per cent. in motor power at high speed with two-spark plugs, because the combustion of the gas is accelerated by igniting the gas simultaneously in two places. To

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