to use two sets of batteries, six terminals are provided on the bottom of the coil case. Two of these are attached directly to the insulated contact point of the timer; two others which are enclosed in hard rubber insulating caps are attached to the spark plugs. The two immediately under the switch are attached to the free terminals of the battery, two sets being provided, one being coupled to each side of the switch.

With a four-unit coil, as shown at D, ten terminals are provided because of the attached switch. Four go to the spark plugs, four to the insulated segments of the timer and two to the battery, or battery and magneto or dynamo, as the case may be. In modern coils the units may be removed from the box without disturbing any internal connection, and a new one slipped in its place if it does not function properly. Special care is taken in insulating the hightension terminal by means of rubber caps which surround the wire, and care is taken to have the vibrator contact points readily accessible for inspection, cleaning, or adjustment.

Action of High Tension Coil Ignition System.-Another explanation of the action of the conventional induction coil and battery system may enable the reader to obtain a clearer understanding of the action of the transformer coil system of intensifying current and can be read to advantage to supplement the explanation previously given. Another diagram, Fig. 29, shows a four terminal coil unit instead of the three terminal coil diagram outlined at Fig. 25, and differs in that the primary and secondary circuits have separate ground connections instead of having a common terminal on the coil. As the internal construction of the induction coil has been previously described, it will be merely necessary to review the action of the complete ignition system outlined.

In the diagram shown the action is as follows: When the switch E is closed and the rotor (f) of the spark-timing device D comes in contact with the terminal (g), the current flows from the positive terminal (m) of the battery to the switch E. From thence to the primary terminal (h) on the coil; and through the vibrator spring (e) across the points (o) which are in contact, to the adjusting screw (i) and into the bridge which supports the adjusting screw. The primary winding (b) is attached to this bridge at (j) and

the current flows through it to the terminal (k), from which terminal it is carried to the point (g) on the commutator and into the rotor (f). A metal brush takes up the current at this point and it is carried to the negative terminal (n) of the battery, passing through the battery it reaches the point (m) from which it started. The current will not flow unless the circuit is complete,

and it cannot be complete unless the rotor (f) touches the terminal (g). This rotor is so set on the engine and so timed in relation to the movement of the piston that it will complete the circuit only at the time the spark is desired in the cylinder. When the current flows through the primary winding it makes a magnet of the core and enables it to attract the armature, as a magnet will attract pieces of iron or steel, and the armature is made of magnetie material.

The vibrator is composed of a piece of spring steel with a small iron button riveted to the end of it. When the circuit is complete and the core is magnetized it attracts the iron button and breaks the contact of the points at (o), thus interrupting or opening the circuit and preventing further flow of the current. The core then loses its magnetism and the vibrator spring pulls the button back and again brings the points in contact to again complete the circuit. This occurs about one hundred times per second and the rapid vibration produces a pronounced buzzing sound at the vibrator.

When the points (o) are in contact and the core is magnetized a very strong magnetic field flows across the wire of the secondary winding (c). When the field becomes strong enough to attract the vibrator button the circuit is broken and the current stops flowing. As soon as the current ceases to flow and the magnetic field or force becomes reduced in intensity, a strong or high voltage current is produced in the secondary winding. This current flows to the spark plug F from the secondary terminal of the coil (s) and it has sufficient power to jump the air gap at (p), causing a spark. The spark plug construction is such that after jumping the air gap the secondary current will flow back to the engine and from the ground terminal (1) to the terminal (t) and then back through the secondary winding to the terminal (s) from which it started.

The magnetic field dying down has thus produced an induced current in the secondary winding, and in addition it will also set up a self-induced current in the primary winding. As the break in the primary circuit is made at the vibrator points, a large spark would occur there and very soon burn them away. To absorb the extra current which causes this spark a condenser is connected across the points by the wires (v) and (w). When the circuit is opened at (o) the self-induced current of the primary winding flows in the same direction as the original battery current. As the condenser has less resistance than the air gap which this current would have to jump at (o) it absorbs the current, and immediately that the condenser is charged, it discharges. The contact points (c) of the vibrator being separated at this time, the current from the condenser cannot pass through them to get to

its other side, but must travel back through the primary winding in the opposite direction to that in which the battery current was flowing, and thus demagnetizes the core.

As the more rapid the change is made from a strong magnetic field to a weak magnetic field, the higher the voltage will be; this will considerably raise the voltage of the secondary winding and give a much better spark at the spark plug. The condenser, therefore, performs two functions, that of absorbing the undesirable spark which without it would occur at the vibrator points, and of giving a much better spark in the spark plug.

Timer and Distributor Forms.-Anyone familiar with the basic principles of internal combustion engine action will recognize the need of incorporating some device in the ignition system, which will insure that the igniting spark will occur only in the cylinder that is ready to be fired and at the right time in the cycle of operations. There is a certain definite point at which the spark must take place, this having been determined to be at the end of the compression upstroke, at which time the gas has been properly compacted and the piston is about to start returning to the bottom of the cylinder again. Timers or distributors are a form of switch designed so that hundreds of positive contacts which are necessary to close and open the circuit may be made per minute without failure.

When the device is employed to open and close a low-tension circuit, it is known as a commutator or timer, and when used in connection with current of high voltage they are called secondary distributors. Certain constructional details make one form different from the other, and while they perform the same functions they vary in design. Such distributing devices are always driven by positive gearing from the engine and are timed so the sparks will occur in the cylinders at just the proper ignition time. The usual construction is to use a fixed case which carries one or more contact members suitably disposed around its periphery and a central revolving member or cam which contacts with the points on the body of the device to close any desired circuit. On a four-cycle engine the cam is revolved at one-half the engine speed and the timer is usually driven from the cam shaft. In two-cycle engines