MAGNETISM
=Magnetism.=--The ancients applied the word “magnet,” _magnes lapes_, to
certain hard black stones which possess the property of attracting small
pieces of iron, and as discovered later, to have the still more remarkable
property of pointing north and south when hung up by a string. At this
time the magnet received the name of _lodestone_ or “leading stone.” It is
commonly, though incorrectly, spelled loadstone.
[Illustration: FIG. 93.--Simple compass. It consists of a magnetic needle
resting on a steel pivot, protected by a brass case covered with glass,
and a graduated circle marked with the letters N, E, S, W, to indicate the
cardinal points; _a b_ is a lever which arrests the needle by pushing it
against the glass when the button _d_ is pressed.]
=Ques. Describe two kinds of magnetism.=
Ans. Magnets have two opposite kinds of magnetism or magnetic poles, which
attract or repel each other in much the same way as would two opposite
kinds of electrification.
=Ques. What is the nature of each kind of magnetism?=
Ans. One has a tendency to move toward the north and the other toward the
south.
[Illustration: FIGS. 94 to 96.--Simple _bar magnet_ and _horse shoe
magnet_ with _keeper_. These are known as _permanent magnets_ in
distinction from _electromagnets_. The horse shoe magnet will attract more
than the bar magnet because both poles act together. A piece of soft iron,
or keeper is placed across the ends of a horse shoe magnet to assist in
preventing the loss of magnetism.]
[Illustration: FIGS. 97 and 98.--Horizontal magnetic needle, and magnetic
“dip” needle. A magnetic needle consists of a small bar magnet, supported
upon a pivot or suspended so that it is free to turn in a horizontal or
vertical plane. The form of magnetic needle illustrated in fig. 97 is
arranged to show the magnetic meridian; the needle moves upon a
perpendicular axis or pivot _ab_. In fig. 98, the needle _sn_ turns upon a
horizontal axis _ab_. This needle indicates the dip or inclination, that
is, the angle which it makes with the horizontal plane, due to the fact
that in most places the lines of force are not horizontal. In the northern
hemisphere the N pole of the needle is depressed, in the southern
hemisphere the S pole is similarly affected. When used, the dip needle
must be set so that the plane in which the needle swings contains the
magnetic meridian, as indicated by the horizontal needle.]
=Ques. Where is the magnetism the strongest?=
Ans. In two regions called the _poles_.
=Ques. Describe the distribution of magnetism in a long shaped magnet.=
Ans. The strongest magnetism resides in the ends, while all around the
magnet half way between the poles there is no attraction at all.
[Illustration: FIG. 99.--Magnetic poles. If a bar magnet be plunged into
iron filings and then lifted, as illustrated in the figure, a mass of
filings will cling to the ends of the magnet but not to the middle. The
ends are called the _poles_ of the magnet.]
=Ques. How are the poles designated?=
Ans. They are called the _north pole_ and the _south pole_.
=Ques. What is the distinguishing feature of each?=
Ans. The north pole points approximately to the earth’s geographical
north, while the south pole of a magnet points approximately to the
earth’s geographical south.
The north pole is the positive (+) pole and the south pole is
the negative. The north and south poles were formerly called in
France, the austral and boreal poles respectively.
=Magnetic Field.=--When a straight bar magnet is held under a piece of
card board upon which iron filings are sprinkled, the filings will arrange
themselves in curved lines radiating from the poles. If a horse shoe
magnet be held at right angles to the plane of the card board, the filings
will arrange themselves in curved lines, as shown in fig. 108. These lines
are called _magnetic lines of force_ or simply _lines of force_; they show
that the medium surrounding a magnet is in a state of stress, the space so
affected being called the _magnetic field_.
[Illustration: FIG. 100.--Badly magnetized bar. Properly magnetized
magnets have only two poles; it is possible, however, by special or
careless magnetization, to produce magnets with more than two poles, but
no process will produce a magnet with a single pole. If an abnormal magnet
with more than two poles be dipped into iron filings, the latter will
adhere at places other than the two ends, as shown in the illustration.
The polarities are alternately N and S; that is, the regions N, B, N, have
north polarity, while A and C have south polarity. These are known as
_consequent poles_.]
[Illustration: FIGS. 101 to 107.--Effect of breaking a magnet into several
parts. If a magnetized needle be broken, each part will be found to be a
complete magnet having a N and S pole. The sub-division may be continued
indefinitely, but always with the same result as indicated in the figure.
This is evidence of the correctness of the molecular theory of magnetism,
which states that _the molecules of a magnet are themselves minute magnets
arranged in rows with their opposite poles in contact_.]
=Ques. What is the extent and character of the magnetic field?=
Ans. The influence of a magnet is supposed to extend in all directions
indefinitely, however, the effect is very slight beyond a comparatively
limited area.
[Illustration: FIG. 108.--The region about a magnet in which its magnetic
forces can be detected is called the _magnetic field_. This can be
represented graphically by placing a piece of cardboard over the magnet
and sprinkling iron filings on the paper, gently tapping at the same time.
Each filing becomes a temporary magnet by _induction_, and sets itself,
like the compass needle, in the direction of the _line of force_ of the
magnetic field.]
[Illustration: FIG. 109.--Tracing lines of force with a suspended magnet.
If a small magnetic needle, suspended by a thread, be held near a magnet,
it will point in some fixed direction depending on the proximity of the
poles of the magnet. The direction taken by the magnet is called the
direction of the force at the point, and if the suspended needle be moved
forward in the direction of the pole, it will trace out a curved line
which will be found to start from one of the poles, and end at the other.
Any number of such lines can be traced; the space filled by these lines of
force is called the _magnetic field_.]
[Illustration: FIG. 110.--Magnetic action: _Unlike poles of magnets
attract each other_.]
[Illustration: FIG. 111.--Magnetic action: _Like poles of magnets repel
each other_.]
=Magnetic Force.=--This is the force with which a magnet attracts or
repels another magnet or any piece of iron or steel. The force varies with
the distance, being greater when the magnet is nearer and less when the
magnet is farther off. The following are the laws relating to magnetic
force:
1. _Like magnetic poles repel one another; unlike magnetic poles attract
one another._
2. _The force exerted between two magnetic poles varies inversely as the
square of the distance between them._
=Magnetic Circuit.=--The path taken by magnetic lines of force is called a
magnetic circuit; the greater part of such a circuit is usually in
magnetic material, but there are often one or more air gaps included. The
total number of lines of force in the circuit is known as the _magnetic
flux_.
=Ques. How is magnetic flux measured?=
Ans. By a unit called the maxwell.
Named after James Clerk Maxwell the Scottish physicist.
=Ques. What is the maxwell?=
Ans. _The amount of magnetism passing through every square centimetre of a
field of unit density._
=Ques. What is the unit of field strength?=
Ans. The gauss.
=Ques. What is a gauss?=
Ans. _The intensity of field which acts on a unit pole with a force of one
dyne. It is equal to one line of force per square centimetre_. Named
after Karl Friedrich Gauss, the German mathematician.
=The Magnetic Effect of the Current.=--Hans Christian Oerstead, the Danish
scientist, discovered in 1819 that a magnet tends to set itself at right
angles to a wire carrying an electric current. He also found that the way
in which the needle turns, whether to the right or left of its usual
position, depends: 1, upon the position of the wire that carries the
current, whether it be above or below the needle, and 2, on the direction
in which the current flows through the wire.
[Illustration: FIG. 112.--Illustrating Maxwell’s “corkscrew rule” for
relative directions of current and lines of force. According to the rule:
_the direction of the current and that of the resulting magnetic force are
in the same relation to each other as is the forward travel and rotation
of an ordinary corkscrew_, Thus, in the figure, if a current flow through
the wire _ab_ in the direction from _a_ to _b_, the magnetic lines will
encircle the wire in the direction of the curved arrow _ro_ which shows
the direction in which the corkscrew must be turned to advance in the
direction of the arrow _n_.]
To keep these movements in mind numerous rules have been suggested, of
which the following will be found convenient:
=Corkscrew Rule=.--_If the direction of travel of a right handed corkscrew
represent the direction of the current in a straight conductor, the
direction of rotation of the corkscrew will represent the direction of the
magnetic lines of force._
[Illustration: FIG. 113.--Experiment showing direction of lines of force
in the magnetic field surrounding a conductor carrying an electric
current. A piece of copper wire is pierced through the center of a sheet
of cardboard, and carried vertically for two or three feet then bent
around to the terminals of a battery or other source of current. If iron
filings be sprinkled over the card while the current is passing, they will
arrange themselves in circles around the wire, thus indicating the form of
the magnetic field surrounding the conductor. Compass needles may also be
used to show the direction of the lines of force at any point.]
[Illustration: FIG. 114.--Right hand rule to determine the direction of
magnetic field around a conductor carrying a current. The thumb of the
right hand is placed along the conductor, pointing in the direction in
which the current is flowing, then, if the fingers be partly closed, as
shown in the illustration, the finger tips will point in the direction of
the magnetic whirls.]
=Ques. What is the effect of a current flowing in a loop of wire?=
Ans. If, in figs. 116 and 117, the current flow in the direction indicated
by the arrow, the lines for magnetic force are found to surround the loop
as shown; all the lines leave on one side of the loop and return on the
other; accordingly, a north pole is formed on one side, and a south pole
on the other.
[Illustration: FIG. 115.--Right hand palm rule to determine the direction
of the magnetic field around a conductor carrying a current: Place the
palm of the outstretched right hand above and to the right side of the
wire with the fingers pointing in the direction of the current, and the
thumb extended at right angles, that is, pointing downward. The direction
in which the thumb points will indicate the direction of the magnetic
whirls.]
[Illustration: FIG. 116.--Lines of force of a circular loop. If a current
flow through the loop in the direction indicated, the lines of force both
inside and outside the loop, will cross the plane of the loop at right
angles, and all those which cross the loop on the inside will pass through
the plane in one direction (downwards in the figure), while all on the
outside will return through the plane in the opposite direction.]
=Solenoids.=--A solenoid consists of a spiral of conducting wire wound
cylindrically so that, when an electric current passes through it, its
turns are nearly equivalent to a succession of parallel circular circuits,
and it acquires magnetic properties similar to those of a bar magnet.
[Illustration: FIG. 117.--Lines of force of a circular loop. If the loop
pass through a piece of cardboard at right angles to its plane, and the
current flow as indicated, the dotted lines on the cardboard will
represent the direction of the lines of force in the plane of the
cardboard. The student should verify the lines of force as here given by
applying the corkscrew rule.]
=Ques. What is the character of the lines of force of a solenoid in which
a current is flowing?=
Ans. The lines of force must be thought of as closed loops linked with the
current. The conductor conveying the current passes through all the loops
of force, and these are, so to speak, threaded or slung on the
current-line of flow, as in fig. 116.
=Ques. What is the distribution of the lines of force?=
Ans. The lines of force form continuous closed curves running through the
interior of the coil; they issue from one end and enter into the other end
of the coil, as shown in fig. 117.
=Ques. What are the properties of a solenoid?=
Ans. A solenoid has north and south poles, and in fact possesses all the
properties of an ordinary permanent magnet, with the important difference
that the magnetism is entirely under control.
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.
[Illustration: 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 passed through the solenoid, the character of the field is
indicated. With the current in the direction shown, it will be found that
wherever small compass 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 passing
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.
[Illustration: 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 passes through any substance. It is defined as: _the ratio
between the number of lines of force per unit area passing through a
magnetizable substance, and the magnetizing force which produces them_.
[Illustration: FIGS. 120 and 121.--Illustrating the effect of introducing
an iron core into a solenoid. In the upper figure, the air space or “air
core” surrounded by the solenoid offers considerable resistance to the
passage of magnetic lines, allowing only a small number to pass 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 passing through a
unit cross section of the iron core divided by the number of lines H,
passing through a unit cross section of the air core is called the
_permeability_ and designated by the Greek letter μ.]
In other words, it is the ratio of flux density to magnetizing force.
Permeability is a measure of the ease with which magnetism passes 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.
[Illustration: 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
passed 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 passing a rectilinear current below the solenoid, it be passed
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 passes 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 passing 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_.
[Illustration: 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).]
=Analogy 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)
=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.
[Illustration: FIGS. 124 and 125.--Experiment illustrating the molecular
theory of magnetism. Coarse steel filings are placed inside a small glass
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 assume 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 glass hard 76,000
Piano steel wire annealed 94,000
“ “ “ normal temper 116,000
“ “ “ glass 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 mass 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
class 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.