Self-induction is fully explained in the chapter following.

CHAPTER XI

INDUCTION COILS

The induction coil has always been a popular piece of apparatus with those interested in electrical science; the experiments which can be performed with its aid are very numerous. It is of considerable importance, especially in its application to such useful purposes as X ray work, wireless telegraphy and _ignition_ for gas engines. The latter has caused manufacturers to give much attention to the development of the induction coil, resulting in many refinements of design and construction.

Induction coils may be divided into two general cla.s.ses:

1. Primary coils; 2. Secondary coils.

The subject of electromagnetic induction has been fully explained in chapter X, but it may be said, with special reference to induction coils, that the operation of the two cla.s.ses just mentioned is respectively due to:

1. Self-induction; 2. Mutual induction.

=Self-induction.=--This is the property of an electric current by virtue of which _it tends to resist any change in its rate of flow_. It is sometimes spoken of as _electromagnetic inertia_ and is a.n.a.logous to the mechanical inertia of matter.

Self-induction is due to the action of the current upon itself during variations in strength. It becomes especially marked in a coil of wire, in which the adjacent turns act inductively upon each other upon the principle of _mutual induction_ arising between two separate adjacent circuits. Self-induction manifests itself by giving "_momentum_" to the current so that _it cannot be instantly stopped when the circuit is broken_, the result being a bright spark at the moment of breaking the circuit. On account of this spark a primary induction coil is used in low tension or "make and break" ignition systems.

[Ill.u.s.tration: FIG. 135.--Diagram ill.u.s.trating the action of mutual induction between two circuits; the one including a source of electrical energy and a switch; the other including a galvanometer, but having no cell or other electrical source. During the increase or decrease in the strength of the current as on closing or opening the key a current is _induced_ in the secondary circuit in a direction opposite to that of the primary current as indicated by the arrows.]

In a single circuit, consisting of a straight wire and a parallel return wire there is little or no self-induction. When a circuit containing a primary induction coil and a battery is closed there is no spark because at the instant of closing the circuit the current is at rest and on account of self-induction _the current cannot at once rise to its full value_.

=Mutual Induction.=--This is a particular case of electromagnetic induction in which the magnetic field producing an electromotive force in a circuit is due to the current in a neighboring circuit.

The effect of mutual induction may be explained with the aid of fig. 135.

If, as ill.u.s.trated, a circuit including a battery and a switch, be placed near another circuit, formed by connecting the two terminals of a galvanometer by a wire, it will be found that whenever the first circuit, 1, is closed by the switch, allowing a current to pa.s.s in a given direction, a momentary current will be induced in the second circuit, 2, as shown by the galvanometer. A similar result will follow on the opening of the battery circuit, the difference being that the momentary induced current occurring at closure moves in a direction opposite to that in the battery circuit, while the momentary current at opening moves in the same direction.

Currents, besides being induced in circuit 2 at _make_ or _break_ of circuit 1, are also induced when the current in 1 is fluctuating in intensity.

The most marked results are observed when the make or break is sudden, _the action being strongest at the break of the current in 1._

The inductive effect of the current in the arrangement shown in fig. 135 is very weak.

=Ques. What name is given to circuit 1?=

Ans. The _primary circuit_.

=Ques. What name is given to circuit 2?=

Ans. The _secondary circuit_.

=Ques. What names are given respectively to the currents in circuits 1 and 2?=

Ans. The _primary_ and _secondary_ or _induced currents_.

=Primary Induction Coils.=--These represent the simplest form of coil, and are used chiefly in low tension ignition to intensify the spark when a battery forms the current source.

A primary coil consists of a long iron core wound with a considerable length of low resistance insulated copper wire, the length of the core and the number of turns of the insulated wire winding determining the efficiency. The effect of the iron core is to increase the self-induction.

[Ill.u.s.tration: FIG. 136.--Primary induction coil as used for low tension ignition. Coils of this type are made in a great variety of form and size.

Ordinarily the winding consists of about six convolutions of No. 14 copper wire. The winding is usually covered and the ends capped with ebonite heads so that the core wires are not exposed.]

The spark produced, as previously explained, is due to self-induction, and it should be remembered that in the operation of the coil, the _spark occurs at the instant of breaking the circuit_, _not at the instant of making_.

=Secondary Induction Coils.=--The arrangement shown in fig. 137, may be considered as a very simple or rudimentary form of secondary induction coil. In the actual coil, the primary and secondary circuits (corresponding to 1 and 2 in fig. 135) are made up of coils of insulated wire, as shown in fig. 143, the primary coil P, being wound over a core C and the secondary coil S being wound over the primary.

The one property of such an arrangement that makes it of great value for most purposes is that _the voltage of the induced currents may be increased or diminished to any extent depending on the relation between the number of turns in the primary and secondary winding._

This relation may be expressed in the following rule:

_The voltage of the secondary current is (approximately) to the voltage of the primary current as the number of turns of the secondary winding is to the number of turns of the primary winding._

[Ill.u.s.tration: FIG. 137.--Production of spark with plain coil. Connect the ends or leads of the secondary winding to fixed insulators and bend the ends so they are from one-sixteenth to one-eighth inch apart. Connect one end of the primary winding to an electric battery, and with the other lead of the primary winding brush against the other terminal of the battery, as indicated. When the contact is broken there will be a spark both at the point of rupture in the primary circuit and at the gap. An electric impulse is also induced in the secondary circuit when the primary circuit is closed and the current flowing in it gradually rises to its maximum value, but this impulse is too feeble to cause a spark to jump across the gap. Only the impulse induced in the secondary during the dying out of the current in the primary is utilized.]

For instance, if the voltage of the primary current be 5 volts, the primary winding have 10 turns and the secondary 100 turns, then

Secondary voltage: 5 :: 100 : 10 from which

Secondary voltage = 50 volts (approximately)

The watts in each circuit are approximately the same; hence: if, for instance, the current strength in the primary circuit be 5 amperes, the watts in primary circuit are 5 5 = 25.

Accordingly, for the secondary circuit the current strength is:

25 watts / 50 volts = 1/2 ampere (approximately)

From this, it is seen that where the voltage is raised in the secondary circuit, the current flow is small as compared to that in the primary circuit; therefore, heavy wire is used in the primary winding and fine wire in the secondary, as indicated in figs. 137 and 143.

For most purposes a very much higher secondary voltage is required than in the example just given.

[Ill.u.s.tration: FIG. 138.--Diagram of battery and coil connections for jump spark ignition as applied to a motor cycle. Coils are usually plainly labeled with the abbreviations: "Bat.," "Pri.," "Sec.," indicating that the wires are to be connected to the battery, the primary circuit or contact maker, and the spark plug. The battery and primary wires being for the low tension circuit are easily distinguished from the secondary wire by the small amount of insulation surrounding them.]

Secondary induction coils may be divided into three general cla.s.ses:

1. Plain coils; 2. Vibrator coils; 3. Condenser coils.

The plain coil gives but one spark when the primary circuit is made and broken, while the vibrator coil gives a series of sparks following each other in rapid succession.

=Plain Secondary Induction Coils.=--Coils of this cla.s.s are very simple and consist of:

1. Core; 2. Primary winding; 3. Secondary winding.

The construction of a plain coil, such as would be suitable for ignition service, is about as follows:

The core is made of soft annealed iron wires (No. 20 B and S gauge) from one-half to three-quarters of an inch in diameter and about six inches long. Over this core is slipped a spool of insulating material (hard rubber or composition), on which is wound first the primary winding of the coil, which consists of several layers of about No. 18 B and S gauge silk insulated magnet wire.