may be used in connection with gasoline motor ignition, but at the same time it is important that the elementary principles be considered to some extent in order that the reader should have a proper

understanding of the very essential ignition apparatus. The first point considered has been the common methods of generating the electricity, then the appliances to utilize it and produce the required spark in the cylinder.

Essential Elements of Simple Ignition System.—The current obtained from the dry or storage battery or low-tension dynamo or magneto is not sufficiently powerful to leap the gap which exists between the points of the spark plug in the cylinder unless it is transformed to a current having a higher potential. The air gap between the points of the spark plug has a resistance which requires several thousand volts pressure to overcome, and as a battery will only deliver six to eight volts, it will be evident that, unless the current value is increased, it could not produce a spark between the plug electrodes.

The low voltage current is transformed to one of higher potential by means of a device known as the induction coil. The current from the battery is passed through the primary coil, which is composed of several layers of coarse wire wound around a core of soft iron wire to form an electro-magnet as shown at Fig. 25. Surrounding this primary coil is one composed of a large number of turns of finer conductor. When a current of electricity of low voltage passes through the primary coil, a current of very high electro-motive force is produced in the secondary winding. One end of each coil is grounded. The free end of the primary coil is coupled to the battery while that of the secondary coil is attached to the insulated terminal of the spark plug.

The arrangement of wiring at Fig. 25 is that employed in a typical transformer coil which is used to increase the voltage of the current sufficiently to cause it to overcome the resistance of the air gap at the spark plug and produce a spark which will ignite the gas. In the primary circuit are included a suitable timer for closing the circuit, a battery of chemical cells to supply the energizing current, and a vibrator or make-and-break mechanism on the coil. The secondary circuit includes the spark plug and the secondary winding of the coil.

When the primary circuit is closed by the cam of the timer making contact with the segment, the current from the battery flows

through the primary coil of the transformer. This magnetizes the core which draws down the trembler blade, this in turn separating the platinum contact point of the vibrator and interrupting the current. As soon as the current is interrupted at the vibrator the core ceases to be a magnet and the trembler blade flies back and once again closes the circuit between the platinum points. Every time the circuit is made and broken at the vibrator an electrical impulse is induced in the secondary winding of the coil.

The vibrator may be adjusted so that it will make and break the circuit many times a minute and as a current of high potential is produced in the secondary winding with each impulse, a small spark will be produced between the points of the spark plug. The condenser is a device composed of layers of tin foil separated from each other by waxed or varnished paper insulation. It is utilized to absorb some of the excess current produced between the vibrator points, which causes sparking. This extra current is induced by the action of the primary coils of wire upon each other and by a reversed induction influence from the secondary coil.

If this current is not taken care of, it will impede the passage of the primary current and the sparks are apt to burn or pit the platinum contact points of the vibrator. When a condenser is provided the extra primary current is absorbed by the sheets of tin foil which become charged with electricity. When contact is made again the condenser discharges the current in the same direction as that flowing through the primary coil from the battery and the value of the latter is increased proportionately. There is less sparking between the vibrator points and a stronger current is induced in the secondary coil which in turn produces a more intense spark between the points of the spark plug.

A typical induction coil such as would be used for firing a onecylinder engine if used with a simple timer, or a multiple-cylinder engine if used in connection with a combined timer and distributor, is depicted in part section at Fig. 26. It will be observed that three terminal screws are provided on the box, one designed to be attached to the battery, the other two to the spark plug and ground, respectively. The terminal to which the battery wire is attached is coupled to the bridge member which carries the contact screw while

the vibrator blade is connected with one of the ends of the primary coil. The other end of the primary coil goes to the terminal which is joined to the ground. The condenser is shunted in between the vibrator points, i. e., one of the leads is attached to terminal No. 1 while the other is soldered to the end of the primary coil which goes on the vibrator spring member. One end of the secondary coil is attached to terminal No. 2, which is grounded on some metal

Fig. 26.-Part Sectional View of Simple Induction Coil, an Important Component of All Battery Ignition Groups and Sometimes Used with Magnetos.

part of the chassis frame, while the other end is secured to terminal No. 3, which is joined to the spark plug electrode. After the various components of the induction coil are assembled in the box and the connections made as indicated, the spaces between the sides of the box and the coil member are filled with an insulating compound composed of bees-wax, pitch and rosin. This holds everything rigidly in place and prevents the wire joints loosening through vibration. The external appearance of a one-cylinder box coil of the vibrator type is shown at Fig. 27.

The method of connecting the members of an induction coil,

shown at Fig. 26, is a conventional one, though the connections will differ with the nature of the circuit of which the coil forms a part and the number of units comprising the coil assembly. When such devices are employed for igniting multiple-cylinder motors, the internal wiring is very much the same as though the same number of

box coils for single-cylinder ignition were combined together by outside conductors. The number of terminals provided will vary with the number of units. Various forms of induction coils are depicted at Fig. 28. That at A is a simple unit form in which the coil is attached directly to the spark plug, which in turn is screwed into the cylinder. On this coil but two primary terminals are attached, one being connected to the insulated contact point on the timer, the other being grounded, or attached to the battery.

Fig. 27.-Three Terminal Box Coil for Single
Cylinder Engine Ignition.

Coils of this type have been very popular in marine application because of the simple and direct wiring possible, but they have not been used in connection with automobile engine ignition to any extent. The form shown at B is a simple dash coil for onecylinder use which has three terminals, one being used for a secondary lead to the spark plug, the other two being joined to the battery and ground respectively, as shown at Fig. 26.

The form of coil shown at C is a two-unit member designed for double-cylinder ignition. As the switch is mounted on the coil box