It was the application of the electric current to the ignition system of the gasoline engine that first enabled these new forms of power plants to be designed with sufficient compactness and to possess enough flexibility to render their use practical on self-propelled vehicles. Without the electric ignition system, the speed and power of the vehicle could not well be controlled, and the explosions would be uncertain and irregular, at best.

Those of us who are familiar with the electric gas lighters that were in popular use a few years ago are furnished with a convincing demonstration of the operation of the first electric ignition systems. By pulling a chain, a wire, or arm was rubbed across a metal point until the contact thus formed was suddenly broken. This arm and the stationary point formed the two terminals of an electric circuit, which caused a flash of blue flame when the contact was broken as the one was "wiped" across the other. The flame thus formed at the instant the contact was broken contained sufficient heat to ignite the gas escaping from the burner to which the device was attached.

Sparks will be formed in the same manner if we hold two wires, connected to the opposite poles of a set of batteries, in both hands and wipe the bare ends across each other. If an arrangement producing this effect is introduced into the gas engine cylinder at the portion in which the charge is compressed, the flash resulting when the terminals are separated will serve to ignite the explosive mixture. The movable terminal is connected to a rod which passes through the cylinder walls and is attached to a mechanism actuated by a cam revolved by the engine. This mechanism is termed the "make-and-break" ignition system for the reason that contact of these terminals is alternately made and broken to produce the flash of electricity that explodes the surrounding charge.

In order to produce a flash of sufficient size when the contact is broken, the nature of the current, obtained from the dry cells or storage battery is changed somewhat by conducting it through a coil of wire surrounding a bundle of bare copper wires. This is known as a spark coil, and while it is generally used with battery ignition of the make-and-break type, magnetos may be designed which produce the proper kind of current direct, without the aid of the coil.

An ordinary set of six dry cells, connected in series—or like with unlike poles—will produce a current of between twenty and twenty-five amperes at a pressure of about nine volts—assuming each battery, when new, to deliver twenty-five amperes at a pressure of one and one-half volts. The "series" wiring gives the entire set the combined voltage of all with the average amperage of one. For the benefit of those who have forgotten their elementary physics, let it be remembered that the ampere is the measure of current amount, or flow, while the voltage is concerned only with the pressure of the current. By the use of various arrangements of windings of wires, the voltage may be raised with a corresponding decrease in the amperage—and vice versa. Thus, if a coil is used that doubles the original number of amperes produced by the battery, the voltage will be halved.

The make-and-break type of ignition has been used successfully for many years, but with the perfection of the magneto, it has been largely supplanted, in automobile practice, at least, by the "jump spark," or "high-tension" system. Because of the fact that the latter system is less expensive to construct and is highly efficient, it will be found also on the majority of the older cars not equipped with a magneto.

It was found, after the general adoption of the make-and-break ignition system, that a flame was not necessary for the combustion of a properly-mixed charge in the engine cylinder. In fact, a tiny spark, scarcely one-sixteenth of an inch long and no larger around than a pin, was discovered to be sufficient to produce the ignition of the charge. Although, of small volume, such a spark generates intense heat, and it is upon this quality, rather than upon area, that the charge depends for its ignition—although it is claimed that a large flame will produce more complete, rapid, and consequently more efficient, combustion. But the jump spark possesses the advantage of requiring no moving parts projecting through the cylinder walls into the combustion chamber, and its greater simplicity over that of the make-and-break system has resulted in its almost universal adoption by automobile manufacturers.

It has been stated in a preceding paragraph that the voltage produced by the average battery set will not exceed nine or ten, and even the pressure generated by the ordinary magneto is not greater than this. But air is not a good conductor of electricity and forms a very high resistance to the passage of a current. It is only when the high resistance of an air gap is encountered in its circuit, however, that a spark will be formed by the current, and consequently the form of electricity used in this system must have resistance-overcoming properties. But it is only by raising the voltage of the current that even a short air gap can be bridged by the spark. In fact, a pressure of somewhat over fifty thousand volts is required to produce a spark less than an inch long in the air.

Although only called upon to jump a gap about a sixteenth of an inch across, the ordinary high-tension current is capable of bridging a space eight or ten times this width in order that ample pressure will always be assured for the formation of the spark. Furthermore, the warm gases in which the spark is formed in the cylinder increase the resistance ordinarily encountered and it is consequently necessary to raise the voltage above the amount that would be needed were the plug exposed to the open air.These conditions make advisable a pressure of from twelve thousand to thirty thousand volts in the ordinary jump spark system, and it is from this voltage that the term "high tension" is obtained. The nine or ten volts delivered by the batteries are transformed to this larger amount by means of an induction coil—or what is more generally termed merely the "coil." This is in reality a "step-up" transformer, since it transforms the current from one of low voltage to another of two or three thousand times its original pressure.

This transformer consists of two coils of wire, one surrounding the other. The inner coil is composed of a comparatively few number of turns of rather coarse wire wound around a soft iron core, and is termed the "primary" winding, since the current from the batteries is led directly through it. The outer coil is composed of many turns of a very fine wire, all of which are thoroughly insulated from each other and from the inner winding. This outer coil is termed the "secondary" winding and is the one from which the high-tension, or transformed, current is taken.

This secondary current is "induced" from the primary winding through which the battery current passes and possesses a voltage that has increased over its original amount in the same proportion that the number of turns in the secondary winding bears to those in the primary. Therefore, if the original battery voltage is ten and there are a thousand times as many turns in the secondary winding as in the primary, the resulting high-tension current will have a pressure of ten thousand volts.

The principle of the coil is dependent entirely upon that peculiar electric property known as "induction." Around every wire through which an electric current passes are invisible "lines of force" similar to those that emanate from an electro-magnet. These lines of force surround the wire throughout its length, and arrange themselves in a spiral formation. Insulation has no effect on these lines of force, and they may be collected from wires which are separated from each other by several thicknesses of current-confining material. It is, of course, necessary to use insulated wires in the construction of these coils, for otherwise the current would merely pass to adjoining turns and would not travel the entire length of the winding—and therefore as great a number of lines could not be collected.

If an additional layer or layers of wire is wound around the first series of turns, the lines of force will be collected, or "induced," by this second coil, and will constitute the secondary current. The induction effect is greatly increased if the primary current is allowed to accumulate, or "pile up," and discharge, alternately, for this surging of the current creates a sort of "overflow" from the original containing wires.

Ohm's Law, which states that the number of amperes in an electric circuit is equal to the voltage divided by the number of ohms of resistance encountered, shows that the current will be changed by its passage through the primary winding. The induced current is further changed, and when collected by the secondary winding and sent through its long coils, we have the high-tension circuit mentioned in the preceding paragraph.

If the reader remembers that it is but one hundred and ten volts that is used to operate our electric lights and that five hundred will run a trolley car, he may wonder why it is not dangerous to handle as great a pressure as the thirty thousand volts that are used in connection with the ignition system of a motor car. But it is the combination of great voltage with high amperage that is dangerous, and if it is remembered that, as the former is increased, the latter is reduced correspondingly, it will be realized that the ordinary high-tension ignition current possesses a quantity, or flow, of scarcely one one-hundredth of an ampere.

If we liken the electric current to a flow of water in a pipe, we have the amperes corresponding to the quantity of the flow, or the number of gallons that will be delivered at the outlet in a given time. Continuing this analogy, the voltage of the electric current will be the pressure, or "head" in the water system, and the current from the batteries before the coil is reached will correspond to a moderate flow of water at a comparatively low pressure. After the coil has transformed the current to the high voltage, we have the conditions of a very small opening in the water pipe containing a tremendous pressure. Such a stream will possess but small flow, but its high pressure will enable it to be "squirted" to a far greater distance than would be the case were its volume larger and its "head" less. Although the pressure is high, its quantity is so low that the stream can do but little damage and would scarcely more than tickle the flesh of a person against whom it is directed.

Thus it is with the ignition current. It can "tickle," rather viciously, sometimes, as many persons will aver, but the amount of electricity involved is so slight as to render the high pressure harmless. Nevertheless, it is well to avoid allowing the fingers or the arm to become a part of the high-tension circuit, for the result may be startling as well as annoying.

But in order that the high voltage shall be induced in the secondary coil, the primary circuit must be alternately made and broken between one stroke and the next. Consequently proper "piling up," or "surging," of the current will be effected. This is accomplished by means of an "interrupter" that either vibrates rapidly or "snaps" once at the formation of each spark. The former is the more common type used with battery ignition and is known as a vibrating coil. A circuit breaker is generally incorporated in the mechanism of a magneto, and consequently when such an instrument is used, the vibrator on the coil is dispensed with. It is the vibrator on each coil that forms the "buzz" that can be heard whenever the box cover is removed, and that often furnishes a simple test for determining the condition of the ignition system of the particular cylinder with which that coil is connected.

The vibrator is a flat, spring steel piece that rests near one end of the soft iron core around which the primary coil is wound. The springy nature of the vibrator ordinarily holds it against a small, adjustable contact point that should be set about an eighth of an inch from the end of the above-mentioned soft iron core. The primary coil is so wired that its current passes through the vibrator steel and the contact point against which it rests. As soon as the current travels through the coil surrounding the soft iron core, however, the latter becomes magnetized and draws the steel vibrator toward it. This breaks the circuit, the magnetism of the iron core disappears, and the vibrator returns to its original position against its contact point. But this action again forms the circuit, and the same operation is repeated as long as the current is allowed to flow toward the coil.

This is the same principle on which an electric bell is rung, but the vibrator of the coil makes and breaks the circuit much more rapidly on account of the less weight of the moving parts. This vibration of the coil interrupter is so rapid—hundreds a second probably—that the resulting spark is practically continuous and shows no effect of the breaks in the circuit.Even though it is the primary current, of low voltage, that is interrupted by the vibrator, the frequency of these interruptions causes a slight sparking, or arcing, at the contact points. These are therefore subjected to rather a high degree of heat, as well as a large amount of wear, and it is necessary that they be made of a material that will resist both. Platinum has been found to be unusually suitable for this purpose, but owing to its high cost, only a small amount in the form of two points, or "buttons," is used. One of these points is placed in the vibrator steel, and the other is embedded in the end of the screw against which the first rests. Thus the actual contact is made against these heat-and-wear-resisting platinum points, and it is evident that upon their proper action depends the formation of the spark in the cylinder with which that particular vibrator is connected.

Notwithstanding the fact that platinum possesses high heat-resisting properties, the constant arcing at the contact points will eventually form a sort of corrosion in which minute particles of the material are carried from one point to the other in the direction in which the current flows. If the current is reversed, the corrosion will take place in the other direction, and consequently the platinum point that formerly lost a part of its material will gradually be "built up" again. This corrosive action is known as "pitting," and while it may be reduced to a certain extent by reversing the terminals of the battery, as described, the platinum will occasionally require additional attention.

A coil having badly pitted contact points on the vibrator will "stick" and will cease to form a spark regularly. It is often difficult to distinguish between trouble arising from badly-pitted contact points and that caused by weak or nearly-exhausted batteries, as either ailment produces the same symptoms of irregular running and "jerking" in the motor. For this reason, a volt and ampere meter for measuring the pressure and amount of the current delivered by the batteries should form a part of every automobile owner's tool equipment.

It is the amperage, rather than the voltage, that is reduced through continued use of the batteries, and when this quantity falls below nine or ten, the cells should be discarded—or recharged, in the case of a storage battery. But if the ignition occurs irregularly when the batteries are delivering the proper amount of current, it is probable that the trouble lies in the pitted condition of the platinum contact points of the vibrator of the coil. Fine emery cloth rubbed over the surfaces of contact should serve to remedy matters. It should be made certain that the resulting surfaces on the platinum points are not only rubbed smooth, but level, as well, in order that the entire area of each will rest in contact and the current will not be concentrated at a small portion.

It is probable that there will be a screw adjustment on the vibrator by means of which the force with which the latter rests against its contact point may be regulated. If the vibrator is set too tight, an undue amount of current will be required to magnetize the core of the coil sufficiently to pull the vibrator away from its contact point, and the batteries will soon "run out." On the other hand, the tension of the vibrator should be sufficient to enable it to spring away from the core of the coil as soon as the circuit is broken, for otherwise the vibrator will lag and will not be as "lively" as is necessary to obtain the best results.

The contact screw should be set so that the vibrator rests about three-thirty-seconds of an inch from the end of the magnetic core. After the tension of the vibrator has been set to approximately the proper amount, the ear must be trusted for the correct adjustment of the contact screw. When the switch is thrown on and the motor turned until current flows through the coil, the resulting buzz emanating from the vibrator should be decided and forceful. If this buzz is exceedingly high-pitched, it is an indication that the vibrator has been set too tight, and its tension should be loosened if unscrewing the contact point slightly does not lower the tone. It must be remembered that the tension of the vibrator can be changed by turning the contact screw. If this screw is turned down so that it forces the vibrator toward the iron core, the tension will be greater than will be the case if the contact point is turned to the left.

If the buzz of the vibrator is pitched lower than was formerly the case, it is an indication that the contact point should be screwed down, or that the tension of the vibrator should be tightened. It is probable that turning the contact screw to the right will produce the proper result. While these changes in the position of the contact screw are being made, the switch should be left turned on so that the variations in the pitch of the vibrator buzz may be detected. When an evenly-pitched, vigorous buzz has been secured, the switch should be thrown on and off several times to make certain that the response of the vibrator is instant and positive. The switch should then be left on and the vibrator allowed to buzz for several seconds in order that it may be determined whether the pitch of the sound will change, or not. If there is a change noticeable, the contact screw should be readjusted until the pitch of the buzz remains constant as long as the circuit is closed.

The coil and batteries or magneto by no means form the entire ignition system, although the generation of the spark depends entirely upon them. The spark must be regulated to occur at the proper point in the stroke of the piston, as a continuous spark would not only waste the current, but would cause the ignition of the charge during the upward stroke and would result in an impulse in the reverse direction that would prevent the motor from running for more than half a turn.

The device by which the time of the occurrence of the spark is regulated is called the timer. This consists, in its essentials, of a hard rubber disc provided with a copper or brass segment. A metal pin, roller, or ball rests against the outer edge of the disc, and as the latter is revolved, the electrical circuit is completed whenever the two metal portions come in contact with each other. The hard rubber being a non-conductor of electricity, prevents the flow of the current at all other times. The disc of the timer, known as the "commutator," is so geared that it revolves in unison with the motor.

Inasmuch as the explosion occurs in each cylinder only at every second stroke of a four-cycle motor, the commutator on this type of engine is geared to revolve at one-half the speed of the crank shaft. In the two-cycle motor, on the other hand, the explosion occurs in each cylinder at every revolution, and consequently the commutator should turn at crank shaft speed.

Although the spark is intended to occur approximately at the extreme upper end of the compression stroke, a few degrees variation both above and below this point is necessary in order to obtain the desired speed and power flexibility of the gasoline motor. At high speeds, the spark should be timed to occur before the piston reaches the extreme top of its stroke, while at slower revolutions of the motor the ignition should take place, in some instances, just after the piston has started to descend. This variation In timing is obtained by swinging the contact piece of the timer—known as the brush—either forward or backward through an arc corresponding to the range of advance and retard.

If this brush is swung in a direction opposite to that of the revolution of the commutator, the metal portions will meet sooner, with the result that the spark will occur earlier, or will be "advanced." If, however, the brush is swung to a point farther along in the direction of rotation of the commutator, the spark will occur later, or will be "retarded." These variations of position of the brush are generally obtained by means of a lever attached to the steering post or wheel.

It is evident that the current must pass from the brush to the metal segment of the commutator in order to complete the circuit through the timer and thus form the spark. It is the primary current, or low-tension current from the battery or magneto, that passes through the timer, and as this is of low voltage and is therefore easily discouraged, it is necessary that the contact points be kept clean in order that its travel may be made easy. Timers are generally protected from dirt, but the particles that will naturally be worn off from the metal and rubber commutator and brush should be cleaned out before its accumulation becomes deposited on the contact points and interferes with perfect electrical connection.

A few years ago, the majority of battery ignition systems employed a separate coil for each cylinder of the motor. Each coil in this system is connected with an individual brush that operates against the same commutator as do the brushes for the other cylinders. With such a system, the primary circuit leads from one terminal of the battery to the primary winding of the coil, through this and the vibrator to the brush of the timer reserved for that particular coil and cylinder, and thence through the switch to the other terminal of the battery. This order may be reversed, or the timer, switch, and coil may be placed in any consecutive position, provided the current passes through all in its travel from one terminal of the battery to the other. The secondary, or high-tension current is led from the terminal of the secondary winding on the coil to the spark plug of the proper cylinder. There should be a "ground" wire to serve for the return of the secondary current. This may lead from any part of the primary circuit to a clean metal connection on the motor.The multiple coil system is still used to a large extent, but an elaboration of it will be found on many of the modern cars. This consists of the use of but a single coil for all of the cylinders of the motor. This is done by means of a distributor, which is a sort of "glorified timer" consisting of a commutator provided with as many segments as there are cylinders in the motor. This distributor receives the current from a single coil and delivers it to the proper cylinder as the various connections are made. The timer still performs its function of completing the circuit from the source of current only at the proper instant, and leaves the distributor to serve the purpose of a "switch" to "sidetrack" the current and deliver it at the various cylinders in turn.

If it should ever become necessary to remove any part of the timer, or to change the length of the spark control rods, the greatest care should be taken to make certain that the motor is properly timed when the various portions are replaced. This can best be done by setting the spark lever in its central position, removing a plug from one of the cylinders, and introducing a rod or long screw driver into the opening for the purpose of determining the exact top of the stroke of the piston. When the flywheel is turned, the top of the stroke should be marked on the rod or screw driver as the latter is forced upward by the piston.

If the spark plug is laid with its large nut resting on the cylinder head, and the switch is thrown, the time of the occurrence of the spark can be readily observed as the motor is turned slowly by hand. This spark should occur in this particular plug just as the piston of that cylinder reaches the top of its stroke, as indicated by the change in the direction of the movement of the rod or screw driver. If the spark occurs too soon or too late, the commutator should be moved backward or forward to remedy the respective trouble. Although if the timer is set properly for one cylinder it is probable that the spark in the others is also timed correctly, it is well to test each to make certain that there has been no uneven wear in the contact segments of the commutator or the brush.