Fig. 94. Diagram of High Tension Coil. ] When a magnetic wave threads or spreads through the turns of a coil of wire, a current of electricity is generated in the coil, the quantity and pressure or voltage of which is proportional to the intensity of the magnetism, and to the number of turns of wire in the secondary coil. Thus it will be seen that at every make and break of the low tension current in the primary coil, a current is generated in the secondary. As the voltage generated in the secondary is roughly proportional to the number of turns in the secondary, and as there are many thousands of turns, it is evident that the voltage in the secondary will be very high. Thus by the use of the induction coil, the low tension battery current is transformed into a high tension current of sufficient voltage to break down the high resistance of the spark gap. The condenser is shown at L which has one wire leading to the vibrator spring A, and one wire to the contact screw M. The function of the condenser is to absorb the spark produced at the vibrator points so that the break is made quickly, producing a maximum spark. The intensity of the spark depends upon the quickness with which the primary current is broken, and if it were not for the condenser the length and intensity of the spark would be greatly reduced. This device consists of alternate layers of paper and tin foil, every other leaf of foil being alternately connected to the vibrator spring and to the contact screw. A method of using two independent sets of battery is shown in the diagram, so that either set may be thrown into circuit by means of the double throw switch O. When handle J is in contact with E, the current of battery set H flows through the coil as shown by the arrows. When J is in contact with F, the battery C is thrown into circuit. The spark gap is shown by X, which represents the spark plug in the cylinder. In practice, the portion of the circuit shown by I-U is generally formed by the frame of the engine, or is grounded. The terminal P of the high tension circuit is always grounded through the threaded shell of the spark plug, the grounded circuit being shown by the dotted lines. Grounding saves wire and many connections, for with P and U connected to ground it follows that one binding post will serve the place of one high tension and one primary post, making three coil connections instead of four. In order that the spark will occur in the cylinder of the engine at the proper time, a switch must be placed in the primary circuit of the coil, that will open and close the circuit at proper intervals. Such a switch is called a timer, and is always driven by the engine. The timer is connected to the engine shaft in such a way that contact is made at, or slightly before, the time at which the explosion is required, and as soon as possible after spark occurs the current is cut off.