_Testing a Cell._--The cells should be frequently tested, to show what loss there is in the amperage. This is done by putting an ammeter in the circuit. If a meter of this kind is not handy, a good plan is to take off one of the wire connections, and snap the wire on the terminal, and the character of the spark will show what energy there is in the cell.
Testing With Instruments.--The method of testing with voltmeter and ammeter, is shown in Fig. 38. The voltmeter is placed in a short circuit between the two terminal wires, whereas the ammeter is placed in circuit with one of the wires. The reason for this is that the voltmeter registers the pressure, the power, or the difference of potential between the two sides of the cell, and the ammeter shows the quant.i.ty of current flowing over the wire.
In practice batteries are not used continuously for igniting. They are temporarily employed, princ.i.p.ally for starting, because their continued use would quickly deplete them.
[Ill.u.s.tration: _Fig. 39. Make and Break, with Battery._]
Simple Battery Make and Break System.--In order to show this method in its simplest form, examine Fig. 39, which diagrams the various parts belonging to the system.
We have ill.u.s.trated it with two cylinders, portions of the heads being shown by the outlines A, A. B, B represent terminals which project into the cylinders, and are insulated from the engine heads. Through the sides of the engine heads are rock shafts C, the ends within the cylinder having fingers D which are adapted to engage with the inner ends of terminals B, B.
On the ends of the rock shafts outside of the cylinders, they are provided with levers E, E, one end of each being attached to a spring F, so that the tension of the spring will normally keep the upper end of the finger D in contact with the terminal B. The cut shows one finger engaging with B, and the other not in contact.
The other end of the lever E rests beneath a collar or shoulder G on a vertical rod H. The lower end of this rod engages with a cam I on a shaft J, and when the cam rotates the rod drops off the elevated part of the cam, and in doing so the shoulder G strikes the end of the lever E and causes the finger to rapidly break away from the terminal B, where the spark is produced.
To Advance the Spark.--For the purpose of advancing or r.e.t.a.r.ding the spark, this rod has, near its lower end, a horizontally-movable bar K, which may be moved to and fro a limited distance by a lever L, this lever being the subst.i.tute in this sketch of the lever on the steering wheel of an automobile.
The spark is advanced or r.e.t.a.r.ded by causing the lower end of the rod H to be moved to the left or to the right, so that it will drop off of the raised portion of the cam earlier or later.
The wiring up is a very simple matter. The battery M has one end connected up with one terminal of a switch N, while the other terminal of the switch has a wire connection with the terminal plugs B, B, in the cylinder heads.
The other end of the battery is connected with the metal of the engine, which may be indicated by the dotted line O which runs to the rock shaft C, and thus forms a complete circuit.
The operation is as follows: When the key P of the switch is moved over so that it contacts with the terminal N, the battery is thrown into the circuit, and the current then pa.s.ses to the plug B of the first cylinder, as the finger D in that cylinder is in contact with that terminal, and it pa.s.ses along the finger D, and rock-shaft C, to the metal of the engine, and pa.s.ses thence to the battery, this course being indicated by the dotted line O.
At the same time, while cylinder No. 2 is also connected up with the battery, the shoulder of the rod H has drawn the finger D from its contact with the plug B, hence the current cannot pa.s.s in that direction.
As the cam I, of cylinder No. 1, turns in the direction of the arrow, the rod drops down and suddenly makes a break in the terminal of this cylinder, causing the ignition, to be followed by a like action in No.
2.
The Magneto in the Circuit.--To insure the life of the battery, so that it may be in service only during that period at the starting, when the magneto is not active, the latter is so placed in the circuit, that, at the starting, when, for instance, the automobile is being cranked, it is cut out by the switch on the dash board.
[Ill.u.s.tration: Fig. 40. Make and Break, with Magneto.]
In Fig. 40, a simple two-pole switch is used. With the magneto it is necessary to have a three-point switch, R, and a plain coil S is placed between the switch and battery.
One side of the magneto T is connected by wire U with one of the points of the switch R, and the other side of the magneto is connected with the metal of the engine, which is indicated by the dotted line V.
In all other respects the mechanism is the same. The starting operation has been explained with reference to the preceding figure, and when the engine has picked up, and is properly started, the switch bar is thrown over so it contacts with the point connected up with the wire U leading to the magneto.
This, of course, cuts out the battery, and the engine is now running on the magneto alone. The object of the coil S is to oppose a rapid change of the current at the moment of the interruption. The coil induces a counter current the moment the break is made, and as the current continues to flow for a very short period after the break a spark of greater intensity is produced than if the circuit should be permitted to go from the battery to the sparker directly, as in the previous ill.u.s.tration.
The best spark is produced by quickly making the break between the points B, D, so that particular attention has been given to mechanism which will do this effectively.
Magneto Spark Plug.--One of the devices to obviate the difficulty of providing moving mechanism outside of the engine cylinder, is shown in Fig. 41. In this the coil A is connected with a terminal B at the head of the device and the other is connected to the plug C which screws into the cylinder head.
[Ill.u.s.tration: _Fig. 41. Magneto Spark Plug._]
Within the core is a pivotally-mounted lever D, the upper end E of which is attracted by the tubular metallic core F, and the lower end having a contact point G, which is adapted to engage with a stationary point H.
The pivot I, on which the lever D is mounted, provides a means whereby the lever swings, and a spring J is so arranged that when the lower end of the lever is disengaged from the contact, the spring will return it to its normal position.
In its operation when a contact is formed by the timing device of the magneto, so as to give a spark, the circuit pa.s.ses to the terminal B, coil A, and plug C, thus forming a complete circuit. This energizes the core A, pulling the upper end of the lever, and at the same time causes the lower end to disengage the two contacts G, H, which breaks the circuit and produces a spark.
The breaking of the circuit deenergizes the core, and the spring again draws the lever back to its normal position, ready for the next completion of the circuit by the timing device.
Such an arrangement is as simple as the spark plug usually employed in the use of the high tension system, although it is more expensive than the plug.
CHAPTER VIII
IGNITION. HIGH TENSION
This system is used to the largest extent, so that we ought to have a full explanation of the devices which are required to do the work. While magnetos are used with the low tension system, for the reasons stated, they are especially necessary with the _Jump Spark_ method.
Magnetos.--The most important element in this system is the magneto, so we shall try and make the subject as explicit as possible. As stated, a magneto is a special type of dynamo which will now be explained. For this purpose it will be necessary to show the elementary operation of an alternating current dynamo.
Alternating Current.--In Fig. 42 A is a bar of soft iron, around which is a coil of wire B, the wire being insulated, so that it will not touch the bar. There is no magnetism in this bar, and this simple form of structure is shown, merely to represent what is called the _field_ of a dynamo.
The object of the coil of wire is to make a magnet of the bar, for the moment a current is sent over the wire, a magnet is formed, and the magnetism leaves the bar the moment the current ceases to flow. If this bar should be of hard steel it would retain the magnetism.
[Ill.u.s.tration: _Fig. 42. Ill.u.s.trating Alternating Current._]
[Ill.u.s.tration: _Fig. 43. Alternating Current. Second position._]
Now, the primary difference between the magneto and the dynamo, is that this field bar is a permanent magnet in the magneto, whereas the field is only a temporary magnet in the dynamo. This should always be kept in mind.
The end of a magnet, whether it is a temporary one, or permanent, has a magnetic field of force at the ends as well as at all parts of it, exterior to the surface of the bar. Such a field is indicated, and in the dynamo, no such field exists unless a current is pa.s.sing over the wire B, which is called the _field winding_.
The U-shaped piece of metal C represents the armature. It is shown hinged to the top of two posts, for clearness in understanding, and is adapted to turn to the right, and in turning the loop pa.s.ses the end of the field bar B, and pa.s.ses through the magnetic field which is indicated by the dotted lines D.
[Ill.u.s.tration: _Fig. 44. Alternating Current. Third position._]
Now, if the loop is simply permitted to remain in the position shown in Fig. 42, a current would flow through the loop, this transference of the current being called induction, and this characteristic of the flow of electricity will be explained and its utility explained.
Cutting Lines of Force.--The loop will now be turned to the right so that it pa.s.ses the magnetic field and goes beyond it in its revolution.
This motion of pa.s.sing the armature through the magnetic field is called _cutting_ the _lines of force_. While the loop was lying within the magnetic field, and also when it was moving through the field, the current set up in the loop flowed in the direction of the darts F, or to the right, through the pivots D.
In Fig. 43 the loop is shown as having made a quarter turn, and it is now vertical, or at right angles to its former position. The loop in thus pa.s.sing away loses its force, until it reaches the position shown in Fig. 44, when there is a surging back of the current to the opposite direction, as indicated by the arrows.
[Ill.u.s.tration: _Fig. 45. Alternating Current. Fourth position._]
When the loop reaches the lowest position, shown in Fig. 45, it again begins to get the influence of the magnetic field, and a reversal back to its former direction takes place, this surging movement back and forth being due to the reversal of the polarity in the coil brought about by the position in which it is placed relative to the magnetic field.
It is now an easy matter to connect the ends of the loop with wire conductors. This is shown in Fig. 46, where a small metal wheel G is placed on each end of the spindle, and in having a strip of metal bearing H on the wheel. These are not commutator brushes, but are merely wiping brushes to take the current from the turning parts. Wires I connect with these wiping bars, and through them the current is transmitted to perform the work.