The induction of the magnetization may be measured by observing the force required to draw apart the two portions of a divided rod or ring when held together by their mutual attraction.
When induction or magnetic flux takes place in a ferromagnetic metal, the metal becomes magnetized, but the magnetization at any point is proportional not to B, but to B - H.
During the second stage small increments of magnetizing force are attended by relatively large increments of magnetization, as is indicated by the steep ascent of the curve.
A thin sheet of magnetic matter magnetized normally to its surface in such a manner that the magnetization at any place is inversely proportional to the thickness h of the sheet at that place is called a magnetic shell; the constant product hI is the strength of the shell and is generally denoted by 4, or 4.
The magnetic susceptibility expresses the numerical relation of the magnetization to the magnetizing force.
The force in the interior is uniform, opposite (6) (II) [[[Terminology And Principles]] to the direction of magnetization, and equal to 3rI.
The availability of the energy of magnetization is limited by the coercive force of the magnetized material, in virtue of which any change in the intensity of magnetization is accompanied by the production of heat.
In the construction of this soft-iron instrument it is essential that the fragment of iron should be as small and as well annealed as possible and not touched with tools after annealing; also it should be preferably not too elongated in shape so that it may not acquire permanent magnetization but that its magnetic condition may follow the changes of the current in the coil.
Changes of Dimensions attending Magnetization.
Effects of Mechanical Stress on Magnetization.
Miscellaneous Effects of Magnetization: Electric Conductivity - Hall Effect - Electro-Thermal Relations - Thermoelectric Quality - Elasticity - Chemical and Voltaic Effects.
Until 1820 all the artificial magnets in practical use derived their virtue, directly or indirectly, from the natural magnets found in the earth: it is now recognized that the source of all magnetism, not excepting that of the magnetic ore itself, is electricity, and it is usual to have direct recourse to electricity for producing magnetization, without the intermediary of the magnetic ore.
With suitable arrangements of iron and coil and a sufficiently strong current, the intensity of the temporary magnetization may be very high, and electromagnets capable of lifting weights of several tons are in daily use in engineering works.
Again, a steel wire through which an electric current has been passed will be magnetized, but so long as it is free from stress it will give no evidence of magnetization; if, however, the wire is twisted, poles will be developed at the two ends, for reasons which will be explained later.
This experiment proves that the condition of magnetization is not confined to those parts where polar phenomena are exhibited, but exists throughout the whole body of the magnet; it also suggests the idea of molecular magnetism, upon which the accepted theory of magnetization is based.
The process of magnetization consists in turning round the molecules by the application of magnetic force, so that their north poles may all point more or less approximately in the direction of the force; thus the body as a whole becomes a magnet which is merely the resultant of an immense number of molecular magnets.
Iron and its alloys, including the various kinds of steel, though exhibiting magnetic phenomena in a pre-eminent degree, are not the only substances capable of magnetization.
Many of the physical properties of a metal are affected by magnetization.
The intensity of magnetization, or, more shortly, the magnetization of a uniformly magnetized body is defined as the magnetic moment per unit of volume, and is denoted by I, I, or „a.
If the magnet is not uni - form, the magnetization at any point is the ratio of the moment of an element of volume at that point to the volume itself, or I = m.ds/dv.
The direction of the magnetization is that of the magnetic axis of the element;'in isotropic substances it coincides with the direction of the magnetic force at the point.
If the direction of the magnetization at the surface of a magnet makes 3 The C.G.S.
an angle e with the normal, the normal component of the magnetization, I cos e, is called the surface density of the magnetism, and is generally denoted by a.
The same would be the case if the magnetization of the filament varied inversely as the area of its cross-section a in different parts.
A magnet consisting of a series of plane shells of equal strength arranged at right angles to the direction of magnetization will be uniformly magnetized.
It can be shown that uniform magnetization is possible only when the form of the body is ellipsoidal.
Since 7ra'I is the moment of the sphere (=volume X magnetization), it appears from (10) that the magnetized sphere produces the same external effect as a very small magnet of equal moment placed at its centre and magnetized in the same direction; the resultant force therefore is the same as in (14).
If the magnetization is parallel to the major axis, and the lengths of the major and minor axes are 2a and 2C, the poles are situated at a distance equal to 3a from the centre, and the magnet will behave externally like a simple solenoid of length 3a.
The internal force F is opposite to the direction of the magnetization, and equal to NI, where N is a coefficient depending only on the ratio of the axes.
This statement, however, is only approximately correct, the distance between the poles depending upon the intensity of the magnetization.
3 In general, the greater the ratio of length to section, the more nearly will the poles approach the end of the bar, and the more nearly uniform will be the magnetization.
For most practical purpose a knowledge of the exact position of the poles is of no importance; the magnetic moment, and therefore the mean magnetization, can always be determined with accuracy.
Inside a magnetized body, B is the force that would be exerted on a unit pole if placed in a narrow crevasse cut in the body, the walls of the crevasse being perpendicular to the direction of the magnetization (Maxwell, § § 399, 604); and its numerical value, being partly due to the free magnetism on the walls, is generally very different from that of H.
The intensity (at any point) of the field due to the magnetization may be denoted by H i, that of the external field by Ho, and that of the resultant field by H.
Magnetization is usually regarded as the direct effect of the resultant magnetic force, which is therefore often termed the magnetizing force.
It is found that when a piece of ferromagnetic metal, such as, iron, is subjected to a magnetic field of changing intensity, the changes which take place in the induced magnetization of the iron exhibit a tendency to lag behind those which occur in the intensity of the field - a phenomenon to which J.
Thus it happens that there is no definite relation between the magnetization of a piece of metal which has been previously magnetized and the strength of the field in which it is placed.
If a bar of hard steel is placed in a strong magnetic field, a certain intensity of magnetization is induced in the bar; but when the strength of the field is afterwards reduced to zero, the magnetization does not entirely disappear.
That portion which is permanently retained, and which may amount to considerably more than one-half, is called the residual magnetization.
The ratio of the residual magnetization to its previous maximum value measures the retentiveness, or retentivity, of the metal.'
Demagnetizing Force.-It has already been mentioned that when a ferromagnetic body is placed in a magnetic field, the resultant magnetic force H, at a point within the body, is compounded of the force H o, due to the external field, and of another force, Hi, arising from the induced magnetization of the body.
Except in the few special cases when a uniform external field produces uniform magnetization, the value of the demagnetizing force cannot be calculated, and an exact determination of the actual magnetic force within the body is therefore impossible.
The residual magnetization I,.
Hence the difficulty of imparting any considerable permanent magnetization to a short thick bar not possessed of great coercive force.
The magnetization retained by a long thin rod, even when its coercive force is small, is sometimes little less than that which was produced by the direct action of the field.
Demagnetization by Reversals.-In the course of an experiment it is often desired to eliminate the effects of previous magnetization, and, as far as possible, wipe out the magnetic history of a specimen.
Forces acting on a Small Body in the Magnetic Field.-If a small magnet of length ds and pole-strength m is brought into a magnetic field such that the values of the magnetic potential at the negative and positive poles respectively are V 1 and the work done upon the magnet, and therefore its potential energy, will be W =m(V2-Vi) =mdV, which may be written W =m d s- = M d v= - MHo = - vIHo, ds ds where M is the moment of the magnet, v the volume, I the magnetization, and Ho the magnetic force along ds.
The small magnet may be a sphere rigidly magnetized in the direction of Ho; if this is replaced by an isotropic sphere inductively magnetized by the field, then, for a displacement so small that the magnetization of the sphere may be regarded as unchanged, we shall have dW = - vIdHo = v I+-, whence W = - 2 I + H2 ° (37) The mechanical force acting on the sphere in the direction of displacement x is 1 Hopkinson specified the retentiveness by the numerical value of the " residual induction " (=47rI).
Since i +47-K' can never be negative, the apparent susceptibility will be positive or negative according as is greater or less than Thus, for example, a tube containing a weak solution of an iron salt will appear to be diamagnetic if it is immersed in a stronger solution of iron, though in air it is paramagnetic.4 Circular Magnetization.
If the wire consists of a ferromagnetic metal, it will become " circularly magnetized by the field, the lines of magnetization being, like the lines of force, concentric circles.
In anisotropic bodies, such as crystals, the direction of the magnetization does not in general coincide with that of the magnetic force.
There are, however, always three principal axes at right angles to one another along which the magnetization and the force have the same direction.
For crystalline bodies the value of or -) is nearly always small and constant, the magnetization being therefore independent of the form of the body and proportional to the force.
principal axes of the crystal, the actual magnetization will be the resultant of the three magnetizations along the axes.
When either the magnetization I or the induction B corresponding to a given magnetizing force H is known, the other may be found by means of the formula B = 41rI + H.
- Intensity of magnetization is most directly measured by observing the action which a magnetized body, generally a long straight rod, exerts upon a small magnetic needle placed near it.
The angle B is indicated by the position of the spot of light upon the scale, and the horizontal intensity of the earth's field H E is known; thus we can at once determine the value of H P, from which the magnetization I of the body under test may be calculated.
Of the three methods which have been described, the first two are generally the most suitable for determining the moment or the magnetization of a permanent magnet, and the last for studying the changes which occur in the magnetization of a long rod or wire wl?E:n subjected to various external magnetic forces, or, in other words, for determining the relation of I to H.
the actual strength of the field as modified by the magnetization of the wire; but if greater accuracy is desired, the value of H, (= NI) may be found by the help of du Bois's table and subtracted from Ho.
The general character of curves of magnetization and of induction will be discussed later.
If it is desired to annihilate the hysteretic effects of previous magnetization and restore the metal to its original condition; it may be demagnetized by reversals.
The magnetometric method, except when employed in connexion with ellipsoids, for which the demagnetizing factors are [[[Magnetic Measurements]] accurately known, is generally less satisfactory for the exact determination of induction or magnetization than the ballistic method.
The second has a very small area, showing that the work done in reversing the magnetization is small; the metal is therefore adapted for use in alternating current trans formers.
The method has been employed by the authors themselves in studying the effects of tension, torsion and circular magnetization, while R.
The results of an example which they quote in detail may be briefly summarized as follows: - It is remarked by the experimenters that the value of the index e is by no means constant, but changes in correspondence with the successive well-marked stages in the process of magnetization.
After pointing out that, since the magnetization of the metal is the quantity really concerned, W is more appropriately expressed in terms of I, the magnetic moment per unit of volume, than of B, he suggests an experiment to determine whether the mechanical work required to effect the complete magnetic reversal i Phil.
Curves of magnetization (which express the relation of I to H) have a close resemblance to those of induction; and, indeed, since B = H+47r1, and 47rI (except in extreme fields) greatly exceeds H in numerical value, we may generally, without serious error, put I = B /47r, and transform curves of induction into curves of magnetization by merely altering the scale to which the ordinates are referred.
A study of such curves as these reveals the fact that there are three distinct stages in the process of magnetization.
During the first stage, when the magnetizing force is small, the magnetization (or the induction) increases rather slowly with increasing force; this is well shown by the nickel curve in the diagram, but the effect would be no less conspicuous in the iron curve if the abscissae were plotted to a larger scale.
Then the curve bends over, forming what is often called a " knee," and a third stage is entered upon, during which a considerable increase of magnetizing force has little further effect upon the magnetization.
When it is not required to determine the residual magnetization there is no necessity to divide the sample bar, and ballistic tests may be made in the ordinary way - by steps 1 S.
Ewing (Magnetic Induction, § 194) has devised an arrangement in which two similar test bars are placed side by side; each bar is surrounded by a magnetizing coil, the two coils being connected to give opposite directions of magnetization, and each pair of ends is connected by a short massive block of soft iron having holes bored through it to fit the bars, which are clamped in position by set-screws.
If a transverse cut is made through a bar whose magnetization is I and the two ends are placed in contact, it can be shown that this force is 27r I 2 dynes per unit of area (Mascart and Joubert, Electricity and Magnetism, § 322; and if the magnetization of the bar is due to an external field H produced by a magnetizing coil or otherwise, there is an additional force equal to HI.
It is, of course, true for permanent magnets, where H = o, since then F = 27rI 2; but if the magnetization is due to electric currents, the formula is only applicable in the special case when the mutual action of the two magnets upon one another is supplemented by the electromagnetic attraction between separate magnetizing coils rigidly attached to them.2 The traction method was first employed by S.
Below is given a selection from Bidwell's tables, showing corresponding values of magnetizing force, weight supported, magnetization, induction, susceptibility and permeability: - A few months later R.
Several instruments in which the traction method is applied have been devised for the rapid measurement of induction or of magnetization in commercial samples of iron and steel.
When there is no magnetization, c c the yoke is in equilibrium; but as soon as the current °'°° is turned on the block C is drawn downwards as far as the screw R will allow, for, though the attractive forces F between B and C and between B' and C' are equal, the former has a greater moment.
The scale is graduated in such a manner that by multiplying the reading by a simple factor (generally 10 or 2) the absolute value of the magnetization is obtained.
[[[Magnetization: Strong Fields (B, H]]) curve for the standard, which is assumed to have been determined; and this same value corresponds to the force H in the case of the test bar.
Unfortunately the effects of magnetization upon the specific resistance of bismuth vary enormously with changes of temperature; it is therefore necessary to take two readings of the resistance, one when the spiral is in the magnetic field, the other when it is outside.
Magnetization In Strong Fields Fields due to Coils.
Mag., 1890, 2 9, 2 53, 2 93) on the magnetization of iron, - nickel, and i cobalt under forces ranging from about 100 to 12 50 units.
These results are of extreme interest, for they show' that under sufficiently strong magnetizing forces the intensity of magnetization I reaches a maximum value, as required by W.
There appears to be no definite limit to the value to which the induction B may be raised, but the magnetization I attains a true saturation value under magnetizing forces which are in most cases comparatively moderate.
Thus the magnetization which the sample of Swedish iron received in a field of 1490 was not increased (beyond the limits of experimental error) when the intensity of the field was multiplied more than thirteen-fold, though the induction was nearly doubled.
This is at once evident when the tractive force due to magnetization is expressed as 27rI 2 -}-HI.
Magnetization In Very Weak Fields Some interesting, observations have been made of the effects produced by very small magnetic forces.
Ann., 1880, 11, 399) that in weak fields the relation of the magnetization I to the magnetizing force H is approximately expressed by an equation of the form I =aH +bH2, or K=I/H =a+bH, whence it appears that within the limits of Baur's experiments the magnetization curve is a parabola, and the susceptibility curve an inclined straight line, x being therefore a known function of H.
unit, the ratio of magnetization to magnetizing force remained sensibly constant at 6.4, wihch may therefore with great probability be assumed to represent the initial value of for the specimen in question.
1,300 [[[Dimensions And Magnetization]] The observations of Baur and Rayleigh have been confirmed and discussed by (amongst others) W.
On the application of a small magnetizing force to a bar of soft annealed iron, a certain intensity of magnetization is instantly produced; this, however, does not remain constant, but slowly increases for some seconds or even minutes, and may ultimately attain a value nearly twice as great as that observed immediately after the force was applied.'
When the magnetizing current is broken, the magnetization at once undergoes considerable diminution, then gradually falls to zero, and a similar sudden change followed by a slow one is observed when a feeble current is reversed.
If now the alternations are performed so rapidly that time is not allowed for more than the first sudden change in the magnetization, there will be no hysteresis loss, the magnetization exactly following the magnetizing force.
Further, if the alternations take place so slowly that the full maximum and minimum values of the magnetization are reached in the intervals between the reversals, there will again be no dissipation of energy.
But at any intermediate frequency the ascending and descending curves of magnetization will enclose a space, and energy will be dissipated.
Changes Of Dimensions Attending Magnetization It is well known that the form of a piece of ferromagnetic metal is in general slightly changed by magnetization.
According to Joule's observations, the length of a bar of iron or soft steel was increased by magnetization, the elongation being proportional up to a certain point to the square of the intensity of magnetization; but when the " saturation point " was approached the elongation was less than this law would require, and a stage was finally reached at which further increase of the magnetizing force produced little or no effect upon the length.
In order to meet the objection that the phenomenon might be due to electromagnetic action between the coil and the rod, Bidwell made some experiments with iron rings, and found that the length of their diameters varied under magnetization in precisely the same manner as the length of a straight rod.
magnetization upon the dimensions of iron rings in directions perpendicular to the magnetization, and upon the volume of the rings.
Hence the changes of volume undergone by a given sample of wrought iron under increasing magnetization must depend largely upon the state of the metal as regards hardness; there may be always contraction, or always expansion, or first one and then the other.
Honda, measured the changes of length of various metals shaped in the form of ovoids instead of cylindrical rods, and determined the magnetization curves for the same specimens; a higher degree of accuracy was thus attained, and satisfactory data were provided for testing theories.
[[[Stress And Magnetization]] magnetized under very heavy loads, the wire was indeed found to undergo slight contraction.
Austin, who found continuous elongation with increasing fields, the curves obtained bearing some resemblance to curves of magnetization.
As regards the effect of magnetization upon volume there are some discrepancies.
Various nickel-steels all expanded under magnetization, the increase being generally considerable and proportional to the field; in the case of an alloy containing 29% of nickel the change was nearly 40 times greater than in soft iron.
Taylor Jones showed in 1897 that only a small proportion of the contraction exhibited by a nickel wire when magnetized could be accounted for on Kirchhoff's theory from the observed effects of pulling stress upon magnetization; and in a more extended series of observations Nagaoka and Honda found wide quantitative divergences between the results of experiment and calculation, though in nearly all cases there was agreement as to quality.
They consider, however, that Kirchhoff's theory, which assumes change of magnetization to be simply proportional to strain, is still in its infancy, the present stage of its evolution being perhaps comparable with that reached by the theory of magnetization at the time when the ratio I/H was supposed to be constant.
It has been suggested 2 that an iron rod under magnetization may be in the same condition as if under a mechanically applied longitudinal stress tending to shorten the iron.
Further, Maxwell's stress is a tension along the lines of force, and is equal to B 2 /87r only when B = H, and there is no magnetization.
Effects Of Mechanical Stress Upon Magnetization The effects of traction, compression and torsion in relation to magnetism have formed the subject of much patient investigation, especially at the hands of J.
Villari in 1868 that the magnetic susceptibility of an iron wire was increased by stretching when the magnetization was below a certain value, but diminished when that value was exceeded; this phenomenon has been termed by Lord Kelvin, who discovered it independently, the " Villari reversal," the value of the magnetization for which stretching by a given load produces no effect being known as the " Villari critical point " for that load.
In the latter case the first application of stress is always attended by an increase-often a very great one-of the magnetization, whether the field is weak or strong, but after a load has been put on and taken off several times the changes of magnetization become cyclic. From experiments of both classes it appears that for a given field there is a certain value of the load for which the magnetization is a maximum, the maximum occuring at a smaller load the stronger the field.
] being to diminish the magnetization; on the other hand, with very weak fields the maximum may not have been reached with the greatest load that the wire can support without permanent deformation.
Analogous changes are observed in the residual magnetization which remains after the wire has been subjected to fields of different strength.
The effects of longitudinal pressure are opposite to those of traction; when the cyclic condition has been reached, pressure reduces the magnetization of iron in weak fields and increases it in strong fields (Ewing, Magnetic Induction, 1900, 223).
The latter found the effect to be enormous, not only upon the induced magnetization, but in a, still greater degree upon the residual.
Even under so " moderate " a load as 33 kilogrammes per square mm., the induced magnetization of a hard-drawn nickel wire in a field of 60 fell from 386 to 72 units, while the residual was reduced from about 280 to io.
Heydweiller, 2 which appeared to indicate a reversal in weak fields (corresponding to I= 5, or thereabouts), have been shown by Honda and Shimizu to be vitiated by the fact that his specimen was not initially in a magnetically neutral state; they found that when the applied field had the same direction as that of the permanent magnetization, Heydweiller's fallacious results were easily obtained; but if the field were applied in the direction opposite to that of the permanent magnetization, or if, as should rightly be the case, there were no permanent magnetization at all, then there was no indication of any Villari reversal.
The effects of longitudinal pressure upon the magnetization of cast cobalt have been examined by C. Chree, 3 and also by J.
Ewing's independent experiments showed that the magnetization curve for a cobalt rod under a load of 16.2 kilogrammes per square mm.
Both observers noticed analogous effects in the residual magnetization.
The effect of tension was subsequently studied by Nagaoka and Honda, who in 1902 confirmed, mutatis mutandis, the results obtained by Chree and Ewing for cast cobalt, while for annealed cobalt it turned out that tension always caused diminution of magnetization, the diminution increasing with increasing fields.
They also investigated the ' magnetic behaviour of various nickelsteels under tension, and found that there was always increase of magnetization.
Tomlinson found a critical point in the " temporary magnetization " of nickel (Proc. Phys.
Thomson (Applications of Dynamics to Physics and Chemistry, 47) that on dynamical principles there must be a reciprocal relation between the changes of dimensions produced by magnetization and the changes of magnetization attending mechanical strain.
Since, for example, stretching diminishes 'the magnetization of nickel, it follows from theory that the length of a nickel rod should be diminished by magnetization and conversely.
The complete reciprocity of the effects of magnetization upon length and of stretching upon magnetization is shown by the following parallel statements: Iron.
Magnetization produces inTension produces increase of crease of length in weak fields, magnetization in weak fields, decrease in strong fields.
Magnetization produces de- Tension produces decrease of crease of length in weak fields, magnetization in weak fields, increase in strong fields.
magnetization in all fields.
Magnetization produces inTension produces increase of crease of length in all fields.
Mag., 1898, 46, 261) have investigated the effects of hydrostatic pressure upon magnetization, using the same pieces of iron and nickel as were employed in their experiments upon magnetic change of volume.
In the iron cylinder and ovoid, which expanded when magnetized, compression caused a diminution of magnetization; in the nickel rod, which contracted when magnetized, pressure was attended by an increase of magnetization.
Under increasing magnetizing force the magnetization first increased, reached a maximum, and then diminished until its value ultimately became less than when the iron was in the unstrained condition.
Experiments on the effect of external hydrostatic pressure upon the magnetization of iron rings have also been made by F.
Frisbie, 5 who found that for the magnetizing forces used by Nagaoka and Honda pressure produced a small increase of magnetization, a result which appears to be in accord with theory.
The relations of torsion to magnetization were first carefully studied by G.
The most interesting of his discoveries, now generally known as the " Wiedemann effect," is the following: If we magnetize longitudinally a straight wire which is fixed at one end and free at the other, and then pass an electric current through the wire (or first pass the current and then magnetize), the free end of the wire will twist in a certain direction depending upon circumstances: if the wire is of iron, and is magnetized (with a moderate force) so that its free end has north polarity, while the current through it passes from the fixed to the free end, then the free end as seen from the fixed end will twist in the direction of the hands of a watch; if either the magnetization or the current is reversed, the direction of the twist will be reversed.
If we twist the free end of a ferromagnetic wire while a current is passing through it, the wire becomes longitudinally magnetized, the direction of the magnetization depending upon circumstances: if the wire is of iron and is twisted so that its free end as seen from the fixed end turns in the direction of the hands of a watch, while 5 Phys.
[[[Temperature And Magnetization]] the current passes from the fixed to the free end, then the direction of the resulting magnetization will be such as to make the free end a north pole.
The twist effect exhibited by iron under moderate longitudinal magnetization has been called by Knott a positive Wiedemann effect; if the twist were reversed, the other conditions remaining the same, the sign of the Wiedemann effect would be negative.
The wire is subject to two superposed magnetizations, the one longitudinal, the other circular, due to the current traversing the wire; the resultant magnetization is consequently in the direction of a screw or spiral round the wire, which will be right-handed or left-handed according as the relation between the two magnetizations is right-handed or left-handed; the magnetic expansion or contraction of the metal along the spiral lines of magnetization produces the Wiedemann twist.
Iron (moderately magnetized) expands along the lines of magnetization, and therefore for a right-handed spiral exhibits a right-handed twist.
Maxwell has also given an explanation of the converse effect, namely, the production of longitudinal magnetization by twisting a wire when circularly magnetized by a current passing through it.
If a longitudinally magnetized wire is twisted, circular magnetization is developed; this is evidenced by the transient electromotive force induced in the iron, generating a current which will deflect a galvanometer connected with the two ends of the wire.
Twisting a circularly magnetized wire produces longitudinal magnetization.
C. Twisting a longitudinally magnetized wire produces circular magnetization.
Magnetization produces change of length.
Longitudinal stress produces change of magnetization.
And, other conditions remaining unchanged, the " sense " of any effect depends upon the nature of the metal under test, and (sometimes) upon the intensity of its magnetization.
Nagaoka and Honda have succeeded in showing that the observed relations between twist and magnetization are in qualitative agreement with an extension of Kirchhoff's theory of magnetostriction.
The effects of magnetization upon the torsion of a previously twisted wire, which were first noticed by Wiedemann, have been further studied by F.
4 Nagaoka' has described the remarkable influence of combined torsion and 'tension upon the magnetic susceptibility of nickel, and has made the extraordinary observation that, under certain conditions of stress, the magnetization of a nickel wire may have a direction opposite to that of the magnetizing force.
The following are the chief results of Hopkinson's experiments: For small magnetizing forces the magnetization of iron steadily increases with rise of temperature till the critical temperature is approached, when the rate of increase becomes very high, the permeability in some cases attaining a value of about i i,000; the magnetization then with remarkable suddenness almost entirely disappears, the permeability falling to about 1.14.
For strong magnetizing forces (which in these experiments did not exceed II= 48.9) the permeability remains almost constant at its initial value (about 400), until the temperature is within nearly i oo of the critical point; then the permeability diminishes more and more rapidly until the critical point is reached and the magnetization vanishes.
For ordinary steel the critical temperature, at which magnetization practically disappeared, was found to be about 830°, and the curious fact was revealed that, on cooling, magnetization did not begin to reappear until the temperature had fallen 40° below the critical value.
The behaviour of cobalt is particularly noticeable; its permeability increased with rising temperature up to a maximum at 500°, when it was about twice as great as at ordinary temperatures, while at 1600°, corresponding to white heat, there was still some magnetization remaining.
As regards the higher temperatures, the chief point of interest is the observation that the curve of magnetization for annealed cobalt shows a small depression at about 450°, the temperature at which they had found the sign of the length-change to be reversed for all fields.
In the case of all the metals tested a small but measurable trace of magnetization remained after the so-called critical temperature had been exceeded; this decreased very slightly up to the highest temperature reached (1200°) without undergoing any such variation as had been suspected by Morris.
no evidence of hysteresis could [[[Temperature And Magnetization]] found to be 780°, 360° and 1090° respectively, but these values are not quite independent of the magnetizing force.
[[[Magnetization: Miscellaneous Effects]] to the publications cited below.'
The magnetization curve was found to be of the same general form as that of a paramagnetic metal, and gave indications that with a sufficient force magnetic saturation would probably be attained.
The effect appeared to be closely connected with the intensity of magnetization, being approximately proportional to I.
They also experimented with constant temperatures of -79°, -185° and -203', and found that at these low temperatures the effect of magnetization was enormously increased.
Attempts have been made to explain these various effects by the electron theory.4 Thermo-electric Quality.-The earliest observations of the effect of magnetization upon thermo-electric power were those of W.
Thomson (Lord Kelvin), who in 1856 announced that magnetization rendered iron and steel positive to the unmagnetized metals.'
In the case of iron and nickel it was found that, when correction was made for mechanical stress due to magnetization, magnetic change of thermo-electric force was, within the limits of experimental error, proportional to magnetic change of length.
to the magnetized cobalt was proportional to the square of the magnetic induction or of the magnetization.
The results of experiments as to the effect of magnetization were for long discordant and inconclusive, sufficient care not having been taken to avoid sources of error, while the effects of hysteresis were altogether disregarded.
In a 29% nickel-steel, magnetization increases the constants by a small amount.
Changes of elasticity are in all cases dependent, not only upon the field, but also upon the tension applied; and, owing to hysteresis, the results are not in general the same when the magnetization follows as when it precedes the application of stress; the latter is held to be the right order.
de Phys., Paris, 1900, p. 561) that the true effect of magnetization is liable to be disguised by secondary or parasitic phenomena, arising chiefly from polarization of the electrodes and from local variations in the concentration and magnetic condition of the electrolyte; these may be avoided by working with weak solutions, exposing only a small surface in a non-polar region of the metal, and substituting a capillary electrometer for the galvanometer generally used.
of magnetization is z = S aK approximately, E l being the electrochemical equivalent and S the density of the metal.
Owing to the difficulty of determining the magnetization I and the susceptibility K with accuracy, it has not yet been possible to submit this formula to a quantitative test, but it is said to afford an indication of the results given by actual experiment.
Sci., 188 7, 34, 4 1 9; 1888, 35, 290) that the transition from the " passive " to the active state of iron immersed in strong nitric acid is facilitated by magnetization, the temperature of transition being lowered.
If, however, the molecules could turn with perfect freedom, it is clear that the smallest magnetizing force would be sufficient to develop the highest possible degree of magnetization, which is of course not the case.
Maxwell (Electricity and Magnetism, § 444), recognizing that the theory in this form gave no account of residual magnetization, made the further assumption that if the deflection of the axis of the molecule exceeded a certain angle, the axis would not return to its original position when the deflecting force was removed, but would retain a permanent set.
Although the amended theory as worked out by Maxwell is in rough agreement with certain leading phenomena of magnetization, it fails to account for many others, and is in some cases at variance with observed facts.
This corresponds to the second stage of magnetization, in which the susceptibility is large and permanent magnetization is set up. A still stronger magnetizing force has little effect except in causing the direction of the needles to approach still more nearly to that of the field; if the force were infinite, every member of the group ‘ would have exactly the same direction and the greatest possible resultant moment would be reached; this illustrates " magnetic saturation " - the condition approached in the third stage of magnetization.
Supposing Ewing's hypothesis to be correct, it is clear that if the magnetization of a piece of iron were reversed by a strong rotating field instead of by a field alternating through zero, the loss of energy by hysteresis should be little or nothing, for the molecules would rotate with the field and no unstable movements would be possible.'
It can be shown that if a current i circulates in a small plane circuit of area S, the magnetic action of the circuit for distant points is equivalent to that of a short magnet whose axis is perpendicular to the plane of the circuit and whose moment is iS, the direction of the magnetization being related to that of the circulating current as the thrust of a right-handed screw to its rotation.
Ferromagnetism was explained by Ampere on the hypothesis that the magnetization of the molecule is due to an electric current constantly circulating within it.
The creation of an external magnetic field H will, in accordance with Lenz's law, induce in the molecule an electric current so directed that the magnetization of the equivalent magnet is opposed to the direction of the field.
The strength of the induced current is - HScosO/L, where 0 is the inclination of the axis of the circuit to the direction of the field, and L the coefficient of self-induction; the resolved part of the magnetic moment in the direction of the field is equal to - HS 2 cos 2 6/L, and if there are n molecules in a unit of volume, their axes being distributed indifferently in all directions, the magnetization of the substance will be-3nHS 2 /L, and its susceptibility - 3S 2 /L (Maxwell, Electricity and Magnetism, § 838).
If the structure of the molecule is so perfectly symmetrical that, in the absence of any external field, the resultant magnetic moment of the circulating electrons is zero, then the application of a field, by accelerating the right-handed (negative) revolutions, and retarding those which are left-handed, will induce in the substance a resultant magnetization opposite in direction to the field itself; a body composed of such symmetrical molecules is therefore diamagnetic. If however the structure of the molecule is such that the electrons revolving around its atoms do not exactly cancel one another's effects, the molecule constitutes a little magnet, which under the influence of an external field will tend to set itself with its axis parallel to the field.
910-915) by the Roman poet, Lucretius (96-5555 B.C.), in which it is stated that the stone can support a chain of little rings, each adhering to the one above it, indicates that in his time the phenomenon of magnetization by induction had also been observed.
He also carried out some new experiments on the effects of heat, and of screening by magnetic substances, and investigated the influence of shape upon the magnetization of iron.
On the one hand he worked out the general theory of the magnetic circuit in the dynamo (in conjunction with his brother Edward), and the theory of alternating currents, and conducted a long series of observations on the phenomena attending magnetization in iron, nickel and the curious alloys of the two which can exist both in a magnetic and non-magnetic state at the same temperature.
Felix Savary (1797-1841) made some very curious observations in 1827 on the magnetization of steel needles placed at different distances from a wire conveying the discharge of a Leyden jar (Ann.
The dependence of the intensity of magnetization on the strength of the current was subsequently investigated (Pogg.
P. Joule found that magnetization did not increase proportionately with the current, but reached a maximum (Sturgeon's Annals of Electricity, 1839, 4).
Assoc. Report, 1867, or Reprinted Papers on Electrostatics and Magnetization, p. 261).
Besides the description of the method of magnetization which still bears his name, this work contains a variety of accurate magnetic observations, and is distinguished by a lucid exposition of the nature of magnetic induction.
It is attracted by a magnet and may be magnetized, but the magnetization is quickly lost.
There is, however, a marked difference between this magnetic rotation and that of a structurally active medium, for in the latter it is always right-handed or always left-handed with respect to the direction of the ray, while in the former the sense of rotation is determined by the direction of magnetization and therefore remains the same though the ray be reversed.
It also discusses electromagnetism, Solar magnetism, dynamo theory, ocean floor magnetization, and the magnetospheres of the Earth and the planets.
remanent magnetization of fresh basalt lavas at the sea floor.
The magnetic properties and magnetization reversal behavior of the bilayer were studied using the Fresnel mode of Lorentz microscopy.
saturation magnetization may also be significant.
With suitable arrangements of iron and coil and a sufficiently strong current, the intensity of the temporary magnetization may be very high, and electromagnets capable of lifting weights of several tons are in daily use in engineering works (see Electromagnetism).
And again if a piece of steel is weighed in a delicate balance before and after magnetization, no change whatever in its weight can be detected; there is consequently no upward or downward resultant force due to magnetization; the contrary parallel forces acting upon the poles of the magnet are equal, constituting a couple, which may tend to turn the body, but not to propel it.
Physical quantities such as magnetic force, magnetic induction and magnetization, which have direction as well as magnitude, are termed vectors; they are compounded and resolved in the same manner as mechanical force, which is itself a vector.
The magnetization at any point inside the ellipsoid will then be I = HN (29) where N=47r (e2t) (-2-eloI- e - t), e being the eccentricity (see Maxwell's Treatise, § 438).
The small magnet may be a sphere rigidly magnetized in the direction of Ho; if this is replaced by an isotropic sphere inductively magnetized by the field, then, for a displacement so small that the magnetization of the sphere may be regarded as unchanged, we shall have dW = - vIdHo = v I+-, whence W = - 2 I + H2 Ã‚° (37) The mechanical force acting on the sphere in the direction of displacement x is 1 Hopkinson specified the retentiveness by the numerical value of the " residual induction " (=47rI).
Soc., 1886, 40, 486), who in 1886 published an account of some experiments in which the relation of magnetization to magnetic field was deduced from observations of the force in grammes weight which just sufficed to tear asunder the two halves of a divided ring electro-magnet when known currents were passing through the coils.
The force required to detach it is measured by a registering spring balance, which is clamped to the upper end of the rod, and thence the induction or the magnetization is deduced by applying the formula (B-H)2/81r = 27r1 2 = Pg/S, where P is the pull in grammes weight, S the sectional area of the rod in square cm., and g=981.
When there is no magnetization, c c the yoke is in equilibrium; but as soon as the current Ã‚°'Ã‚°Ã‚° is turned on the block C is drawn downwards as far as the screw R will allow, for, though the attractive forces F between B and C and between B' and C' are equal, the former has a greater moment.
The problem of determining the magnetization of iron and other metals in the strong fields formed between the poles of an electromagnet was first attacked by J.
Joule also made experiments upon iron wires under tension, and drew the erroneous inference (which has been often quoted as if it were a demonstrated fact) that under a certain critical tension (differing for different specimens of iron but independent of the magnetizing force) magnetization would produce no effect whatever upon the dimensions of the wire.
The curve showing the circumferential (or longitudinal) changes was also plotted, and from the two curves thus obtained it was easy, on the assumption that the metal was isotropic in directions at right angles to the magnetization, to calculate changes of volume; for if circumferential elongation be denoted by 1 1, and transverse elongation by 1 2, then the cubical dilatation (40r -) = l l 2/ 2 approximately.
If 1 1 were exactly equal to - 212 for all values of the magnetizing force, it is clear that the volume of the ring would be unaffected by magnetization.
The experimental results agreed in sign though not in magnitude with those calculated from the changes produced by simple longitudinal magnetization, discrepancies being partly accounted for by the fact that the metals employed were not actually isotropic. Heusler's alloy has been tested for change of length by L.
Magnetization produces deTension produces decrease of crease of length in all fields.
When the wire is free from twist, the magnetization at any point P is in the tangential direction PB (see fig.
Thus in A the twist may be right-handed or left-handed; in B the polarity of a given end may become north or south; in C the circular magnetization may be clockwise or counter-clockwise; in D the length may be increased or diminished; in E the magnetization may become stronger or weaker.
For ordinary steel the critical temperature, at which magnetization practically disappeared, was found to be about 830Ã‚°, and the curious fact was revealed that, on cooling, magnetization did not begin to reappear until the temperature had fallen 40Ã‚° below the critical value.
The behaviour of cobalt is particularly noticeable; its permeability increased with rising temperature up to a maximum at 500Ã‚°, when it was about twice as great as at ordinary temperatures, while at 1600Ã‚°, corresponding to white heat, there was still some magnetization remaining.
As regards the higher temperatures, the chief point of interest is the observation that the curve of magnetization for annealed cobalt shows a small depression at about 450Ã‚°, the temperature at which they had found the sign of the length-change to be reversed for all fields.
In the case of all the metals tested a small but measurable trace of magnetization remained after the so-called critical temperature had been exceeded; this decreased very slightly up to the highest temperature reached (1200Ã‚°) without undergoing any such variation as had been suspected by Morris.
no evidence of hysteresis could [[[Temperature And Magnetization]] found to be 780Ã‚°, 360Ã‚° and 1090Ã‚° respectively, but these values are not quite independent of the magnetizing force.
Miscellaneous Effects Of Magnetization Electrical Conductivity.-The specific resistance of many electric conductors is known to be temporarily changed by the action of a magnetic field, but except in the case of bismuth the effect is very small.
They also experimented with constant temperatures of -79Ã‚°, -185Ã‚° and -203', and found that at these low temperatures the effect of magnetization was enormously increased.
When such precautions are adopted it is found that the " electromotive force of magnetization " is, for a given specimen, perfectly definite both in direction and in magnitude; it is independent of the nature of the corrosive solution, and is a function of the field-strength alone, the curves showing the relation of electromotive force 'to field-intensity bearing a rough resemblance to the familiar I-H curves.
This illustrates the first stage in the process of magnetization, when the moment is proportional to the field and there is no hysteresis or residual magnetism (see ante).
This is due to the strong remanent magnetization of fresh basalt lavas at the sea floor.
Where the material is used to generate a magnetic field or to create a force then the saturation magnetization may also be significant.
Magnetization in Strong Fields.
Magnetization in Weak Fields.
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