Motor-Car Principles

Appendix

Operation and Care of Low and High Tension Magnetos and Magneto Ignition Systems

General Principles

The red-painted toy magnet that is one of the properties of childhood and with which everyone is familiar, may well be used as the beginning of a study of the magneto, for with it the characteristics of magnetism may be observed. A little experimenting will show that the magnet will attract, or “pick up”, iron and steel objects only, having no effect on copper, brass, lead, wood, or, for practical purposes, any other substance. Furthermore, it illustrates the fact that when iron and steel are in contact with it, they in turn become magnetic, able also to attract tacks and other bits of the same metals. Iron, however, is shown by experiment to be magnetic only when in actual contact with the magnet, losing its magnetism as soon as the contact is broken, while when steel is magnetized by touching it to a magnet it remains magnetic. This fact is illustrated by the magnet itself, which is of steel and therefore capable of retaining its power for a greater or less time, depending on its quality and hardness.

If iron filings are scattered on a piece of paper laid over a magnet they will not fall evenly and regularly, but will collect most thickly at the ends, or poles, of the magnet, showing that there the magnetic attraction is stronger than at any other points. If the filings are examined closely it will be seen that they have taken up definite positions, forming lines and curves extending between the two poles (Fig. 1). This is the simplest method by which the magnetism may be made visible, and it illustrates the fact that the power of a magnet acts in a series of lines passing from one pole to the other. If a piece of iron or steel is placed across the poles of a magnet these lines, or as many of them as possible, will use it as a bridge or conductor, because they can pass through it more easily than through air. Such a piece of iron or steel is called a keeper, and by its use the magnet will retain its strength for a much greater time than if the lines are obliged to make their way through the far greater resistance that the air presents to their passage.

These lines, which are known as magnetic lines of force, always move in the same direction, passing through the air or the keeper from the north pole of the magnet to the south pole, and passing through the metal of the magnet itself from the south pole to the north pole.

The strength of a magnet depends on the number of these lines of magnetic force that it possesses. If two magnets, one strong and the other weak, are placed under sheets of paper on which iron filings are scattered, their comparative strengths are clearly shown by the difference in the number of lines of force that the filings show them to possess.

The space through which a magnet makes itself felt is known as the magnetic field, and this is large or small, according to the number of lines of force. The stronger the magnet, the larger will be the sweep of the curves of its lines of force, and the greater will be the field that they form.

When a piece of iron or steel is placed in contact with a magnet, the lines of force flow into it, and it becomes magnetized, throwing out lines of force and forming its own magnetic field, which is quite distinct from the magnetic field of the original magnet. If the piece is of iron, its lines of force and its field die away as soon as it is separated from the magnet, but a piece of steel once magnetized will retain its magnetism and, of course, its lines of force.

A wire of nonmagnetic metal, such as copper, for instance, will not have the slightest attraction for iron filings, but when an electric current is passed through it the filings will act as if the copper were a magnet, clinging to it as long as the current passes, and dropping as soon as the circuit is broken. As a matter of fact, an electric current sets up lines of magnetic force exactly similar to those of a permanent magnet, their number being in proportion to the strength of the current.

As has been stated, iron becomes magnetized when magnetic lines of force flow into it. If, therefore, a wire through which an electric current passes is wound around an iron rod, the lines of force set up by the current will pass into the rod and magnetize it, so that it sets up its own magnetic field. This magnetic field is created when the electric current starts flowing in the wire and dies out when the flow of the current is stopped, for then the lines of force due to the current die out, and the iron, which depends on them for its magnetism and which has not the ability to retain its lines of force, returns to its original nonmagnetic condition. An arrangement of this sort, consisting of a soft iron core, around which is wound a number of layers of insulated wire, forms an electro-magnet, and will produce a magnetic field whenever an electric current passes through the wire. The action is practically instantaneous, the magnetic field appearing and dying out on the making and breaking of the electric circuit.

A magnetic field may thus be produced by the action of an electric current, and an electric current in turn may be produced by the action of a magnetic field. To generate a current by this method it is only necessary to place a conductor forming a closed circuit in a magnetic field, and to change the strength of the field, making it stronger or weaker. For an example, a length of insulated wire may be wound on an iron bar, and the bar then touched with a magnet. As soon as the lines of force flow into the bar it becomes magnetized and sets up a magnetic field in which the lines of force follow the law and flow from one pole to the other through the air. The formation of this field is exceedingly rapid, but during the time that the field is forming and increasing to its full strength, an electric current will flow in the wire winding. When the field has reached its full strength, the flow of the current ceases. On separating the bar from the magnet it loses its magnetism, and the field set up by it dies away; or in other words, its strength undergoes another change, now growing weaker as the bar returns to a nonmagnetic condition. This dying out of the field generates another momentary flow of current in the wire, which moves in the direction opposite to the flow of the current generated while the strength of the field was increasing.

The current generated is called an induced current, and the method of producing it is called induction.

The intensity of the induced current in any given winding depends on the extent of the change in the strength of the field and upon the rapidity with which it occurs. A bar of iron is limited as to the strength to which it can be magnetized; or in other words, it can only set up a limited number of lines of force. Increasing the strength of the field from nothing to this point, or reducing its strength from this point to nothing, gives the greatest change in strength possible to obtain, and if this change occurs in the shortest possible time, then the current induced will be of the greatest intensity that can be obtained from a conductor of the size and length used.

For an understanding of the action of a magneto it is necessary to bear in mind the following points:

First. That a magnetic field is composed of lines of magnetic force that flow from one pole of the magnet to the other.

Second. That the lines of force will take the path that presents the least resistance.

Third. That an electric current will be generated, or induced, in a conductor placed in a magnetic field whenever the strength of the field changes.

Fourth. That the current flows only while the change in strength is taking place, ceasing to exist when the field becomes uniformly strong or weak.

To make a practical application of the laws governing the production of an induced current, a conductor forming a closed circuit is placed in a magnetic field, and the strength of the field caused to change, alternately becoming strong and weak. In magnetos, the magnetic field is due to two or more powerful steel magnets, and the conductor, a length of insulated copper wire, is wound on a soft iron core and revolved between the poles of the magnets where the field is strongest. The magnets are known as the field of the magneto, and the wire on its core the armature. The shape of the iron armature core is shown in Fig. 2, the winding being indicated by heavy black lines. On the inside of the poles of the field are pole pieces, which are blocks of soft iron hollowed out to receive the armature. As the successful operation of the magneto requires the lines of force to have as easy a path as possible, the air space between the armature heads and pole pieces is very small, being in the neighborhood of 1/100 of an inch.

When the armature is not in position, the lines of force will be required to pass from one pole piece to the other through the air, and as they will seek the path of lowest resistance, most of them will pass between the lower points of the pole pieces, where the air gap is short, and where the lines of force are present in the greatest number (Fig. 3). When the armature is placed between the pole pieces, however, it gives the lines of force a path of still lower resistance, and they will therefore follow it, whatever its position may be. If the armature is free to turn, it will take a horizontal position, as shown in Fig. 4, for then the heads are entirely in contact with the pole pieces, and the greatest possible number of lines of force can take the path offered by the neck of the armature.

When the armature is in this position, the neck is magnetized by the flow of lines of force through it and sets up a powerful magnetic field which is quite distinct from the field thrown out by the field magnets. If the armature is revolved, it takes the lines of force, or most of them, with it, and they continue to flow through the neck as long as one head of the armature is in contact with the north pole piece and the other head in contact with the south pole piece, for even this long and distorted path, as shown in Fig. 5, is of less resistance than the air gap. The lines of force, however, resist this lengthening of their path, and tend to hold the armature with the neck horizontal, when their passage is easiest. If the armature is turned by hand, it will be noticed that it becomes harder to turn as the neck approaches the vertical position, and if the magnets are sufficiently powerful, a great effort will be necessary. Once vertical, however, the armature hangs, and a still greater effort is required to continue the revolution, for the lines of force have found new paths of low resistance (Fig. 6). Each armature head now forms a bridge between the pole pieces, and the lines of force divide, some going through the upper head and some through the lower. The lines entirely abandon the neck, and in consequence its magnetic field dies out. When these paths are broken by continuing the revolution of the armature, the lines of force again flow through the neck, and its magnetic field is again established (Fig. 7). This action occurs twice during each complete revolution of the armature, and if the armature is revolved by hand, the two points when the neck is horizontal and the movement easy will be very distinct from the hard points when the neck is going over the vertical position.

In one revolution the neck is twice magnetized and demagnetized; or in other words, the magnetic field set up by the neck as the lines of force pass through it twice changes its strength, being strong when the neck is horizontal and weak when it is vertical. The winding on the armature is affected by this magnetic field, and electric currents are induced in it by these changes in its strength.

The greatest change in the strength of the field occurs as the armature moves into the vertical position, when its heads form bridges between the two pole pieces (Fig. 6). Up to this point the armature neck is strongly magnetized, but its magnetic field dies out as the neck becomes vertical. It is at this point then that the induced current is at its greatest intensity and becomes sufficient for ignition purposes.

In making an armature the channels are wound full of wire, and this is retained against the action of centrifugal force by two or three short lengths of wire bound around the armature and lying in grooves cut in the heads for that purpose. Disks of brass are also screwed to the ends of the armature core, and assist in making it dust and water proof. The magneto must have a base, and this must be of some nonmagnetic metal, like brass, for if it were made of iron it would provide a convenient path for the lines of force, and they would have no interest in passing through the armature. There must also be bearings for the armature shaft, and these are either plain or ball, set in brass plates screwed to the ends of the pole pieces. A zinc or aluminum plate covers the space over the armature and between the upper edges of the pole pieces, so that the armature revolves in a tunnel that is proof against the entrance of dust and water.

There are, of course, two ends to the wire wound on the armature, but the simplicity of a magneto is increased by grounding one end on the metal of the armature, the other end being brought to the single terminal (Fig. 2). The circuit is therefore complete when this terminal is connected to any metal part of the magneto; or, as the magneto is mounted directly on the metal of the engine, to any metal part of the engine or frame of the car. The live end of the armature winding is brought out by means of a metal rod passing lengthways through the shaft of the armature, the rod being insulated from the shaft by means of a hard rubber bushing or tube. The terminal of the winding is therefore found at one end of the armature shaft, and the current flows from this revolving part to the stationary binding post by means of a carbon or steel spring that is kept pressed against the end of this rod.

Magnetos are classified as L. T. (low tension) and H. T. (high tension) according to the current that they deliver, the word tension being used to indicate the pressure or voltage of the current, but more accurate expressions would be primary and secondary magnetos. The magneto already described is of the low-tension type, and is used for the make-and-break ignition system, its winding being so proportioned that at maximum speed it delivers a current of from 100 to 150 volts. What is often spoken of as a high-tension magneto is employed for the jump spark system, a magneto of the type described delivering a current that flows through the primary winding of a secondary induction coil; but this use of the term is erroneous, for while the system delivers a high-tension spark, this is from the coil. The magneto itself is not only of the low-tension type, but its current must be so feeble that the danger of burning out the coil is obviated. A true high-tension magneto has two windings on the armature; one, the primary, consisting of a few layers of coarse wire, over which the very great number of layers of fine wire forming the secondary is wound. This may give a current of from 10,000 to 20,000 volts, and is used for the jump spark ignition system.

These types and their applications will be discussed in the succeeding chapters.

Magnetos in General

One of the great advantages resulting from the use of a magneto is that for ordinary running it does away with the necessity for the hand advance of the spark. The greater the speed at which a magneto runs, the more abrupt is the change in the strength of the magnetic field, and in consequence the greater is the intensity of the current delivered. When running at slow speed, the spark produced in the cylinder will be weak and thin; it will be sufficient to ignite the mixture, but the ignition will occur slowly. At high speeds, on the contrary, the intensity of the current produced will be such that a flame rather than a spark will be produced, and ignition will occur much more rapidly and positively. When starting an engine on the magneto, the spark control lever must be advanced more than is necessary for ignition by battery, and the engine must be cranked at such a speed that the magneto will produce a current sufficient for ignition. Once started, it is rarely necessary to move the spark control lever, except for high speeds, for speeding the engine up by opening the throttle will increase the speed of the magneto, and the flaming spark will result in a quicker ignition of the mixture.

Because a magneto does not deliver a continuous current, it cannot be driven by a belt or by friction, for a slight slip would throw it out of time with the engine. The best drive is by gears, for this is positive, and there is a minimum of lost motion. In some cases the magneto is driven by chain and sprocket, and while this prevents slipping, there is considerable lost motion when the chain is loose enough to run smoothly, and the magneto cannot be timed as accurately as is possible with gears.

Magneto Troubles

If the ignition fails, and the question arises as to the reason, the condition of the magneto may be tested quickly and in a most satisfactory manner. It either gives a current or it does not, and to learn its condition it is only necessary to disconnect it from the ignition system, and to connect one end of a length of wire to its terminal. Holding the free end of this wire in the bare fingers of one hand, and cranking the engine with the other hand, also bare, a shock will be felt as the armature revolves, if it is in good condition. If this test is too strenuous, the free end of the wire may be held lightly against the teeth of a gear while an assistant cranks the engine briskly. As the wire falls from tooth to tooth, a time will come when the point of maximum current coincides with the breaking of the circuit at the gear, and if the magneto is in good condition, a flaming spark will appear.

There is little about a magneto of the type described to get out of order. Oil or dirt between the spring or brush and the end of the conducting rod may prevent the flow of current, or, what is more unlikely, the wire of the armature may be broken. The last trouble is of rare occurrence, for when winding the wire on the armature it is shellacked, and for all practical purposes the whole becomes a solid mass. The only point where the wire can break is the half-inch of it that is connected to the inside end of the conducting rod that passes through the shaft, and the condition of this may be learned by removing the dust cover and looking. If broken, a drop of solder, carefully applied, will repair the damage.

A question that is frequently asked is regarding the liability of the field magnets to lose their magnetism. If made of the proper material, and handled and used under proper conditions, they should hold their magnetism indefinitely. The strength of a toy magnet may be increased by tearing its keeper sharply away from the poles, and as sharply replacing it, the operation being repeated. When the armature of a magneto revolves, it performs the same office for the field magnets, and it has the effect of keeping them up to strength indefinitely. If the magneto is mishandled, however, it is another story, and an inquisitive or careless worker can almost instantly weaken a field by removing the armature without taking the proper precautions. The armature, when in position, acts as a keeper, and provides a path of low resistance for the passage of the lines of magnetic force. If the keeper is taken away, the lines of force are required to traverse the higher resistance of the air, and many of them then being overcome, their number decreases and the field becomes greatly weakened. This takes place instantly on the removal of the armature. It is rarely necessary for a chauffeur to remove the armature of a magneto, but when it is required, the first step is the placing of a heavy plate of iron under the arch of the magnets, and in close metallic contact with the pole pieces, the dust plate being removed. This will act as a keeper, and the armature may then be removed. If such a plate of iron is not at hand, both of the end plates may be unscrewed and one of them removed, and then, as the armature is drawn out slowly, small steel tools, or short lengths of iron rod, well cleaned, may be fed in after it, so that the cavity is well filled. If these precautions are taken the armature may be removed with safety, but it is better not to attempt this, and it is never advisable to detach the magnets from the base or pole pieces. So much damage may be done to a magneto by an unskilled man, that some of the manufacturers go to the length of equipping their magnetos with seals, the breaking of which is evidence that the machine has been tampered with, and that someone is directly to blame if it does not deliver current.

When an engine is equipped with a double system of ignition, or when there are two sources of current, one being a magneto, the greatest care must be taken to prevent even the momentary flow of the battery current through the armature winding, for this will result in the demagnetization of the field. The same trouble will result if the magneto current is led through the winding of a primary induction coil in the hope of intensifying the current. Should the battery current flow through the armature winding, the core would be magnetized, for it would then be an electro-magnet, and the magnetic field set up by the armature under such conditions would overcome the less powerful field set up by the field magnets and their strength would instantly be reduced.

It is very essential to keep the armature bearings thoroughly lubricated in order that there may be as little wear as possible. The clearance between the armature heads and the pole pieces is so slight that a trifling amount of wear in the bearings would permit the two to rub or strike. The best magnetos are equipped with ball bearings, provided with wick feeds that keep them lubricated, but even with these the lubrication must be watched. Oil cups are provided, which must be kept filled, but it must be remembered that an excess of oil may make trouble in working its way into the armature winding and destroying the insulation. In some cases the oil cups are provided with an overflow, which prevents excessive oiling and the collection of surplus oil around the bearings.

Low-Tension Ignition System

In utilizing the current of a magneto for ignition purposes, the circuit is so arranged that it is first closed and then opened, the closing of the circuit permitting the current to flow, and the breaking of the circuit resulting in a spark at the break. The closing and opening of the circuit is due to the action of the igniter, which is a device located in the cylinder wall, one end projecting into the combustion space and the outside end bearing the parts by which it is operated. The two chief parts of an igniter are the stationary and movable electrodes, so arranged and connected that when the movable electrode comes into contact with the stationary electrode the circuit is closed, and when it moves out of contact the circuit is broken.

The stationary electrode is a steel pin or screw passing through the cylinder wall, and carefully insulated from it by a tube or bushing of mica or other insulant that is proof against the heat and pressure produced in the combustion space. The current developed by the magneto is led to this stationary electrode. The movable electrode rocks in a bearing in the cylinder wall, the inside end of its short shaft carrying a finger that touches or separates from the stationary electrode as the shaft is rocked. The outer end of the short shaft carries one or two short arms by which the shaft is moved. The operation of the igniter is due to the action of a cam on the half-time shaft, which moves a tappet rod that in turn acts on the arm controlling the movable electrode.

When the cam is not acting on the tappet, the movable electrode is held out of contact with the stationary electrode, and there is consequently no circuit for the current. As the tappet is moved by the revolution of the cam, the movable electrode is freed and is brought into contact with the stationary electrode by the action of a light spring. This closes the circuit, and the current flows. The further revolution of the cam separates the movable electrode from the stationary, and the sudden breaking of the circuit results in the production of a spark between the two.

There is a great variety in the construction of igniters, but the operation is the same in all. Once in its revolution the cam operates the movable arm of the igniter, permitting it to touch the stationary electrode, and this establishment of the circuit is followed by its rupture as the continued movement of the cam causes the tappet again to operate the movable electrode. A better spark for ignition purposes is obtained when the movable electrode is moved out of contact with the stationary electrode sharply than when it is moved slowly, and for this reason the parts are so arranged that the tappet strikes the movable electrode a blow instead of pushing or pressing it. This construction is shown in Fig. 8. When the cam is not acting on the tappet, the head on the tappet holds the movable electrode away from the stationary, against the action of a light spring. As the cam lifts the tappet, the tappet head releases the movable electrode, and the light spring draws it upward and against the stationary electrode, establishing the circuit. When the tappet drops off the cam, it is brought down sharply by the tappet spring, and its head strikes the movable electrode a sharp blow that results in the sudden breaking of the circuit.

In some types, the movable electrode comes into contact with the end of the stationary electrode, which is then a screw, and the distance between them may be adjusted by screwing it in or out. In other types, contact is made with the side of the stationary electrode, in which case the adjustment of the distance is provided for on the movable electrode. This distance should be about one sixteenth of an inch, for this is ample for the production of a satisfactory spark, and a greater distance will result in the more rapid burning of the contact points.

In multicylinder engines, the current from the live end of the armature winding is led to a bus-bar of conducting material that serves to distribute it to the igniters. The stationary and insulated electrode of each igniter is connected to this bus-bar by a short length of wire that either leads through a switch of ordinary construction, or is provided with a plug on its end that fits into a socket on the bus-bar and serves the purpose of a switch by which the igniters may be thrown out of circuit when it is desirable. As the magneto is grounded when it is placed on the engine, and as the movable electrode of the igniter is also grounded on the engine, a circuit for the current is provided only when a movable electrode is in contact with a stationary electrode. The construction of the cam shaft is such that only one igniter at a time makes contact, which occurs during the compression stroke, so that the rupturing of the circuit will occur at the point in the compression stroke when the spark is required for ignition.

To cut off the ignition circuit for the purpose of stopping the engine, it would be bad practice to break the contact between the magneto and the bus-bar, for while this would bring the desired result it might result in the injury of the magneto, as there would then be no circuit for the current that the magneto would still be developing. The best practice is to short-circuit the magneto, and this is provided for by means of a simple switch, one point of which is grounded, and the other connected to any insulated part of the ignition system; the magneto terminal, bus-bar, or other. When this switch is closed, a circuit is provided for the magneto current, which, in the arrangement shown in Fig. 8, flows from the magneto to the bus-bar, to the switch (when closed), to ground, and back to the magneto. If this circuit is provided, the current will follow it, abandoning the paths across the igniters as they operate, and the production of sparks ceasing, the engine will come to a stop.

Setting Up the L. T. System

To obtain successful results, the spark must be produced at the correct point in the compression stroke, and this requires such adjustment of the igniters that the movable electrode breaks the circuit at this point. Because the magneto delivers a fluctuating current, it is clear that the most satisfactory spark will be produced when it is so set that it will be delivering its maximum current at the instant that the igniters separate. As most of the running of a car is done on an advanced spark, the point of maximum advance must be determined in order to set or adjust a low-tension ignition system. This point varies with different engines; in a large proportion the piston has a “lead” of one half inch, while in a smaller number the “lead” varies from this to three quarters of an inch. By this it is meant that when advanced, the spark occurs when the piston still has one half inch to travel to reach top dead center of the compression stroke, or anything up to three quarters of an inch, as the case may be. Inquiry of the manufacturers regarding this point will secure the information, but if this is not possible, the point may be found by experiment.

The first step is to bring one of the pistons to within one half inch of top dead center of the compression stroke. This position may be determined by dropping a stiff wire through a compression relief cock or other opening in the cylinder head, so that it rests on the piston and is moved by it. The crank shaft of a four-cycle engine must make two revolutions to complete the cycle in one of the cylinders, and during these two revolutions the piston will twice pass top dead center, as will be shown by the movements of the wire. It is necessary, however, to distinguish between top dead center of the compression stroke and top dead center of the exhaust stroke, and this may be accomplished by watching the stem of the exhaust valve. When the wire shows that the piston is moving toward top dead center, and the exhaust valve stem shows the valve to be open, the piston is known to be making the exhaust stroke, and to get it to top dead center of the compression stroke it is necessary to continue the revolution of the crank shaft while the piston moves downward on the inlet stroke, and again upward on the compression stroke. During this second upward stroke the exhaust valve stem will not move, showing the valve to be closed. Cranking must stop when the lack of movement of the wire shows the piston to be at top dead center, and then the crank shaft must be turned backward by means of the flywheel until the piston has moved down one half inch. This may be determined by making a mark on the wire and holding a rule firmly in such a position that as the wire follows the piston the mark will move along the graduations. When the mark has moved down one half inch below top dead center, the movement of the crank shaft must be stopped so that the piston is held in that position.

Place the spark control lever in the advanced position, when the nose of the cam should be just ceasing to act on the tappet. The movable electrode must then be adjusted so that it is in the act of separating from the stationary electrode, and this is performed by shortening or lengthening the tappet, or otherwise setting the parts so that the tappet head is in contact with the outside arm of the movable electrode and beginning to move it away from the stationary electrode. This point may be accurately determined by the use of an electric bell circuit. Connect a bell with three or four dry cells, grounding the free terminal of the battery on the engine. Then breaking any connection between the igniter terminal and the ignition system, connect the free terminal of the bell to the igniter. The circuit will then consist of battery, bell, igniter, and ground return, and the bell will ring when the movable electrode is making contact with the stationary. Having adjusted the movable electrode so that it is approximately correct, it may be tested by cranking the engine. The bell should begin to ring during the compression stroke, and should stop ringing when the marks of the wire show the piston to be one half inch from top dead center. The igniter of each cylinder must be adjusted separately, and when tests have shown them to be correct, the lock nuts must be set up to hold them in position.

In attaching the magneto to the engine, means must be provided for driving it at the proper speed. In a four-cylinder engine, two revolutions of the crank shaft are necessary for the production of a cycle in each of the cylinders, two power strokes occurring in each revolution. As the magneto delivers its maximum current twice in each revolution of the armature, it must be driven at the speed of the crank shaft, and as the drive is usually taken from the cam shaft, the gear on the magneto must have half as many teeth as the gear on the half-time shaft with which it meshes. To set the magneto, the crank shaft should be revolved until one of the pistons (it makes no difference which) is in the firing position; that is, the position for which the igniter is set to open. The armature should then be revolved by hand until it is just past the vertical position of Fig. 6, with about one sixty-fourth of an inch of space showing between its rear edge and the edge of the pole piece from which it is moving. This can most easily be accomplished by removing the dust cover and reaching under the arch of the field with the fingers. Holding the armature in this position, its driving gear should be slipped into mesh with the gear on the half-time shaft.

In some makes of magnetos the gear is keyed to the armature shaft, which determines its position in relation to the armature. In this case, the gears must be meshed as close to the indicated position as possible. In other designs the magneto is driven by a positive clutch, so that it is impossible to mesh it in any but the correct position. In Bosch magnetos there is no key, but the shaft is tapered to fit a tapered hole in the gear, so that a very exact setting of the magneto is possible.

When the gear is keyed to the armature shaft, the closeness of the setting depends on the thickness of the teeth; the thinner the teeth, the closer it is possible to adjust the magneto. If the armature gear has an uneven number of teeth, however, it is possible to make the setting by one half the thickness of a tooth. If it is found that the thickness of the teeth is such that the armature cannot be set correctly, give the armature and its gear a half turn, when, if the number of teeth is odd, it will be found that a tooth on one side corresponds to the space between two teeth on the opposite side.

Having set the magneto and drawn up the nut securing the gear to the armature shaft, the system may be wired by running a wire from the terminal of the magneto to the bus-bar, and other wires from the bus-bar to each of the igniter terminals. The short circuiting switch should also be connected up, the running of its wires depending on convenience.

The system installed, it should be tested. With the system adjusted as indicated, the engine should start, and a very short run will show if it is correct. If it does not respond to advancing the spark, or if its operation is not satisfactory, a readjustment should be made, giving the piston a lead of one eighth of an inch more, or five eighths of an inch in all, with the spark advanced. If this shows an improvement, but the engine still does not develop its full power, the operation may be repeated, the piston being given more lead by one sixteenth of an inch.

If the compression is not the same in all of the cylinders, a closer adjustment of the igniters may be made that will improve the operation of the engine, the tappet of the high compression cylinder being adjusted to operate the movable electrode slightly before the others, so that the larger volume of mixture will have more time in which to burn. The switches through which the igniters are connected to the bus-bar may now be brought into use, and when three of them are open the engine should run on the single cylinder of which the circuit is complete. Testing the cylinders in this way, running one at a time, gives an opportunity for comparing their action, and for noting those in which the power is weak. If the weakness is due to ignition trouble, it may be corrected by a further adjustment of the tappet.

Troubles

Failure of ignition due to magneto trouble has already been described.

Any short circuit of the system will prevent ignition, for the current will not flow across the igniters if another path is open to it. Short circuiting may be due to the chafing of the insulation of the wires, to a frayed end of a stranded cable making contact with the metal of the engine, to the carbonization or breaking of the insulation around a stationary electrode, or to the sticking of a movable electrode in contact with a stationary. Operating the igniters by hand will show the presence of this last named condition, and inspection of the insulation and terminals the presence of any defects. Loss of power is frequently due to the leakage of compression around the plate carrying the igniter, around the stationary electrode, or through the bearing in which the movable electrode rocks. The side of the bearing toward the combustion space is built like a valve, and when worn may be ground to a seat.

If the engine runs well at low speed, but misses at high speed, the fault may be located in faulty insulation of one of the stationary electrodes, which holds the low-pressure current developed by the magneto at low speed, but fails to retain the higher pressure developed when the speed is increased. The faulty igniter may be located by running the engine one cylinder at a time.

The heat developed by the spark is very great, and provisions must be made to preserve the contact points from undue corrosion. These points are usually made of platinum, or of an alloy of platinum and iridium, and must be kept clean and smooth. The spring that draws the movable electrode against the stationary must not be too strong or it will bring the contacts together with sufficient force to batter them out of shape. This spring should be of sufficient strength to bring them together promptly, but without undue force. The tappet spring should be of considerable strength, in order that the separation of the electrodes may be quick and positive.

After long running the contacts will become worn to such an extent that the spark occurs too late in the stroke to permit the engine to develop its full power. This may be corrected by readjustment of the tappets, so that ignition takes place earlier. A wearing of the outside arm of the movable electrode, or of the tappet head, will have the same effect.

Another cause of failure will be the weakness of the spring of the movable electrode, which is so weakened that it will not do its work at high speed.

L. T. Magneto with Secondary Coil

The advantages of a magneto as a current producer for the low-tension system pointed the way to its adoption for the jump spark system also, but it was at once recognized that it would be impossible to lead the current direct to the primary winding, as the current from a battery is used. The reason for this is that while the battery delivers a current of constant value, the current obtained from a magneto is fluctuating, its intensity depending on the positions of the armature as it revolves, and the speed at which it is driven. A current is induced in the secondary winding of the coil as the magnetic field set up by the core changes its strength, and as has been explained, the induced current is strongest when the greatest change in the strength of the field occurs in the shortest possible time. The effect of the flow of the magneto current through the primary winding of the coil would be to magnetize and demagnetize the core slowly, as the magneto current increased to its maximum and died away to the minimum, and these gradual changes in the magnetic field of the core would induce currents in the secondary that would be too feeble to produce ignition of the charge. When a battery is used with a coil, the operation of the vibrator produces rapid magnetizations and demagnetizations of the core, but as the field set up by the core dies away more rapidly than it is established, the greatest current is induced by the breaking of the circuit. In the best known method of applying a magneto to the operation of a secondary coil, the magnetization of the core is caused to occur with exceeding rapidity, and the current induced in the secondary winding as this occurs is sufficient for ignition.

This is known as the Eisemann system, and is in very general use. A diagram illustrating the connections is shown in Fig. 9.

The magneto is of the usual type, but the winding is so proportioned that the current is of lower voltage than is delivered by what has been described as the low-tension type of magneto. One end of the winding is grounded on the metal of the armature, and the live end is carried to a contact screw. The current flows to this screw, and from there two paths are presented by which it may flow back and complete its circuit. One of these paths leads through the primary winding of a secondary coil, and the other through a lever that for the greater part of the revolution of the armature touches the contact screw. The lever is grounded on the metal of the magneto, and when it touches the contact the circuit that is then completed is short and of low resistance, and the current follows it in preference to the circuit of higher resistance through the primary winding of the coil. The flow of current is thus from the armature winding to the contact screw, which is insulated from the metal of the magneto, to the lever, and by the ground back to the winding. Attached to the end of the armature shaft, and so placed that it operates the lever, is a cam with two projections, these projections being arranged to move the lever away from the contact screw. This arrangement is called the interruptor, and by its operation it closes and opens the low-resistance circuit by which the magneto current may flow.

When the armature is horizontal, and during the time that it is approaching the vertical, the circuit is closed through the interruptor, but when the armature reaches the position in which it gives its maximum current, the interrupted is opened by the cam, and the current, losing its low-resistance circuit, is required to flow through the primary winding of the coil, for that is the only path by which it can return to the armature winding. The sudden flow of this current through the primary winding results in the production of a powerful magnetic field around the core, and this rapid growth in the strength of its field induces a current in the secondary winding that is sufficient for the ignition of the charge. As the magneto current is at its maximum twice during each revolution of the armature, the cam is arranged to open the low-resistance circuit at such periods that each maximum is required to flow through the primary of the coil.

In applying this system to a multicylinder engine, it is necessary to distribute the secondary current to the different cylinders, so that a spark will be produced in each as it is required. This is accomplished by means of a secondary distributor, consisting of a revolving conductor to which the secondary current is led, and a stationary contact for each cylinder arranged in a circle, to be touched as it revolves. These stationary contacts are connected with the spark plugs, according to the firing order of the engine. During one revolution of the distributor, a spark is produced in each of the cylinders, and as two revolutions of the crank shaft are necessary in order to have power strokes occur in all of the cylinders, the distributor must revolve at half the speed of the crank shaft. The magneto produces a current that is sufficient for ignition twice during each revolution of the armature, and therefore, for four cylinder engines, must run at the speed of the crank shaft. For the sake of compactness, the distributor is built into the magneto, its shaft being carried in bearings that support it above the armature and under the arch of the field magnets. It is geared to the armature so that the two revolve in a fixed relation to each other, and in order that the distributor may run at half the speed of the armature, its gear has twice the number of teeth of the gear on the armature shaft that drives it.

The secondary induction coil used is similar in construction to the coils used for ignition by battery, except that it is heavier, and has no vibrator. The period during which the magneto is delivering its maximum current is so brief that there would hardly be time for a vibrator to get into action, and even if this were not the case, the making and breaking of the circuit by the vibrator is not necessary, as the single change in the strength of the magnetic field set up by the core is so abrupt that the current induced by it is sufficient for the purpose.

The strength of this secondary current is such that it is necessary to provide a circuit for it if by any accident or oversight the circuit through the distributor or spark plugs is interrupted, as would be the case should one of the secondary wires become disconnected. This is taken care of by what is known as the safety spark gap, which is a gap provided between the secondary terminals of the coil, or between the live end of the secondary winding and the ground. This safety spark gap, in the case of the coil furnished by the Eisemann company, takes the form of two flat pieces of brass, each in contact with one of the secondary terminals of the coil, their pointed ends being separated by a distance of about half an inch. When there is no interruption of the secondary circuit, and the parts of the secondary system do not present undue resistance to the flow of the current, the higher resistance of the air space between the points of the safety spark gap prevents the current from jumping between them, but should one of the secondary cables become disconnected, the pressure of the current will rise to a point that will enable it to jump the gap between the two points. If it were not for this safety gap, an interruption of the secondary circuit would make the pressure of the current rise to such an extent that there would be great danger of a spark passing inside of the coil, rupturing the insulation.

As the magneto is received from the makers, the parts will be in a fixed relation to each other; when the armature is vertical, and held in that position by the lines of force, the cam is in the act of opening the primary interruptor, and the revolving part of the secondary distributor has moved into contact with one of the stationary contacts about one thirty-second of an inch.

The interruptor of the magneto can be utilized as the timer for an ignition system by battery and coil, the secondary current being distributed to the cylinders by the distributor. A better arrangement, however, requires the use of two coils, and a timer which is carried on the rear end of the distributor shaft. The coil for the battery system is provided with a vibrator, and connections are made in the manner usual with a system composed of these parts. The coil is in a case that also contains the non-vibrator coil for the magneto system, and the two wirings are separate and distinct with the exception of the secondary connection between the coil box and the revolving parts of the distributor, which is common to both.

While little change in the position of the spark is required for this system, provisions are made by which it may be advanced or retarded. In one system, the interruptor is operated by a groove in the face of the cam disk, a pin on the end of the interruptor lever following the groove and being moved by the irregularities. In this case, the advance and retard of the spark is obtained by rotating a plate carrying the interruptor in one direction or the other, so that the lever is moved earlier or later in the revolution of the cam. In another type the gear driving the armature is not attached direct to the armature shaft, but to a sleeve surrounding the shaft, and the pulling out of a second sleeve between the two alters the position of the armature in relation to the driving gear, resulting in the advancing or delaying of the moment when the maximum current is produced.

Setting Up the System

In setting up a system of this description, the firing order of the engine must be ascertained, and this may be done by watching the order in which the exhaust valve stems move as the engine is slowly cranked. Piston No. 1 must then be brought to top dead center of the compression stroke, and about one thirty-second of an inch down on the power stroke, being stopped in this position. With the magneto screwed into place, the covers of the interruptor and distributor may be removed, and the armature revolved by hand in the direction in which it will be driven, until the interruptor opens, the spark control being in the most retarded position. A mark on the interruptor will come into line with a mark on the casing at the instant that the interruptor breaks contact. The magneto gear should then be meshed with the gear that drives it, and the lock nut set up.

The connection between the live end of the armature winding and the interruptor is arranged by the maker, but it is necessary to make connections between the magneto and the coil. A terminal will be found in the casing close to the stationary contact screw, and this is to be connected with one of the primary terminals of the coil, the other primary terminal either being grounded on the engine or the magneto, according to the construction. The moving part of the secondary distributor may then be connected with the secondary terminal of the coil. If the coil has but three terminals, two primary and one secondary, there are no further coil connections to be made, but if, as is sometimes the case, there are two secondary terminals, the free one is to be grounded on the engine.

An inspection of the distributor will show that the revolving part is beginning to make contact with one of the stationary points, and this point is to be connected to the spark plug of the cylinder in which the piston is at top dead center of the compression stroke. The next distributor point in the direction of rotation is to be connected with the spark plug of the next cylinder to fire, the remaining distributor points and spark plugs being connected in the order of firing.

Magnetos of this type are built to run in either clockwise or counter clockwise, and the two are not interchangeable. The direction in which the magneto is to be driven is indicated by an arrow stamped on the gear casing or end plate.

When the system is properly installed, the motor should start on a quarter-turn of the crank shaft, the crank being given a quick upward jerk. If it does not, the setting is incorrect, the connections are improperly made, or there is too great a distance between the points of the spark plugs. This distance should be one sixty-fourth of an inch.

Care

The bearings of the armature and secondary shafts must be kept well lubricated, as well as the parts of the interruptor that require it.

Every few weeks, depending on the use, the carbon or steel brush bearing against the live end of the armature shaft must be wiped off with gasoline, and the same care must be given to the distributor contacts and to the platinum contacts of the interruptor.

When the platinum contacts of the interruptor become worn down after long use, or if for any other reason they require readjustment, they must be set so that the distance between them at the moment that the cam holds them apart is not more than one one-hundredth of an inch. This space may appear to be very small for the work that is to be performed, but it will be sufficient, and a greater distance will result in sparking that will burn down the contacts. In the Eisemann magneto, the adjustable contact is held by two nuts, and small open end wrenches are provided to fit. A wrench should be applied to each of these nuts, and the lower or lock nut loosened. This will free the upper nut by which the adjustment may be made, and the lock nut may then be tightened. The contact faces of the platinums should be kept flat and smooth, and true to each other, so that they come together squarely. If they are pitted after long use they may be faced off with a dead smooth file, but this is a job that requires great care, and must only be done under the most favorable conditions. When completely worn down they should be replaced with new ones.

Troubles

Should the engine show a tendency to miss, the first suspicion, if the magneto contacts are known to be correct, should fall on the spark plugs, and the easiest method of testing them out is to replace them with new ones. After considerable use, the spark plug points will burn off, and the size of the gap will increase to such an extent that the spark will find less resistance in traversing the safety spark gap than the gap in the plug.

In order that the ignition may be cut off for the purpose of stopping the engine, a switch is always provided by which the magneto may be short circuited, one pole of the switch being connected to the magneto terminal, or live wire, and the other pole grounded, as shown in Fig. 9. When this switch is closed, the magneto current flows through it in a closed circuit, and as it abandons its path through the interruptor there will be no further action in the coil, the ignition of course ceasing. This switch is often located in the rim or arm of the steering wheel, so arranged that pressing on a button closes the circuit and diverts the magneto current from the coil.

Any accidental short circuit of the magneto current will produce the same effect, and in case of the abrupt cessation of ignition, this is one of the probable causes. A short circuit in the secondary will usually make itself known by the snapping of the sparks as they pass through broken insulation or from a frayed cable end.

The remarks on general magneto troubles and care already made on page 254 also apply to a magneto of this type, and the same rules regarding the protecting of the circuit of the lines of the field must be borne in mind.

H.-T. Magneto Systems

A high-tension magneto differs from the types described in that the armature has a double winding; one, the primary winding, consisting of a few layers of coarse wire, and the secondary winding, placed over the primary, consisting of many layers of very fine wire. As the armature revolves, a current is induced in the primary winding, the circuit of which is kept closed until the armature reaches the position in which the induced current is at its greatest value. An interruptor operated by a cam then opens the primary circuit, and the sudden demagnetization of the armature core that results induces in the secondary winding a current of high pressure that is quite sufficient to jump the gap in the spark plug and to cause ignition of the mixture. The construction of the armature is more delicate than is the case with a low-tension magneto, for the secondary winding must have a large number of layers, and to permit this in the limited space, the wire must be fine. In addition, it must be insulated with the greatest care, for the intensity of the current produced will enable it to seek out and to break down any weakness that exists. There must also be two brushes, one for the primary current and one for the secondary, which complicates the construction.

A magneto of this type is shown in diagram in Fig. 10. In this case, the inner ends of both the primary and secondary windings are grounded, the live end of the primary being connected with the insulated contact of the interruptor, and the live end of the secondary leading to the rotating part of a secondary distributor. For the greater part of the revolution of the armature, the cam on the armature shaft permits the interruptor to maintain a closed circuit, but as the current reaches a maximum the cam breaks the circuit, and the flow of the primary current ceases. During this time, the secondary circuit has been open, the moving part of the distributor being out of contact with the stationary points. Under the law of induction, a current will only be induced in a closed circuit, and there is therefore no action in the secondary winding. When the primary current is at its maximum and its circuit is broken, another condition exists, for then the secondary distributor is making contact, the only gap being in the spark plug, but the sudden demagnetization of the armature core as the primary circuit is broken, and the additional effect as the armature passes the vertical position, induces in the secondary winding a current of great intensity that passes to the spark plug by way of the distributor, returning to the winding by the ground.

The parts of the interruptor are carried on one of the end plates of the magneto, and the secondary distributor revolves in bearings that support it under the arch of the magnets.

In the diagram shown provisions are made for a double ignition system, the operator having his choice of ignition by high-tension magneto direct or by battery, timer and coil. In practice, the three switches shown operate together. When magneto ignition is desired, the switch between the battery and ground is opened, and the switch at the secondary distributor thrown into contact with the point connected with the secondary terminal of the magneto. The third switch is thrown to make contact between the primary terminal of the magneto and the interruptor. If battery ignition is desired, the switch on the magneto primary is thrown to the point through which it is grounded, and the primary and secondary switches of the battery circuit closed. The upper contact points of the interruptor are then utilized to close primary circuit of battery, and the secondary current induced in coil is led to distributor. This complication of switches is necessary to prevent possibility of the battery current flowing through the armature winding, and to provide a short circuit for the primary of the magneto. If the engine is running on magneto ignition, it is stopped by throwing the primary switch to the point by which the primary is grounded. If running on battery, the circuit is broken in the usual way.

The current from a high-tension magneto is of such intensity that the small points of an ordinary spark plug would quickly be burned, and it is necessary to provide plugs with heavy points, which will stand the work. The distance between the points should be about one sixty-fourth of an inch.

The Bosch magnetos, which are perhaps in more general use than any other, are built on similar lines to the magneto just described, except that the grounded end of the secondary winding, instead of being attached to the metal of the armature, is connected to the live end of the primary winding, as shown in Fig. 11. The live end of the primary is brought out through a conducting rod passing lengthways through one end of the armature shaft, from which the current flows to the stationary contact of the interruptor. While in other magnetos the interruptor parts are stationary and operated by a moving cam, in this type the interruptor parts revolve with the armature, and the moving arm is operated by two fiber wheels as it drags around the inside of a stationary ring. By rotating the ring and the two wheels the interruptor is caused to operate sooner or later in its revolution, this giving the advance and retard of the spark.

The other end of the armature shaft carries the collecting ring for the secondary current, this consisting of a hard rubber disk around the surface of which is a metal ring to which the live end of the secondary winding is connected. A carbon brush is kept pressed against this metal ring by a light spring, and the current is thus led off to the revolving part of a secondary distributor.

The interruptor keeps the primary circuit closed until the current induced in the primary winding is at its maximum, when the opening of this circuit causes the rapid demagnetization of the core, and a powerful current is induced in the secondary, which flows to the spark plug with which the distributor is making contact.

A safety spark gap is applied between the brass strip that conducts the secondary current from the collecting brush to the distributor, and the metal of the magneto. The necessity for this device has already been explained, and it is essential that a magneto of this type should be so provided, because of the intensity of the current.

Setting

In setting up a system of ignition with a magneto of this type, the firing order of the engine must be observed, and then one of the pistons brought to the point in the compression stroke that it will occupy when the spark is at the greatest advance. As has been explained in the case of engines equipped for the make and break system, this point will usually be from one-half to three-quarters of an inch before top dead center, the greater proportion of designs placing it at one-half an inch. The driving gear of a Bosch magneto is not keyed to the armature shaft, but shaft and gear are tapered, so that when the nut is drawn up tight there is no possibility of slipping. After the magneto is secured in position its gear may be meshed with the gear that is to drive it, but without drawing up the nut. The armature may then be turned by hand until it is in the vertical position, where it will be retained by the lines of force (Fig. 12), and the gear nut drawn up tight. If this position of the armature cannot be obtained by the sense of touch, it may be exposed to sight by the removal of the bridge that carries the safety spark gap, and of the dust cover that protects the space between the upper sides of the pole pieces.

The loosening of the three-armed frame on the front of the magneto will permit the removal of the covers of the interruptor and timer, and it will be seen that the moving part of the distributor is beginning to make contact with one of the stationary pieces; this should be connected to the spark plug of the cylinder that is in the firing position. The other contacts of the distributor should then be connected to the remaining spark plugs in the order of firing.

Ignition is cut out by short-circuiting the primary winding, and a binding post will be found on the front of the magneto under the three-pronged frame to which is connected the wire that leads to a switch making a ground connection when closed.

Care

The armature and distributor shafts run on ball bearings, and these require the application of a few drops of oil twice a month. The interruptor is designed to operate without oil, and there is therefore little danger of oil working its way into the winding.

The carbon brushes must be wiped off with a little gasoline occasionally, and these are arranged in such a manner that they are easy of access.

The platinum contacts of the interruptor must be kept free from dirt and oil, and when, after long use, they become worn and uneven, they must be faced off by the use of a dead smooth file. The distance between them when separated by the action of the fiber rollers must not be more than one sixty-fourth of an inch, and this may be adjusted by means of the nuts. Accompanying each magneto is a small open end wrench for this purpose, to which is pivoted a leaf of steel that is to be used as a gauge for this distance.

This same measurement applies to the spark plugs, the points of which must provide a gap of not more than one sixty-fourth of an inch.

Troubles

If there is an abrupt failure of ignition, it is probable that the primary is short-circuited, and this can only occur on the wire that leads to the short-circuiting switch, for the other parts of the primary circuit are inclosed and protected against injury.

In case of the missing of one cylinder only, the trouble will usually be found in the plug, which may be short-circuited by a carbon deposit, or because the intense heat of the spark has fused the metal, resulting in the formation of a globule of metal between the spark points. Too great a distance between the points, more than one sixty-fourth of an inch, will prevent the passing of the spark, which will then be seen in the safety spark gap.

A miss in different cylinders may be due to defective insulation of the wires, but if these are in good condition the fault may be looked for in the magneto. The place where trouble is usually experienced is in the interruptor, which may become dirty, or the contacts may be worn down or loosened from the vibration. Fouled contacts in the distributor may also lead to a miss-fire, and this will be corrected by wiping the parts with gasoline. If all of these parts have been examined and found correct, it is inadvisable to examine further into the magneto, for inexperienced handling will be likely to injure the delicate parts. It is better to place it in the hands of a man whose shop is equipped for the work.

Four-Spark Magneto

All of the magnetos described have been of the type in which the armature revolves, and two ignitions are secured per revolution. These are known as two-spark magnetos, or crank-shaft speed magnetos, from the fact that for four cylinder engines the magneto runs at the speed of the crank shaft. This is in distinction to the Bosch four-spark, or cam-shaft speed, magnetos, in which the armature as well as the field is stationary. This magneto for its simplicity, freedom from trouble, and workmanship, has few equals for the high-tension ignition of an internal combustion engine.

The armature in this magneto is of the usual type, and is stationary, with the armature neck in a vertical position. Around the armature, and between it and the pole pieces, revolves a soft iron shield in two sections, these being the length of the armature and the same width as the heads. As it revolves, it forms bridges between the pole pieces and the core of the armature, the lines of force flowing through it as well as through the armature. The diagrams of Fig. 13 show the positions of the shield as it revolves about the armature. In the first position, one segment of the shield forms a bridge between the pole piece and the upper armature head, while the other segment is bridging the space between the lower head and the other pole piece. When in this position, the lines of force flow through the neck of the armature and magnetize it so that it sets up its own field. When the shield revolves so that it covers the heads, the lines of force abandon the neck and flow between the pole pieces by the segments, the field established by the neck dying out. In the third position, the lines of force again flow through the neck, while in the fourth position, with the shield completing a half revolution, the lines of force again pass directly across. In this half revolution there are therefore two periods when a magnetic field forms around the armature neck and dies out, which will result in the induction of two currents in the winding of the armature. If the revolution is continued it will be seen that the same conditions are repeated, the positions of the two segments of the shield being reversed, and that one revolution of the shield about the armature will produce four waves of current in the winding. The magneto may therefore be driven at cam shaft speed for a four cylinder engine, and in addition has the advantage of the wire windings being stationary. The action of the magneto is the same as that of the two-spark type already described, but the construction is simplified because the slow speed of the shield will permit the secondary distributor to be attached directly to the armature shaft, the secondary shaft being done away with.

A diagram of the wiring is shown in Fig. 14. The primary wiring is grounded, and the live end brought out to the stationary contact point of the interruptor. The moving part of the interruptor is pivoted in the center, and the lower end, a polished steel knob, bears against the face of a disk that has four ribs running from center to edge. When the knob is on the space between two of the ribs, the interruptor closes the circuit, but as the disk revolves and a rib touches the knob, the arm moves on its pivot, and the circuit is broken.

The grounded end of the secondary winding is attached to the live end of the primary winding, and its other terminal passes to a contact point carried on a hard rubber disk placed on the revolving shaft immediately in front of the interruptor. The diagram shows a face view of this distributor, as well as a side view. A ring-shaped plate is set on one side of this disk, and is in connection with a contact piece carried on the edge of the disk. A carbon brush to which the live end of the secondary winding is attached is kept pressed against the ring-shaped plate, and the secondary current is thus led to the contact piece on the edge of the disk. As the disk revolves with the shaft of the shield the contact piece comes into successive contact with four carbon brushes, which are connected to the spark plugs.

When the shield is in such a position that the lines of force flow through the neck of the armature, the interruptor is closed, but when the intensity of the current increases to the maximum the interruptor opens, breaking the primary circuit, and producing a rapid demagnetization of the armature core. This dying out of the field induces a powerful current in the secondary winding, which is led by the distributor to one of the spark plugs, where it jumps the gap and returns to the winding by the ground connection.

Setting

In setting the magneto the firing order of the engine is noted, as has been explained, and the magneto secured in position with its gear in mesh, but loose on the shaft. The cover over the space between the pole pieces will be found to be in the form of a shallow aluminum box, which is held in position by a single screw with a large head, and when this is removed the stationary armature and revolving shield will be exposed to view. One of the pistons must then be brought into the position it will take with a fully advanced spark, and the shield then revolved until it is in the position shown in Fig. 15. The nut securing the gear to the shaft may then be drawn up tight. It will be noticed that one of the segments of the shield has a deep notch at each edge. Remembering the direction in which the shield will be driven, the notch at the rear edge of the shield will indicate the position of the contact piece of the distributor which will then be coming into contact with the carbon brush of one of the stationary distributor contacts. The spark plug of the cylinder that is in the firing position should be connected to the brush that the notch indicates as making contact, and the remaining spark plugs and brushes connected according to the firing order.

Care

The proper operation of the magneto requires the free and perfect action of the interruptor, and this may be observed by means of a brass slide that operates on the arm or the spark control lever. When this slide is raised, the action of the interruptor may be watched. Like the two-spark Bosch magneto, the distance between these points when the disk separates them should be one sixty-fourth of an inch, and contact should be made and broken without undue sparking. The condenser that will be found in the shallow aluminum box covering the space between the pole pieces prevents sparking at the interrupter, and is thrown into circuit automatically when it is placed in position.

The four-spark magneto is provided with a safety spark gap, and when the points of one of the spark plugs are separated too far, or when there is any interruption of the secondary circuit, a spark will show there. Sparks should not be permitted to pass in the safety gap for any considerable period, as damage will be done, and when they indicate by their presence that something is wrong, the trouble should be located and repaired without delay.

The regulation and care of the four-spark magneto is the same as has been described for the two-spark machine.

Dual Ignition Systems

It has been the custom to equip jump-spark engines with two complete systems of ignition, one operated by a magneto and the other by coil and battery. Each system has its own spark plugs, timing device, wiring, etc., the two being connected only at the switch. Many of these duplicate parts have been eliminated by the introduction of dual ignition systems, and the mechanism is correspondingly simplified. The Bosch dual system is a good example of the principles involved, and a diagram of the connections is shown on Fig. 16. The magneto used is of the usual two-spark type, except that the secondary current, instead of being led from the collecting ring direct to the distributer, is taken to the switch that is located in the coil box on the dash. In addition to the regulation interrupter illustrated on Fig. 11, the magneto is provided with a second interrupter or circuit breaker that controls the flow of the battery current, and takes the place of a timer. While the magneto interrupter revolves with the armature, and is operated by stationary cams, the battery circuit breaker is stationary, and is operated by a two-nose cam that revolves with the armature shaft. The stationary and revolving cams are so set that the interrupter and circuit breaker break their circuits at the same instant.

The coil is contained in a brass case on the dash, and in design and construction is a duplicate of the magneto armature. Attached to the coil is a handle that projects through a slot in the case, by means of which the coil may be turned partly around in the case. The terminals of the coil windings are attached to metal knobs located on the under side of a fiber plate that forms the bottom of the coil, and turning the coil by the handle moves these knobs in and out of contact with corresponding knobs on the bottom of the case. These knobs form the switch that cuts off all current, or permits the engine to be run on magneto or battery. When the switch is in such a position that the magneto produces secondary current, this flows from terminal 3 of the magneto to terminal 3 of the switch; thence by the switch connection to terminal 4 and to the secondary distributer. When the battery is switched in, the magneto primary is grounded, and secondary current from the coil flows over wire 4 to the secondary distributer.

By pressing a button on top of the coil case, a vibrator is cut into the circuit to permit starting the engine on the spark. When the engine starts the button is released, and ignition is produced by the single spark that results from the demagnetizing of the core that follows the breaking of the battery circuit by the circuit breaker. Throwing the switch breaks the battery circuit and cuts in the magneto.

Inductor Magnetos

In types of magnetos that have recently been produced, the winding is stationary and surrounds a moving core that is magnetized and demagnetized during its revolution between the poles of the field magnets. The Remy magneto is of this type, and Fig. 17 is a diagram of the revolving core, or inductor, with the coil surrounding it. As the inductor revolves, the magnetism flows from one pole of the field magnets into the wing of the inductor nearest to it, through the central cylindrical part, and by the second wing to the other pole, the central part of the inductor then being magnetized. The magnetism of this portion dies away when the wings are in such a position that they bridge across from one pole to the other, but reappears when the central cylindrical part again becomes the only path by which the magnetism may flow. The constant changes in the magnetic strength of the central part affect the coil, and currents are induced in it. These currents, which are of low voltage, are led to the primary winding of a secondary induction coil similar to that used in the Eisemann system, the secondary current flowing to the plugs through a distributer located on the magneto. In the Remy system, the interrupter does not short circuit the magneto winding, as is the case with the Eisemann, but the flow of current is from the magneto winding to the grounded side of the interrupter, to the insulated screw, to the primary winding of the coil, and back to the magneto. The interrupter is closed and the current flows in this circuit while the strength of the current is increasing, but when the current is at a maximum the interrupter breaks the circuit, the core of the coil loses its magnetism, and a secondary current is induced. The current reaches a maximum twice in each revolution.

In other magnetos, the form of the inductor is such that six or more waves of current are produced during each revolution. An inductor of this type is shown in Fig. 18. There are six arms radiating from the central cylindrical portion, three at each end, and equally spaced. The coil is usually made of flat copper ribbon instead of wire, and it surrounds the central cylindrical part between the two sets of arms. In the inductor shown, the magnetism of the central part will change strength six times to each revolution.

Magnetos of this type are used to replace batteries in jump spark systems using vibrator coils. They are driven by belt, or by friction against the fly wheel, and are given a speed at least three times that of the crank shaft. By this means the waves of current follow one another at such brief intervals that the flow is practically continuous, and it is therefore unnecessary to time the magneto to the engine. These magnetos have no interrupters, and the wiring consists of one wire leading to the switch on the coil box, the usual connections between the coil box and timer, and a wire from the second magneto terminal to ground.

Magnetic Make-And-Break System

The flaming spark produced by the make-and-break system has many advantages for ignition over the jump spark produced by the high-tension system, but these are largely offset by the mechanical complication of cams, tappets, springs, etc. A make-and-break system in which the igniters are operated by magnetism has been perfected by the Bosch company, and is giving good results. The magneto used is of the ordinary low-tension type, equipped with an interrupter, while all of the igniter parts are contained in a plug that screws into the cylinder wall like a jump-spark plug. The plug is shown in diagram in Fig. 19. The sleeve that screws into the cylinder supports an iron core, the two being thoroughly insulated by blocks and washers of heat-proof insulating material. The upper portion of the core has a flat side, on which is formed a projection that serves as a pivot. A hammer bar is balanced on this pivot, its long end passing through the plug and coming into contact with an anvil attached to the screwed sleeve. A long horseshoe-shaped spring holds the two in light contact, and the hammer bar then forms the only path by which an electric current may pass from the core to the screwed sleeve and to the engine ground. When the hammer bar is moved on its pivot this connection is broken. Surrounding the core is a coil consisting of a few layers of wire covered with heat-proof insulation. The upper part of the plug is incased in a light iron casing screwed to the core, which completely incloses the core, hammer bar, spring and coil, and in which there is no opening that would permit the escape of compression from the combustion chamber. One terminal of the coil is connected to the binding post, and the other is grounded on the iron casing. The current generated in the magneto passes to a distributer and thence to the binding post of the plug, as shown in Fig. 20. It flows through the coil to the core, and thence by the hammer bar to the engine ground and back to the magneto. As the current establishes itself in the coil, the core becomes magnetized, and attracts to it the flat end of the hammer bar. When the hammer bar moves on its pivot, its connection with the anvil is broken, and this break in the circuit produces a flaming spark that is in every way identical with the spark of the ordinary mechanical make-and-break igniter. The casing that contains the coil may be unscrewed from the core without disturbing the screwed sleeve, and this makes it an easy matter to give the moving parts the inspection and cleaning that they should receive every few weeks. The spark occurs in the plug at the instant that the interrupter operates, and the setting up of the system is identical with that of the high-tension magneto systems.