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 (o) 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

the revolving member of the timer turns at engine speed, and should be driven directly from and at the same speed as the crank shaft.

Simple timer forms suitable for one-cylinder motors are shown at Fig. 30. The simplest one, depicted at A, consists of a rocking member of fiber or other insulating material which carries a steel spring that is normally out of engagement with the surface of the cam. When the point of the cam brushes by the contact spring,

Fig. 30.-Simple Forms of Contact Breakers or Timers Used on One Cylinder Engines. A-Wipe Contact. B-Touch Contact.

any circuit in which the device is incorporated will be closed and current will flow from the battery or dynamo to the transformer coils and spark plugs which are depended on to furnish a spark of sufficient intensity to insure ignition of the gas. It is desirable that the member which carries the contact spring be capable of a certain degree of movement, in order that the spark time may be advanced or retarded to suit various running conditions. In the form shown if the top of the casing is pushed in the direction of the arrow, the contact spring will come in contact with the point of the cam which is turning in the direction indicated sooner than

it will if the base member is rocked in a reverse direction and the contact spring pulled away from the point of the cam instead of being moved forward to meet it. The wipe contact form is the simplest, but the spring is liable to wear at the point of contact and may break off and cause trouble. Such a device is more suitable for low-speed engines that it is for those which have high crank-shaft velocity.

The single-cylinder timer depicted at B is a form that is widely used on high-speed engines and contact is made between a pair of platinum contact points which just touch each other instead of wiping. Platinum is a material that is not affected by the arcing or heat of the spark as much as steel or brass would be and provides a more positive contact. In the wipe contact form the continual brushing action of the cam against the spring tends to keep the contact surfaces clean, but this condition does not obtain in the simple touch contact of the form shown at B. The casing is rocked in the direction of the arrow to advance a spark in either case. The form shown at B is more economical of current because the contact is shorter and is more suitable for high-speed engines. While the forms considered prove practical in their application to simple one- and two-cylinder engine forms, they are very heavy or clumsy appliances when used for four-cylinder engines, as it is very hard to assemble the spring elements so that the contact will take place at the proper point in all cylinders.

When a timer is to be used in connection with a four- or sixcylinder engine the compact form shown at Fig. 31, A, is usually adopted. This has many desirable features and permits of timing the spark with great accuracy. The contact segments are spaced on quarters for a four cylinder and are imbedded in a ring of fiber which is retained in a casing of aluminum. The central revolving element carries a lever which has a roll at one end and a tension spring designed to keep the roller in contact with the inner periphery of the fiber ring at the other. The segments are of steel and are accurately machined and hardened, and as the surface of the roller is also hardened, this form of timer is widely used because it provides a positive contact and works smoothly at all engine speeds.

A secondary distributor which is employed to distribute both high and low tension current is shown at Fig. 31, B. This consists of a primary timing arrangement in the lower portion, and a secondary current-distributing segment at the upper portion. The central revolving member carries as many rolls as there are cylinders to be fired, these being spaced at the proper points in the

Fig. 31.-Timers Employed on Four Cylinder Engines. A-Four Contact Device for Commutating Primary Current. B-Combined Timer and Distributor, Directs Both High and Low Tension Energy.

circle to insure correct timing. One primary contact member is screwed into the casing, this contacting with the rolls as they revolve. At the upper portion of the case a number of terminals are inserted from which wires lead to plugs in the cylinders. When a timer of the form shown at A is used, a separate induction coil is needed for each cylinder and the number of units in the coil box and contact points on the timer will be the same as the number of cylinders to be fired. When a secondary distributor