[Illustration: FIG. 147 to 161.--Wiring diagrams showing connections of some standard spark coils. Key: B, to battery; C, to commutator or timer; G, to ground (engine frame); P, to plugs; S, to switch. 1, 6 terminal standard non-vibrator coil; 2, 3 terminal standard vibrator coil; 3 and 4, terminal standard vibrator coil; 5, standard double vibrator coil; 6, standard triple vibrator coil; 7, standard quadruple vibrator coil; 8, single dash coil; 9, single dash coil with switch; 10, double dash coil; 11, double dash coil with switch; 12, triple dash coil; 13, triple dash coil with switch; 14, quadruple dash coil; 15, sextuple dash coil.] 3. A half turn of the adjusting screw on a coil will often increase the strength of the current four or five times the original amount, hence, the necessity of carefully adjusting the vibrator. When the adjustment is not properly made it causes, 1, short life of the battery, 2, burned contact points, and 3, poor running of the engine. 4. In adjusting a multi-unit coil, if any misfiring be noticed, hold down one vibrator after another until the faulty one is located, then screw in its contact screw very slightly. 5. The number of cells in the circuit should be proportioned to the design of the coil. If the coil be described by the maker as a 4 volt coil, it should be worked by two cells of a storage battery or four dry cells. The voltage of the latter will be somewhat higher, but since their internal resistance is also greater, the current delivery will be about the same. Most coils are made to operate on from 4 to 6 volts. 6. It is a mistake to use a higher voltage than that for which the coil is designed, because it does not improve the spark and the contact points of the vibrator will be burned more rapidly, moreover, the life of the battery will be shortened. CHAPTER XII THE DYNAMO The dynamo is a machine which converts mechanical energy into electrical energy by electromagnetic induction. The word dynamo is used to designate a machine which produces _direct current_ as distinguished from the _alternator_ or machine generating an _alternating current_. In a broader sense, the word _generator_ is used to denote any machine generating electric current by electromagnetic induction; the term therefore includes both dynamos and alternators.