IGNITERS.--In the low tension system an igniter must be placed in the head of the engine cylinder which will mechanically make and break the circuit; but in the high tension device a spark plug is available, the points of which are stationary and in close contact with each other. For the foregoing reasons, therefore, while the low tension is very simple so far as the wiring is concerned, the mechanical devices necessary to make and break, are somewhat difficult and complicated. The high tension wiring is much more complex, but it has the advantage that no mechanism is necessary in the engine except the spark plug. HIGH TENSION COILS.--Before proceeding to an explanation of the systems referred to, we shall explain the action and operation of the high tension coils. These coils depend for their action on what is called _inductance_. Suppose two wires lie side by side, but not touching each other, and a current of electricity is sent through one of these wires, which we will call the primary, the other, called the secondary, will take a current from the primary. If the wires are the same size, and of the same material, the current in the two wires will be of substantially the same _potentiality_. By this is meant that they will have the same amperage and voltage. INDUCTANCE.--But assuming that the primary wire is larger than the secondary, then the current carried by inductance across the space between the two wires will be changed in the secondary so that it has a larger voltage, but a correspondingly lower amperage. This is what high tension means. The convenient way to arrange these wires parallel with each other, is to wind the two different size wires on the same core, in which the coarse wire, which forms the primary, is first wound around the core, and on this is wound the fine wire. CONSTRUCTING A COIL.--Such a coil is shown in section in Fig. 89, in which the core A is a hard rubber or fiber tube, with disk ends B of the same material. The primary wire C is large in cross section, and carefully insulated. The opposite ends are brought out through the disk heads, and run to the generator, that is, the battery or dynamo. The fine wire D, which constitutes the secondary winding, is also of insulated wire, wrapped over the primary, and its ends are connected up with the sparking mechanism, as will be more fully explained hereinafter. [Illustration: Fig. 89. Induction Coil.] A SIMPLE HIGH TENSION SPARKING SYSTEM.--With such a coil, of proper size, and adapted to receive the required current, several things are necessary in order to produce a sparking effect.