THE SPARK PLUG.--The mechanical device with the separated conductor ends, where the spark is produced, is called the spark plug, and must be located within the cylinder of the engine. The gap is between the separated ends of the conductor within the plug, is usually about one thirty-second of an inch. HOW PRODUCED.--The low tension may be produced either by a primary or a storage battery, or by a magneto designed for the purpose. This requires some consideration of the meaning and construction of a magneto. [Illustration: Fig. 87. Dynamo Connection.] [Illustration: Fig. 88. Magneto.] THE MAGNETO.--This device is simply a dynamo, structurally, but it differs in this respect: What is called the field, or the cores around which the wires of the field are wound, are made of permanent magnets. The ordinary dynamo has merely soft iron, which is demagnetized as soon as the current ceases to flow in the field windings. The permanent magnet cores are made of hardened steel, the same as is done with horse shoe magnets, and others of that class, whereby they are enabled to retain the magnetic charge. A dynamo must have its fields energized. DIFFERENCE BETWEEN DYNAMO AND MAGNETO.--Fig. 87 will give an idea of the difference between the two. In the dynamo the pole pieces A of the field have the ends of their windings B connected to the brushes C, and the circuit wires D for the electric lights are connected with the brushes. On the other hand, the magneto with its field 1 of a permanent magnet, the armature 2 is in a permanent magnetic field, so that the current can be taken directly from the brushes 3 by the wires 4, as in Fig. 88. ADVANTAGES OF MAGNETO.--Owing to the permanent magnetized character of the field, it operates more satisfactory for ignition purposes on an automobile than a regular type of dynamo. The dynamo should be driven at a regular speed, whereas the magneto can be driven at any speed, as it is not self regulating, like the magneto. However, dynamos are used for the purpose, but in that case they are provided with mechanical means for giving them a regular motion. DIFFERENT KINDS OF MAGNETOS.--There are two general types of magnetos; first those which have rotating armature; and second, those with stationary armatures and revolving inductors. The _high tension_ type is provided with a self-contained coil, or it may have a high tension coil separate from the magneto. The low tension magneto has an armature of fairly thick wire, one end of the wire being grounded to the armature core and the other connected with a terminal which is insulated from the magneto. From these two points the current is distributed. In the high tension magneto two coils are necessary, one called the _primary_, and the other the secondary. The primary generates a low pressure current, and the secondary a high tension, and the spark is produced by the latter.