If the fuel, lubricating oil, and air are not supplied in proper proportions, soot will be deposited on the lower surface of the porcelain, and as soot is an excellent conductor of high tension current, the current will follow the soot rather than the high resistance of the spark gap, a condition that will result in misfiring or a complete stoppage of the motor. Carbonized lubricating oil or moisture have the same effect. Preventing the deposits of soot, moisture and carbonized oil is the chief object of plug manufacturers, many of whom have brought out designs of merit. In fact the problem of elimination of soot is the principal cause of the many types of plugs now on the market. While many plugs differ in minor refinement of detail from the typical plug shown, the connections and general construction are the same in all types, the spark being produced in a gap of fixed length which is insulated from the cylinder. A well known form of plug, the Bosch, is shown by Fig. 99 a-b. In this plug a special material known as Steatite is used instead of the usual porcelain. The three external electrodes surrounding the center electrode is a particularly efficient arrangement, especially for magnetos. A peculiar form of pocket minimizes the soot problem. As porcelain is brittle and is easily broken by the effects of heat or blows, mica insulation is often used in place of the porcelain. The central core of a mica plug is formed by a stack of mica washers, which are held in place by the central electrode and the upper lock nuts. A poorly constructed mica plug is easily destroyed by a weak, stretching, electrode, or by an overheated cylinder. The latter causing the washers to shrink and admit oil between the layers of mica washers causes a short circuit. As soon as the mica washers loosen and separate, they should be forced together by means of the mica lock nuts on the top of the plug. If by any reason the mica core becomes saturated with oil, it is best to obtain a new one, as it is almost impossible to remove the oil by simple means open to the average operator. The chief value of a mica plug lies in its toughness and mechanical strength, a good mica plug being practically indestructible. When heated, porcelain does not expand at the same rate as the metal sleeves, hence in poorly designed or imperfect plugs, heavy strains are thrown on the delicate porcelains which causes them to crack. When a crack develops it provides a lodging place for soot and carbon which of course causes a short circuit. Should a compression leak occur through faulty packing between the porcelain and sleeve, it should be immediately tightened up for eventually it will leak enough to destroy the plug or reduce the output of the engine.