It is simply a case A, usually attached to the gear box of an engine, which serves as the journal bearing for a shaft B, which enters at one side, and drives a cam C. Within the case is a spring finger D, attached to a binding post E, and the free end of the spring has an A-shaped contact point F which is designed to enter the V-shaped notch of the cam, as the latter turns. To prevent the A-shaped projection from coming into contact with the cam when the V-shaped portion is opposite, an adjustable screw G is provided, which screws through a bushing of insulating material secured to the case. [Illustration: Fig. 94. Contact Breaker.] The current is through the adjusting screw, spring finger D, and binding post E. By this construction the circuit is broken during the entire revolution of the cam, except when the notch in the cam appears at the A-shaped contact point. THE CONTACT BREAKER.--Compare this with the contact breaker shown in Fig. 94. The case is also provided to receive the end of a journal A, which rotates a cam. In this case the cam B has an A-shaped projection C. This projection comes into contact only momentarily with the anti-friction wheel D on one end of a lever E, which is pivoted midway between its ends to the case. The free end of the lever is normally held out of contact with a terminal F, by means of a spring G. The terminal is insulated from the case. By this arrangement the circuit is closed at all times except during that short period when the point C is in contact with the wheel D. SPARKING PLUGS.--Much of the difficulty of satisfactory running is due to the sparking plug which contains the small points, on which everything in the power system depends. The intense heat generated at that point by the secondary coil tends to destroy them, so that the points should be larger when used with a magneto, and they should be closer together than if used wholly with a battery. TESTING PLUGS.--This is a simple matter, and in so-called engine troubles, this is generally the first thing considered. It should be unscrewed and laid on the cylinder so it is in metallic contact. The character of the spark exhibited, when the engine is cranked, will show whether or not the fault is due to the plug or to the electrical source. If no spark is obtained then the electrical system must be examined. Commence at the battery. When the engine is on the sparking point and the primary switch closed, the terminals of the suspected wires may be touched by a test wire and if a current then flows it will indicate a break at that point. SHORT CIRCUITING FAULTS.--A _short circuit_ is one where the path of the current is from the lead to the return wire at some point between the battery, or source of electrical energy, and the coil or other mechanism which is to be operated by the electricity.