=Hysteresis.=--The term hysteresis has been given by Ewing to the subject of _lag of magnetic effects behind their causes_. Hysteresis means to "lag behind," hence its application to denote the _lagging of magnetism, in a magnetic metal, behind the magnetizing flux which produces it_.

=Ques. What is the cause of hysteresis?=

Ans. It is due to the friction between the molecules of iron or other magnetic substance which requires an expenditure of energy to change their positions.

[Ill.u.s.tration: FIGS. 124 and 125.--Experiment ill.u.s.trating the molecular theory of magnetism. Coa.r.s.e steel filings are placed inside a small gla.s.s tube and the contents magnetized. It will be found that filings which at first had no definite arrangement will rearrange themselves under the influence of magnetic force, and a.s.sume symmetrical positions, each one lying in line with, or parallel to its neighbor, as shown in the lower figure.]

=Ques. When do the molecules change their positions?=

Ans. Both in the process of magnetization and demagnetization.

=Ques. What becomes of the loss of energy due to hysteresis?=

Ans. It is converted into heat in changing the positions of the molecules during magnetization and demagnetization.

Ewing gives the value for the energy in ergs dissipated per cubic centimetre, for a complete cycle of doubly reversed strong magnetization for a number of substances as follows:

Substance Energy dissipated (ergs) Very soft annealed iron 9,300 Less " " " 16,300 Hard drawn steel wire 60,000 Annealed " " 70,000 Same steel gla.s.s hard 76,000 Piano steel wire annealed 94,000 " " " normal temper 116,000 " " " gla.s.s hard 117,000

Approximately 28 foot pounds of energy are converted into heat in making a double reversal of strong magnetization in a cubic foot of iron.

=Residual Magnetism.=--When a ma.s.s of iron has once been magnetized, it becomes a difficult matter to entirely remove all traces when the magnetizing agent has been removed, and, as a general rule, a small amount of magnetism is permanently retained by the iron. This is known as _residual magnetism_, and it varies in amount with the quality of the iron.

Well annealed, pure wrought iron, as a rule, possesses very little residual magnetism, while, on the other hand, wrought iron, which contains a large percentage of impurities, or which has been subjected to some hardening process, such as hammering, rolling, stamping, etc., and cast iron, possess a very large amount of residual magnetism.

Residual magnetism in iron is of great importance in the working of the _self-exciting_ dynamo, and is, indeed, the essential principle of this cla.s.s of machine.

That is, without residual magnetism in the field magnet core, the dynamo when started would not generate any current unless it received an initial excitation from an external source.

CHAPTER X

ELECTROMAGNETIC INDUCTION

The word _induction_, introduced by Faraday, has various meanings so far as it relates to electricity. It signifies, in general, phenomena produced in bodies by the influence of other bodies, having no necessary material connection with them.

_A body charged with electricity causes or "induces" charges on neighboring bodies._ The process in this case is called _electrostatic induction_.

A magnet induces magnetism in neighboring ma.s.ses of iron or other magnetic materials by the process of _magnetic induction_.

Again, a moving magnet induces electric currents in neighboring conductors by the process of _electromagnetic induction_.

=Faraday's Discovery.=--All dynamos of whatever form, are based upon the discovery made by Faraday[11] in 1831, which may be stated as follows:

_Electric currents are generated in conductors by moving them in a magnetic field, so as to cut magnetic lines of force._

=Ques. What does the expression "cut lines of force" mean?=

Ans. A conductor, forming part of an electric circuit, _cuts lines of force_ when it moves across a magnetic field in such manner as to _alter_ the number of magnetic lines of force which are embraced by the circuit.

It is important to clearly understand the meaning of this expression, which will be later explained in more detail.

[Ill.u.s.tration: FIG. 126.--Faraday's dynamo which embodies his discovery in 1831 of _electromagnetic induction_, the principle upon which all dynamos work, as well as induction coils, transformers, and other electrical apparatus.]

=Faraday's Machine.=--After various experiments, Faraday made his "new electrical machine" as shown in fig. 126. This piece of apparatus is preserved and was shown in perfect action by Prof. S. P. Thompson in a lecture delivered April 11th, 1891, after an interval of sixty years.

It consists of a horse shoe magnet and a copper disc attached to a shaft and supported so as to turn freely. The magnet is so placed that its inter-polar lines of force traverse the disc from side to side. There are two copper brushes, one bears against the shaft, and the other against the circ.u.mference of the disc. A handle serves to rotate the disc in the magnetic field.

Now, if the north pole of the magnet be nearest the observer and the disc be rotated clockwise, the current _induced_ in the circuit will flow out at the brush which touches the circ.u.mference, and return through the brush at the shaft.

=Faraday's Principle.=--The principle deduced from Faraday's experiment may be stated as follows:

_When a conducting circuit is moved in a magnetic field so as to alter the number of lines of force pa.s.sing through it, a current is induced therein, in a direction at right angles to the direction of the motion, and at right angles also to the direction of the lines of force, and to the right of the lines of force, as viewed from the point from which the motion originated._

Faraday's principle may be extended as follows to cover all cases of electromagnetic induction:

_When a conducting circuit is moved in a magnetic field, so as to alter the number of lines of force pa.s.sing through it, or when the strength of the field is varied so as to either increase or decrease the number of lines of force pa.s.sing through the circuit, a current is induced therein which lasts only during the interval of change in the number of lines of force embraced by the circuit._

=Ques. Explain just what happens when a current is induced by electromagnetic induction.=

Ans. In order to induce an electromotive force by moving a conductor across a uniform magnetic field, it is necessary that the conductor, in its motion, should so cut the magnetic lines as to alter the number of lines of force that pa.s.s through the circuit of which the moving conductor forms a part.

=Ques. What is the proper name for a "conductor" which moves across the magnetic field?=

Ans. An _inductor_, because it is that part of the electric circuit in which induction takes place.

In the case of a dynamo, an inductor may be either a copper wire or copper bar.

=Ques. How may a conducting circuit be moved across a magnetic field without having a current induced therein?=

Ans. If a conducting circuit--a wire ring or single coil, for example--be moved in a uniform magnetic field, as shown in fig. 127, so that only the same number of lines of force pa.s.s through it, no current will be generated, for since the coil is moved by a motion of translation to another part of the field, as many lines of force will be left behind as are gained in advancing from its first to its second position.

[Ill.u.s.tration: FIG. 127.--Electromagnetic induction: In order to induce a current by electromagnetic induction, a conductor must be so moved through a magnetic field _that the number of lines of force pa.s.sing through it (that is, embraced) are altered_. If a coil be given a simple motion of translation in a uniform magnetic field as indicated in the figure, no current will be induced _because the number of lines of force pa.s.sing through it are not changed_, that is, during the movement as many lines are lost as are gained.]

=Ques. Describe another movement by which no current will be induced.=

Ans. If the coil be merely rotated on itself around a central axis, that is, like a fly wheel rotating around a shaft, the number of lines of force pa.s.sing through the coil will not be altered, hence no current will be generated.

=Ques. State the essential condition for current induction in a uniform field.=

Ans. The coil in which a current is to be induced, must be tilted in its motion across the uniform field, or rotated around any axis in its plane as in fig. 128, _so as to alter the number of lines of force which pa.s.s through it_.

[Ill.u.s.tration: FIG. 128.--Electromagnetic induction: If a coil be given a motion of rotation from any point within its own plane so that it pa.s.ses through a uniform magnetic field, a current will be induced in the coil _because the number of lines of force pa.s.sing through it is altered_.]

=Ques. In what direction will the current flow in the coil, fig. 128?=