Since a solenoid carrying a current attracts and repels by its extremities the poles of a magnet, two such solenoids will attract and repel each other.

[Ill.u.s.tration: FIG. 118.--Magnetic field of a solenoid. This is best observed by cutting a piece of cardboard and fitting it around the solenoid, as shown. If iron filings be sprinkled on the cardboard and a current pa.s.sed through the solenoid, the character of the field is indicated. With the current in the direction shown, it will be found that wherever small compa.s.s needles are placed, the direction in which their north poles turn is along arrows marked on the card. The card only exhibits the field in one of the sectional planes of the coil, but it is obvious that the field is the same for all sectional planes.]

=Ques. How does the magnetic strength of a solenoid vary?=

Ans. It is proportional to the strength of the electric current pa.s.sing through it.

=Ques. On what, besides the current strength, does the magnetizing power of a solenoid depend?=

Ans. _The magnetic effect or the magnetizing power is proportional to the number of turns of wire composing the coil._

=Ques. How may the magnetizing power of a solenoid be increased?=

Ans. By inserting in the solenoid an _iron core_ or round bar of soft iron.

[Ill.u.s.tration: FIG. 119.--Right hand rule for polarity of a solenoid: If the solenoid be grasped in the right hand, so that the fingers point in the direction in which the current is flowing in the wires, the thumb extended will point in the direction of the north pole.]

=Ques. Describe the action of an iron core.=

Ans. At first, the presence of an iron core greatly increases the strength of the field; after a time, however, as the strength of the current flowing in the exciting coils is increased, the _conductibility_ of the iron for the lines of force appears to decrease, until a point is eventually reached when the presence of the iron core appears to have no effect in increasing the strength of the field.

=Permeability.=--Permeability is a measure of the ease with which magnetism pa.s.ses through any substance. It is defined as: _the ratio between the number of lines of force per unit area pa.s.sing through a magnetizable substance, and the magnetizing force which produces them_.

[Ill.u.s.tration: FIGS. 120 and 121.--Ill.u.s.trating the effect of introducing an iron core into a solenoid. In the upper figure, the air s.p.a.ce or "air core" surrounded by the solenoid offers considerable resistance to the pa.s.sage of magnetic lines, allowing only a small number to pa.s.s through.

If a piece of iron be introduced, as in the lower figure, the number of lines will be greatly increased. The number of lines B pa.s.sing through a unit cross section of the iron core divided by the number of lines H, pa.s.sing through a unit cross section of the air core is called the _permeability_ and designated by the Greek letter .]

In other words, it is the ratio of flux density to magnetizing force.

Permeability is a measure of the ease with which magnetism pa.s.ses through any substance. The permeability of good soft wrought iron is sometimes 3000 times that of air, varying with the quality of the iron.

=Ques. What is the effect of increasing the magnetization?=

Ans. The magnetic permeability decreases as the magnetization increases.

=Ques. What is magnetic saturation?=

Ans. The state of a magnet which has reached the highest degree of magnetization.

[Ill.u.s.tration: FIG. 122.--Action of currents on solenoids. To demonstrate this fact experimentally, a solenoid is constructed as shown, so that it can be suspended by two pivots in the cups _a_ and _c_. The solenoid is then movable about a vertical axis, and if a rectilinear current QP be pa.s.sed beneath it, which at the same time traverses the wires of the solenoid, the latter is seen to turn and set at right angles to the lower current; that is, in such a position that its circuits are parallel to the fixed current; moreover, the current in the lower part of each of the circuits is in the same direction as in the rectilinear wire. If, instead of pa.s.sing a rectilinear current below the solenoid, it be pa.s.sed vertically on the side, an attraction or repulsion will take place, according as the two currents in the vertical wire, and in the nearest part of the solenoid, are in the same or in contrary directions.]

A magnet, just after being magnetized, will appear to have a higher degree of magnetism than it is able to retain permanently; that is, it will appear to be super-saturated, since it will support a greater weight immediately after being magnetized than it will after its armature has been once removed.

For all practical purposes, magnetic saturation may be defined as: That point of magnetization where _a very large increase in the magnetizing force does not produce any perceptible increase in the magnetization_.

From tests it has been shown that permeability increases with the flux density up to a certain point and then decreases, indicating that the iron is approaching a state of saturation.

=Magnetomotive Force.=--This is a force similar to electromotive force, that is, magnetic pressure. When a coil pa.s.ses around a core several times, its magnetizing power, or magnetomotive force, (m.m.f.) is proportional both to the strength of the current and to the number of turns in the coil. The product of the current pa.s.sing through the coil multiplied by the number of turns composing the coil is called the _ampere turns_.

It is known by experiment that one ampere turn produces 1.2566 units of magnetic pressure, hence:

magnetic pressure = 1.2566 turns amperes

that is,

magnetomotive force (m.m.f.) = 1.2566 n I.

The unit of magnetic pressure is the _gilbert_ (named after William Gilbert, the English physicist) and is equal to

1 1.2566 ampere turn = .7958 ampere turn.

=Reluctance.=--The magnetic pressure (magnetomotive force) acting in a magnetic circuit encounters a certain opposition to the production of a magnetic field, just as electromotive force in an electric circuit encounters opposition to the production of a current. In the magnetic circuit this opposition is called the _reluctance_; it is simply _magnetic resistance_ and may be defined as: _the resistance offered to the magnetic flux by the substance magnetized, being the ratio of the magnetomotive force to the magnetic flux_.

The unit of reluctance or magnetic resistance is the _oersted_ (named after Hans Christian Oersted, the Danish physicist) and is defined as: _the reluctance offered by a cubic centimetre of vacuum_.

[Ill.u.s.tration: FIG. 123.--Mutual action of solenoids. When two solenoids traversed by a current are allowed to act on each other, one of them being held in the hand and the other being movable about a vertical axis, as shown in the figure, attraction and repulsion will take place just as in the case of two magnets (see figs. 110 and 111).]

=a.n.a.logy Between Electric and Magnetic Circuits.=--The total number of magnetic lines of force, or magnetic flux, produced in any magnetic circuit will depend on the magnetic pressure (m.m.f.) acting on the circuit and the total reluctance of the circuit, just as the current in the electrical circuit depends upon the electrical pressure and the resistance of the circuit.

To make this plain, Ohm's law states that

electric current = electromotive force / resistance or I = E/R

expressed in units

amperes = volts / ohms

The resistance, as already explained, depends on the materials of which the circuit is composed, and their geometrical shape and size.

Similarly, in the magnetic circuit, the total number of magnetic lines produced by a given magnetizing solenoid depends on the magnetic pressure, the material composing the circuit, and its shape and size.

That is,

magnetic flux = magnetomotive force / reluctance

expressed in units, the equation becomes:

maxwells = gilberts / oersteds

_The gilbert is the unit of magnetomotive force, equivalent to the magnetomotive force of .7958 ampere turn._

It should be noted that in the electric circuit resistance causes heat to be generated and therefore energy to be wasted, but in the magnetic circuit reluctance does not involve any similar waste of energy.

=Ques. Upon what does the reluctance of a magnetic circuit depend?=

Ans. _The reluctance is directly proportional to the length of the circuit, and inversely proportional to its cross sectional area_.

The reluctance of a magnetic circuit is calculated according to the following equation:

reluctance = length in centimetres / (permeability cross section in square centimetres)