The spark plug shown here is equally well adapted for high or low tension ignition. It may be made by any handy workman from an old mica plug by taking out the steel wire down the center and putting in its place a piece of brass tubing, 3/16 of an inch outside diameter and 1/8 of an inch inside diameter. A and B are the terminals forming the spark gap, C is the mica, and D the threaded body of the plug. The upper end of the terminal A is riveted to a small valve E, which is pressed down by a weak spring and thereby closes the upper end of the tube, as shown in the sketch. A valve chamber is screwed on the upper end of the tube, thus holding the mica together. The valve operates in the chamber, being fastened thereon by the screw F. The valve and its seat should be ground fiat, and then the terminal A should be riveted to it. There is a small hole G in the cover of the air chamber. The cover should be fastened to the chamber with a screw or other simple means, so that it may easily be detached for the purpose of cleaning the valve. In use the terminal A vibrates rapidly, like the trembler of a coil, and in this way strikes the terminal B. During the compression stroke the compressed gases travel up the central tube, lift the small valves, and instantly fill the valve chamber to the same pressure as the cylinder. With the same pressure on both sides of the valve, it will be forced down by its spring. But as the gases in the chamber C escape through the hole G, the pressures will be thrown out of balance, and the valve will again be raised by the pressure in the cylinder. This action will continue during the entire compression stroke. The hole G should be extremely small, and the amount of gas that will escape during compression will not perceptibly lessen the power of the engine. It will be clear that as the valve is raised, A approaches B and also that A moves away when the valve is closed, because the fulcrum is virtually at the screw F.

Fig. 324 - Sectional view of the spark plug.

As A keeps striking B while it vibrates, the low-tension contact, or touch spark, may be used with this plug. This plug gives many contacts, and therefore many sparks, instead of the single contact given by the usual hammer and anvil. This plug is far simpler than the hammer and anvil system, and besides it does not leak and lose compression like the latter does when worn. It will work with high-tension ignition with a trembler coil. As the terminal vibrates, it keeps itself clean, and an excess of oil or soot will not affect it like an ordinary plug. If used for high-tension ignition, the period of vibration should not be the same as that of the coil trembler, or an odd spark will be missed, owing to A being sometimes in contact with B when the high-tension current is in the act of flowing. The terminal A may be made to vibrate as rapidly or slowly as desired by strengthening or weakening the valve spring