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

Fig. 54.-Double Pole Spark Plug and Method of Applying It to Obtain Two Sparks in Cylinder.

large capacity, more power may be obtained by setting fire to the mixture at two different points instead of but one. This may 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

fit a double-spark system successfully, one of the plugs must be a double pole member to which the high-tension current is first delivered, while the other may be one of ordinary construction.

A typical double-pole plug is shown in section at Fig. 54, A. In this member two concentric electrodes are used, these being well insulated from each other. One of these is composed of the usual form passing through the center of the insulating bushing, while the other is a metal tube surrounding the tube of insulating material which is wound around the center wire. The current enters the plug through the terminal at the top in the usual manner, but it does not go to the ground because the sparking points are insulated from the steel body of the plug which screws into the cylinder. After the current has jumped the gap between the sparking head and the point, it flows back to the terminal plate at the top, from which it is conducted to the insulated terminal of the usual type plug.

The method of wiring these plugs is shown at Fig. 54, B. The secondary wire from the coil or magneto is attached to the central terminal of the double-pole plug, and another cable is attached to the insulated terminal plate below it and to the terminal of the regular type plug. One is installed over the inlet valve, the other over the exhaust valve, if the system is fitted to a T head cylinder. Before the current can return to the source it must jump the gap between the points of the double-pole plug as well as those of the ordinary plug, which is grounded because it is screwed into the cylinder. When a magneto of the high-tension type furnishes the current a double distributor is sometimes fitted, which will permit one to use two ordinary single-pole plugs instead of the unconventional double-pole member. Each of the plugs is joined to an individual distributor, and as but one primary contact breaker or timer is used to determine the time of sparking at both plugs, the ignition is properly synchronized and the sparks occur simultaneously.

Sometimes a spark plug of the special form shown at Fig. 53, C, is used in connection with a regular spark plug of the form shown at A, the special plug being placed first in the circuit and joined to the regular plug by a length of wire bridging the free terminal

of the plug at C with that on top of the insulator of the regular pattern. As the plugs are in series, the current must jump the gap of both plugs and thus two sparks occur, which is said to increase power by accelerating the rate of flame propagation, which of course results in more energetic ignition. The insulator is shaped to form a double V, the sides being slightly concave and

larger than the center V, which ends in a sharp point. This construction is said to cause the point to be self-cleaning by the explosion. Two electrodes pass through the insulating member instead of one, these being insulated from each other and the plug body as well. The high tension current enters one terminal and passes down one of the electrodes, jumps the air gap, and can only reach the ground if the terminal connected to the second electrode is in electrical connection with the terminal of an ordi

nary form of spark plug or if it is bridged down to the plug body by the keeper B. When this keeper is in place, as indicated, the plug will act the same as a single electrode sparker. When the plug is to be used for double ignition in connection with one of the regular forms, the keeper B should be removed and a short

Fig. 56.-Method of Employing Single Vibrator Coil to Fire Four Cylinders when Secondary Current is Distributed Instead of Battery Energy.

wire used to join the terminal to which the keeper was attached to the terminal of the regular pattern spark plug.

Typical Battery Ignition Systems.-The components of typical battery ignition systems may be easily determined by studying the illustrations given at Figs. 55, 56 and 57. The four-cylinder ignition group shown at Fig. 55 depicts the conventional method of wiring two sets of batteries, a four-point timer or commutator, and a four-unit induction coil together. It will be seen that eight dry cells are wired together in series and are used as an auxiliary