CHAPTER IX
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.
=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.
=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.
=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_.
=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.
=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.
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._
=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.
=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.
=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.
=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.
=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_.
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.
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_.
=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.
=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.