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Ewing's theory of induced magnetism
~48
Electrical
Sectwn.
[ J . 1~. I.,
It is clear t h a t the d e s t r u c t i o n of the boiler iron is c a u s e d b y the f o r m a t i o n of m a g n e s i u m h y d r a t e from the chloride, according to the e q u a t i o n MgCI~ + 2H20 ~--- MgH~O2 + 2HC1 If a sufficient q u a n t i t y of Na~CO 3 be a d d e d to the water, there is no p r e c i p i t a t e f o r m e d at ordinary t e m p e r a t u r e ; b u t at 80 ° C. a flocculent deposit follows of m a g n e s i u m or calcium carbonates. A n a l y s i s /? s h o w s t h a t the tri-sodium p h o s p h a t e h a d v e r y slight effect. It w o u l d not be well to reject it on this account, b e c a u s e it m a y not have b e e n i n t r o d u c e d properly. B u t it seems to me t h a t the c a r b o n a t e should be recomm e n d e d in such cases• T h e latter is on trial at G a l v e s t o n now, and after some time a n o t h e r sample of t h e scale will be s u b m i t t e d . EWING'S
THEORY
OF I N D U C E D
MAGNETISM.
BY PROF, HENRY CREW.
• [Read at N~e pneeling of lhe Electrical 5"eclion, ]zeht ~kfay 5, I89z.]
T h e labors of R o w l a n d , Stoletow, W i e d e m a n n and E w i n g h a v e b r o u g h t to light so large a 0number of m a g n e t i c phenomena, that a n y general principle on w h i c h they m a y be explained w o u l d receive a w a r m w e l c o m e at the h a n d s of all i n t e r e s t e d in this rapidly g r o w i n g subject. A n y generalization of this kind w o u l d be called u p o n to explain the following p h e n o m e n a , w h i c h are selected each as .typical of a large class of well-known facts• (I) The behavior o f a piece o f iron when placed in a magnetic field whose strength is made to pass throngh a cycle o f changes. (a) F o r m a g n e t i z i n g forces r a n g i n g from 1 to #~ of a C. G. S. unit, the i n t e n s i t y of m a g n e t i z a t i o n is proportional to the m a g n e t i z i n g force (Rayleigh, Phil. Mag., March, I887). (b) F o r a r a n g e of m a g n e t i c forces e x t e n d i n g (roughly) from -~ to 5 C. G. S. units the m a g n e t i z a t i o n increases with a r a p i d i t y e n o r m o u s l y g r e a t e r t h a n the s t r e n g t h of field. .(c) F r o m this point on; as the force continues to i n c r e a s e
Aug., i89I. ]
Electrical Section.
149
the p e r m e a b i l i t y decreases and the i n d u c t i o n a p p r o a c h e s a limit which, the work of ]Swing and L o w (P/dl. Traus., I889), leads us to think, has for a value a b o u t 4o,ooo. (d) If n o w the m a g n e t i z i n g force be diminished, the m a g n e t i z a t i o n diminishes, but not so rapidly, until, w h e n all the force is w i t h d r a w n , there still r e m a i n s a perfectly definite magnetization, w h i c h is used as a m e a s u r e of the " r e t e n t i v e n e s s " of the iron u n d e r the conditions of t h e experiment.
I
4
~
'
/ J
FIG. I.
(e) If next the m a g n e t i z i n g force be reversed the magnetization will rapidly diminish, and at a certain n e g a t i v e w,lue of the m a g n e t i z i n g force, the m a g n e t i z a t i o n of the iron will p r e s e n t l y reach a zero value ; but, if the o ~ r e n t be broken the m a g n e t i z a t i o n will " s p r i n g b a c k " along the line ~/ B ( F ~ . z). In order to d e s t r o y the m a g n e t i z a t i o n of the iron we m u s t p u r s u e our w a y along the curve to a point at C, such t h a t when the c u r r e n t is w i t h d r a w n and the m a g n e t i z a t i o n
Electrical Section.
15o
[J. F. I.,
falls, it will fall along the line C O, from C to O. T h e iron here, however, will h a v e m a g n e t i c properties q u i t e different from those w h i c h it h a d before magnetization. (f) H a v i n g r e a c h e d a large n e g a t i v e value we o b t a i n , on g r a d u a l l y d i m i n i s h i n g the force and allowing it to increase in the opposite direction, a c u r v e c o r r e s p o n d i n g to ( e ) a n d (d), and nearly s y m m e t r i c a l on t h e o t h e r side of the axes of I and H. T h e curve a, b, c, d, e, f is a graphical s u m m a r y of the n o w well-known facts of hysteresis. (II) Nearly all reversals of sign in the change of the magnetizing force are accompanied by small changes in the magnetiza-
I
Fro. 2.--Illustrating the effect of a small superimposed cycle in the magnetizing force.
tion. In the magnetization curve any reversal starts out along a nearly horizontal line. (See Fig. 2.) (III) The f a c t that a piece o f iron submitted to vibrations or mechanical shocks is magneti~ed and demagnetized more readily and w i t h a smaller hysteresial area than i f it remains undisturbed. T h e I H curve t h e n takes a form of w h i c h Fig. 3 is a type. (IV) The phenomenaof" time lag" in magnetizatzon. E w i n g finds his m a g n e t o m e t e r needle creeping up long after the t i m e w h e n F o u c a u l t currents or self-induction c o u l d h a v e a n y effect. R a y l e i g h (Phil. 3/fag., March, 1887) finds t h a t witfi a second coil he can balance the effect of a m a g n e t i z i n g coil and iron core, at the end of any given time, say five seconds.
Electrical Section.
Aug., t89z. ]
I5 I
If balance is o b t a i n e d at the m o m e n t of m a k i n g the m a g n e t i z i n g current, t h e n at the end of five seconds t h e r e will be a deflection of the m a g n e t o m e t e r in one direction. But if t h e b a l a n c i n g coil is so a r r a n g e d as to give balance at the end of 5 seconds, t h e n there will be an initial deflection in t h e other direction. (V) Phenomena o f stress and magnetism. T h e s e are u s u a l l y g r o u p e d u n d e r j t w o heads, viz: (a) T h o s e w~hich occur w h e n a b o d y has first been placed in m a g n e t i c field and the stress t h e n m a d e to vary. (b) T h o s e w h i c h occur w h e n a b o d y is first placed u n d e r a c o n s t a n t stress a n d the m a g n e t i z i n g force is m a d e to vary.
2r
S !
I
J
ZY
FI~. 3.--Illustrating the effect of tapping on the ZZ-Zcurve.
T h e hysteresis between m a g n e t i z a t i o n a n d stress m a y he best i l l u s t r a t e d by the curve in Fig. 4, derived b y E w i n g (t'hil. Trans., 1885) from e x p e r i m e n t s on a piece of iron which h a d been previously strained. If, however, the wire be a freshly a n n e a l e d and u n s t r e t c h e d one, t h e n the hysteresis is m u c h larger, a n d we obtain a curve like t h a t in Fig. 5, the m a x i m u m on t h e down curve corresponding to a n e g a t i v e load, L e., to a pressure. T h e effect of a constant load on the I H curve m~,2 be seen in Fz~,~.6. F r o m this it will be seen t h a t 10ngitudinal stress aids m a g n e t i z a t i o n at first, b u t for larger values of _~r it diminishes it. (VI) Effects o f heat on magnetizatio n. In genera!, perma-
152
Electrical Section.
[ J. F . I.,
n e n t m a g n e t i s m is decreased by h e a t i n g t h r o u g h a n y cycle of t e m p e r a t u r e s . But in the case of i n d u c e d m a g n e t i s m we m u s t d i s t i n g u i s h b e t w e e n two cases : (a) W h e n the heating' is not h i g h the effect is to increase the susceptibility. (b) W h e n the h e a t i n g is excessive, it decreases the susceptibility. T h e foregoing are a few of t h e h u n d r e d s of facts w h i c h e x p e r i m e n t has recently b r o u g h t to light. A n y tolerably complete explanation of t h e m m u s t include two parts, viz : (a) A n explanation of the amp~rian c u r r e n t s (or of molecular m a g n e t s on some other hypothesis.'
Z
.£ o a o~ FIG. 4.--Effect of loading an already strained iron wire in a magnetic field.
(b) T h e arrangement of W e b e r ' s m o l e c u l a r m a g n e t s in t h e mass. W e are h e r e concerned only w i t h t h e second and appare n t l y less difficult part of the problem. T h e first a t t e m p t at a t h e o r y of m a g n e t i c induction is t h a t of W e b e r , who supposes the molecules of freshly a n n e a l e d iron to be t u r n e d at r a n d o m in all directions, and to be held in their respective .positions b y constant forces a c t i n g along their respective axes. T h i s h y p o t h e s i s explains t h e existence of an upper limit to m a g n e t i z a t i o n ; it also explains t h e e x p e r i m e n t of Beetz, in w h i c h he obtains a s t r o n g m a g n e t by the electrolytic deposition of iron in a
Aug., I89I. ]
Electrical Section.
I 53
m a g n e t i c field; b u t is c o n d e m n e d b y its failure to explain t h e p h e n o m e n o n of p e r m a n e n t m a g n e t i s m . Maxwell, therefore, modifies W e b e r ' s t h e o r y b y supposing t h a t w h e n the axis of a m a g n e t i c molecule is displaced t h r o u g h an angle g r e a t e r than a certain l i m i t i n g angle it receives a certain set. T h u s amended, the h y p o t h e s i s of W e b e r covers a m u c h larger n u m b e r of facts, e. g'., p e r m a n e n t m a g n e t i s m , and t h e p r o d u c t i o n of p e r m a n e n t m a g n e t i s m b y m o d e r a t e l y high forces only. Maxwell's t h e o r y fails, however, to explain such a p h e n o m e n o n as that of " time lag," which has b e e n so e l e g a n t l y w o r k e d o u t b y R a y l e i g h and E w i n g (Roy. Soc. Proc., June, 1889). In the Philosophical 3/Zaga~iJte for S e p t e m b e r , 189o, Prof. E w i n g offers the a p p a r e n t l y bold h y p o t h e s i s of no constraint
Y a#
~"
~
~
~
Zoerd FIG. 5.--Effect of loading a freshly annealed iron wire in a magnetic field.
at all acting on the m o l e c u l a r magnets, save only their magnetic action on one another. A large model, c o n s t r u c t e d on the lines s u g g e s t e d in the paper j u s t mentioned, was here e x h i b i t e d to t h e Section. A helix, a b o u t three feet long and one foot in diameter, c o n t a i n e d within it three tiers of m a g n e t s m o u n t e d on three glass decks. E a c h m a g n e t was b a l a n c e d on a needle-point, the whole a r r a n g e m e n t s h o w i n g in a striking m a n n e r the interaction of the m a g n e t s and the m u l t i t u d e of different positions of e q u i l i b r i u m w h i c h t h e y m a y assume. T h e sufficiency of this h y p o t h e s i s m a y b e s t b e tested b y a p p l y i n g it to explanations of the six typical facts j u s t mentioned. (I) Stirring the magnets up, with no current in the helix,
154
Electrical Section.
[J. F. I.,
we observe that they arrange themselves very much at random, yet more of them pointing north than in any other direction. (The north poles were fitted with damping vanes.) This, of course, represents the magnetization of iron by the earth's field. Balancing the earth's field by a slight current through the helix, and again stirring up the magnets, we have a piece of freshly annealed iron, with practically no external moment to start with. A slight increment of current deviates the magnets to some extent, thus giving an external moment, but no new position of equilibrium is assumed, so that on bringing the current back to its initial value, we find the iron without any residual magnetism, which corresponds exactly to the fact of nature. Increasing the current a little more, one observes a stray magnet now and then passing into a new position of equilibrium which corresponds to the part a of the curve in Fig. r. A little more current and these changes occur in a m o r e wholesale manner, giving the part b of Fig. ±. Increasing the strength of field to about two C. G. S. units, we find that all of the magnets have set themselves nearly in the direction of the field, an' a r r a n g e m e n t which corresponds to the asymptotic part of the I t f curve. Diminishing the magnetizing force by gradual steps to zero there remains considerable external moments, which, however, is destroyed and generally reversed by a comparatively small negative force. In fact, the cyclic curve thus obtained bears a striking resemblance to those actually obtained from a piece of iron. In the Electrical World., May 16, 1891, Mr. Arthur Hoopes has three curves, taken from this model under different conditions. (II) If for any given value of magnetizing field we reverse the sign of change in the current, the magnets will be observed to change direction very slightly, falling back into their nearest position of equilibrium, where they are held by each other until subjected to a change of more con-
Electrical Section.
Aug., I891.]
t 55
siderable size. This, of course, is the second p h e n o m e n o n m e n t i o n e d above. (ITI) One has only to shake the model, even slightly, to see the effect of t a p p i n g a piece of soft iron d u r i n g m a g netization or demagnetization. If the m a g n e t s b e not v e r y close together, it takes careful h a n d l i n g not to t h r o w t h e m into a n e w position of equilibrium. (IV) Allow the m a g n e t s to come to perfect rest and then turn one of t h e m w i t h the h a n d t h r o u g h an angle of 4 °0 or 50% T h e d i s t u r b a n c e will be p r o p a g a t e d along the line at a c o m p a r a t i v e l y slow rate. In this way, E w i n g explains " t i m e lag." 2"
~Zoad'= 0 , Zo~d= I&~Zo.
bl FIG. 6 . - - I l l u s t r a t i n g the effect o f a c o n s t a n t | o a d on the _T_,~ curve.
T h e g r e a t while occupied b y t h e m a g n e t s in c o m i n g to rest after t h e y h a v e once b e e n d i s t u r b e d b y a c h a n g e of current, continually increasing or conti, nually decreasing, their external m o m e n t , chimes in b e a u t i f u l l y w i t h the w o r k of R a y l e i g h m e n t i o n e d in " I V " above. -IV (V) T h e effect of stress on m a g n e t i z a t i o n is s t a t e d as follows in terms of t h e model b y the a u t h o r of t h e t h e o r y (-/)/dL ~ag-., S e p t e m b e r , 189o): " W h e n pulling stress is applied, those rows of m o l e c u l a r m a g n e t s which lie m o r e or less along the direction of t h e stress have their s t a b i l i t y
156
Electrical Sectzon.
[ J. v. I.,
reduced by the lengthening of the lines of centres ; similarly, rows which lie more or less normal to the stress have their stability increased. The resulting effect on the general ,susceptibility of the material will depend on which of these conflicting influences preponderates. Let pull be applied before magnetization begins, while the metal is still in a neutral state. The stretching of longitudinal lines and the contraction of transverse lines will not only alter the stability of those molecules which continue to lie in their original rows. but will tend to make the memb'ers of those rows which are much lengthened swing round, and form transverse lines in which they will be more stable than before. W e may, therefore, reasonably expect that the permeability with regard to strong fields will be reduced by pull, as it actually is both in iron and in nickel, though with regard to weak fields the permeability m a y be increased, as it is in iron. " Again, the theory explains well why the effects of stress are by no means the same ( i ) w h e n the stress is applied first and the magnetic force after, and (2) when the magnetic force is applied first and the stress after. " L e t a moderate magnetizing force be applied and then begin to apply stress. The first effects are in general large for the strain precipitates into instability those molecular magnets which were already on the verge of instability. T h i s is ~.~eautifully apparent in iron (see Phil. Trans., I885, part ii, plates 63 and 64); and the theory shows why t h e first effects are not reversible, why they do not disappear when the stress is removed, and why it is only in subse
Aug., I891.]
Electrical Sectwn.
I57
it m a y produce r o t a t i o n of the molecules and t h u s give t h e effect of zero e x t e r n a l m o m e n t . This m u c h at least m a y be said, t h a t n e a r l y the whole group of m a g n e t s m a y be placed in rapid r o t a t i o n by quickly reversing a s t r o n g c u r r e n t t h r o u g h the helix. Indeed, m o s t a n y violent disturbance, such as a shock, will set a large n u m b e r in rotation, w h i c h state t h e y seem to maintain for a long while. Some facts in the d y n a m i c a l t h e o r y of h e a t lead one to t h i n k t h a t this supposition of r o t a t i n g molecules m a y not be so wild as at first it m i g h t appear. T h e f o r e g o i n g are some of the facts which fall into line, on the supposition of no c o n s t r a i n t a m o n g the molecules except their own m a g n e t i c a t t r a c t i o n and repulsion. Prof. E w i n g has g i v e n m a n y more, and everyone should read his paper. But there are a n u m b e r of o u t s t a n d i n g facts w h i c h d o not s u b m i t so easily to explanation, e. g., the difference of m a g n e t i c b e h a v i o r w i t h reference to t e m p e r a t u r e in t h e cases of nickel and cobalt. [Ency., Brit., art. " M a g n e t i s m , " P. 257.] It m u s t not be f o r g o t t e n t h a t this model r e p r e s e n t s only one single phase of molecular a r r a n g e m e n t . Chemical a n d spectroscopic p h e n o m e n a i n d i c a t e a v a s t l y more complicated structure for the molecule t h a n has y e t been d r e a m e d of. However, it is i n t e r e s t i n g to note t h a t Prof. E w i n g ' s hypothesis leads to conclusions w h i c h have a l r e a d y been derived from other lines of work. l?or instance, the distinction b e t w e e n h a r d a n d soft iron in terms of the model is t h a t in the former case the molecules are g r o u p e d more i n t i m a t e l y t h a n in the l a t t e r ; while Barus and S t r o u h a l (Bulletin of U. S. Geol. Survey, No 14, p. 96), from a s t u d y of the viscous and electrical properties of h a r d and soft iron, h a v e been led to t h i n k t h a t hardness is due to h e t e r o g e n e i t y of i n t e r i o r a r r a n g e m e n t ; hard t e m p e r b e i n g a condition in which the molecules are !ocked up into a new- position of equilibrium, c o r r e s p o n d i n g to a g r e a t e r storage of p o t e n t i a l e n e r g y t h a n in the soft condition.
Electrical
Sectwn.
[ J . 1~. I.,
It is clear t h a t the d e s t r u c t i o n of the boiler iron is c a u s e d b y the f o r m a t i o n of m a g n e s i u m h y d r a t e from the chloride, according to the e q u a t i o n MgCI~ + 2H20 ~--- MgH~O2 + 2HC1 If a sufficient q u a n t i t y of Na~CO 3 be a d d e d to the water, there is no p r e c i p i t a t e f o r m e d at ordinary t e m p e r a t u r e ; b u t at 80 ° C. a flocculent deposit follows of m a g n e s i u m or calcium carbonates. A n a l y s i s /? s h o w s t h a t the tri-sodium p h o s p h a t e h a d v e r y slight effect. It w o u l d not be well to reject it on this account, b e c a u s e it m a y not have b e e n i n t r o d u c e d properly. B u t it seems to me t h a t the c a r b o n a t e should be recomm e n d e d in such cases• T h e latter is on trial at G a l v e s t o n now, and after some time a n o t h e r sample of t h e scale will be s u b m i t t e d . EWING'S
THEORY
OF I N D U C E D
MAGNETISM.
BY PROF, HENRY CREW.
• [Read at N~e pneeling of lhe Electrical 5"eclion, ]zeht ~kfay 5, I89z.]
T h e labors of R o w l a n d , Stoletow, W i e d e m a n n and E w i n g h a v e b r o u g h t to light so large a 0number of m a g n e t i c phenomena, that a n y general principle on w h i c h they m a y be explained w o u l d receive a w a r m w e l c o m e at the h a n d s of all i n t e r e s t e d in this rapidly g r o w i n g subject. A n y generalization of this kind w o u l d be called u p o n to explain the following p h e n o m e n a , w h i c h are selected each as .typical of a large class of well-known facts• (I) The behavior o f a piece o f iron when placed in a magnetic field whose strength is made to pass throngh a cycle o f changes. (a) F o r m a g n e t i z i n g forces r a n g i n g from 1 to #~ of a C. G. S. unit, the i n t e n s i t y of m a g n e t i z a t i o n is proportional to the m a g n e t i z i n g force (Rayleigh, Phil. Mag., March, I887). (b) F o r a r a n g e of m a g n e t i c forces e x t e n d i n g (roughly) from -~ to 5 C. G. S. units the m a g n e t i z a t i o n increases with a r a p i d i t y e n o r m o u s l y g r e a t e r t h a n the s t r e n g t h of field. .(c) F r o m this point on; as the force continues to i n c r e a s e
Aug., i89I. ]
Electrical Section.
149
the p e r m e a b i l i t y decreases and the i n d u c t i o n a p p r o a c h e s a limit which, the work of ]Swing and L o w (P/dl. Traus., I889), leads us to think, has for a value a b o u t 4o,ooo. (d) If n o w the m a g n e t i z i n g force be diminished, the m a g n e t i z a t i o n diminishes, but not so rapidly, until, w h e n all the force is w i t h d r a w n , there still r e m a i n s a perfectly definite magnetization, w h i c h is used as a m e a s u r e of the " r e t e n t i v e n e s s " of the iron u n d e r the conditions of t h e experiment.
I
4
~
'
/ J
FIG. I.
(e) If next the m a g n e t i z i n g force be reversed the magnetization will rapidly diminish, and at a certain n e g a t i v e w,lue of the m a g n e t i z i n g force, the m a g n e t i z a t i o n of the iron will p r e s e n t l y reach a zero value ; but, if the o ~ r e n t be broken the m a g n e t i z a t i o n will " s p r i n g b a c k " along the line ~/ B ( F ~ . z). In order to d e s t r o y the m a g n e t i z a t i o n of the iron we m u s t p u r s u e our w a y along the curve to a point at C, such t h a t when the c u r r e n t is w i t h d r a w n and the m a g n e t i z a t i o n
Electrical Section.
15o
[J. F. I.,
falls, it will fall along the line C O, from C to O. T h e iron here, however, will h a v e m a g n e t i c properties q u i t e different from those w h i c h it h a d before magnetization. (f) H a v i n g r e a c h e d a large n e g a t i v e value we o b t a i n , on g r a d u a l l y d i m i n i s h i n g the force and allowing it to increase in the opposite direction, a c u r v e c o r r e s p o n d i n g to ( e ) a n d (d), and nearly s y m m e t r i c a l on t h e o t h e r side of the axes of I and H. T h e curve a, b, c, d, e, f is a graphical s u m m a r y of the n o w well-known facts of hysteresis. (II) Nearly all reversals of sign in the change of the magnetizing force are accompanied by small changes in the magnetiza-
I
Fro. 2.--Illustrating the effect of a small superimposed cycle in the magnetizing force.
tion. In the magnetization curve any reversal starts out along a nearly horizontal line. (See Fig. 2.) (III) The f a c t that a piece o f iron submitted to vibrations or mechanical shocks is magneti~ed and demagnetized more readily and w i t h a smaller hysteresial area than i f it remains undisturbed. T h e I H curve t h e n takes a form of w h i c h Fig. 3 is a type. (IV) The phenomenaof" time lag" in magnetizatzon. E w i n g finds his m a g n e t o m e t e r needle creeping up long after the t i m e w h e n F o u c a u l t currents or self-induction c o u l d h a v e a n y effect. R a y l e i g h (Phil. 3/fag., March, 1887) finds t h a t witfi a second coil he can balance the effect of a m a g n e t i z i n g coil and iron core, at the end of any given time, say five seconds.
Electrical Section.
Aug., t89z. ]
I5 I
If balance is o b t a i n e d at the m o m e n t of m a k i n g the m a g n e t i z i n g current, t h e n at the end of five seconds t h e r e will be a deflection of the m a g n e t o m e t e r in one direction. But if t h e b a l a n c i n g coil is so a r r a n g e d as to give balance at the end of 5 seconds, t h e n there will be an initial deflection in t h e other direction. (V) Phenomena o f stress and magnetism. T h e s e are u s u a l l y g r o u p e d u n d e r j t w o heads, viz: (a) T h o s e w~hich occur w h e n a b o d y has first been placed in m a g n e t i c field and the stress t h e n m a d e to vary. (b) T h o s e w h i c h occur w h e n a b o d y is first placed u n d e r a c o n s t a n t stress a n d the m a g n e t i z i n g force is m a d e to vary.
2r
S !
I
J
ZY
FI~. 3.--Illustrating the effect of tapping on the ZZ-Zcurve.
T h e hysteresis between m a g n e t i z a t i o n a n d stress m a y he best i l l u s t r a t e d by the curve in Fig. 4, derived b y E w i n g (t'hil. Trans., 1885) from e x p e r i m e n t s on a piece of iron which h a d been previously strained. If, however, the wire be a freshly a n n e a l e d and u n s t r e t c h e d one, t h e n the hysteresis is m u c h larger, a n d we obtain a curve like t h a t in Fig. 5, the m a x i m u m on t h e down curve corresponding to a n e g a t i v e load, L e., to a pressure. T h e effect of a constant load on the I H curve m~,2 be seen in Fz~,~.6. F r o m this it will be seen t h a t 10ngitudinal stress aids m a g n e t i z a t i o n at first, b u t for larger values of _~r it diminishes it. (VI) Effects o f heat on magnetizatio n. In genera!, perma-
152
Electrical Section.
[ J. F . I.,
n e n t m a g n e t i s m is decreased by h e a t i n g t h r o u g h a n y cycle of t e m p e r a t u r e s . But in the case of i n d u c e d m a g n e t i s m we m u s t d i s t i n g u i s h b e t w e e n two cases : (a) W h e n the heating' is not h i g h the effect is to increase the susceptibility. (b) W h e n the h e a t i n g is excessive, it decreases the susceptibility. T h e foregoing are a few of t h e h u n d r e d s of facts w h i c h e x p e r i m e n t has recently b r o u g h t to light. A n y tolerably complete explanation of t h e m m u s t include two parts, viz : (a) A n explanation of the amp~rian c u r r e n t s (or of molecular m a g n e t s on some other hypothesis.'
Z
.£ o a o~ FIG. 4.--Effect of loading an already strained iron wire in a magnetic field.
(b) T h e arrangement of W e b e r ' s m o l e c u l a r m a g n e t s in t h e mass. W e are h e r e concerned only w i t h t h e second and appare n t l y less difficult part of the problem. T h e first a t t e m p t at a t h e o r y of m a g n e t i c induction is t h a t of W e b e r , who supposes the molecules of freshly a n n e a l e d iron to be t u r n e d at r a n d o m in all directions, and to be held in their respective .positions b y constant forces a c t i n g along their respective axes. T h i s h y p o t h e s i s explains t h e existence of an upper limit to m a g n e t i z a t i o n ; it also explains t h e e x p e r i m e n t of Beetz, in w h i c h he obtains a s t r o n g m a g n e t by the electrolytic deposition of iron in a
Aug., I89I. ]
Electrical Section.
I 53
m a g n e t i c field; b u t is c o n d e m n e d b y its failure to explain t h e p h e n o m e n o n of p e r m a n e n t m a g n e t i s m . Maxwell, therefore, modifies W e b e r ' s t h e o r y b y supposing t h a t w h e n the axis of a m a g n e t i c molecule is displaced t h r o u g h an angle g r e a t e r than a certain l i m i t i n g angle it receives a certain set. T h u s amended, the h y p o t h e s i s of W e b e r covers a m u c h larger n u m b e r of facts, e. g'., p e r m a n e n t m a g n e t i s m , and t h e p r o d u c t i o n of p e r m a n e n t m a g n e t i s m b y m o d e r a t e l y high forces only. Maxwell's t h e o r y fails, however, to explain such a p h e n o m e n o n as that of " time lag," which has b e e n so e l e g a n t l y w o r k e d o u t b y R a y l e i g h and E w i n g (Roy. Soc. Proc., June, 1889). In the Philosophical 3/Zaga~iJte for S e p t e m b e r , 189o, Prof. E w i n g offers the a p p a r e n t l y bold h y p o t h e s i s of no constraint
Y a#
~"
~
~
~
Zoerd FIG. 5.--Effect of loading a freshly annealed iron wire in a magnetic field.
at all acting on the m o l e c u l a r magnets, save only their magnetic action on one another. A large model, c o n s t r u c t e d on the lines s u g g e s t e d in the paper j u s t mentioned, was here e x h i b i t e d to t h e Section. A helix, a b o u t three feet long and one foot in diameter, c o n t a i n e d within it three tiers of m a g n e t s m o u n t e d on three glass decks. E a c h m a g n e t was b a l a n c e d on a needle-point, the whole a r r a n g e m e n t s h o w i n g in a striking m a n n e r the interaction of the m a g n e t s and the m u l t i t u d e of different positions of e q u i l i b r i u m w h i c h t h e y m a y assume. T h e sufficiency of this h y p o t h e s i s m a y b e s t b e tested b y a p p l y i n g it to explanations of the six typical facts j u s t mentioned. (I) Stirring the magnets up, with no current in the helix,
154
Electrical Section.
[J. F. I.,
we observe that they arrange themselves very much at random, yet more of them pointing north than in any other direction. (The north poles were fitted with damping vanes.) This, of course, represents the magnetization of iron by the earth's field. Balancing the earth's field by a slight current through the helix, and again stirring up the magnets, we have a piece of freshly annealed iron, with practically no external moment to start with. A slight increment of current deviates the magnets to some extent, thus giving an external moment, but no new position of equilibrium is assumed, so that on bringing the current back to its initial value, we find the iron without any residual magnetism, which corresponds exactly to the fact of nature. Increasing the current a little more, one observes a stray magnet now and then passing into a new position of equilibrium which corresponds to the part a of the curve in Fig. r. A little more current and these changes occur in a m o r e wholesale manner, giving the part b of Fig. ±. Increasing the strength of field to about two C. G. S. units, we find that all of the magnets have set themselves nearly in the direction of the field, an' a r r a n g e m e n t which corresponds to the asymptotic part of the I t f curve. Diminishing the magnetizing force by gradual steps to zero there remains considerable external moments, which, however, is destroyed and generally reversed by a comparatively small negative force. In fact, the cyclic curve thus obtained bears a striking resemblance to those actually obtained from a piece of iron. In the Electrical World., May 16, 1891, Mr. Arthur Hoopes has three curves, taken from this model under different conditions. (II) If for any given value of magnetizing field we reverse the sign of change in the current, the magnets will be observed to change direction very slightly, falling back into their nearest position of equilibrium, where they are held by each other until subjected to a change of more con-
Electrical Section.
Aug., I891.]
t 55
siderable size. This, of course, is the second p h e n o m e n o n m e n t i o n e d above. (ITI) One has only to shake the model, even slightly, to see the effect of t a p p i n g a piece of soft iron d u r i n g m a g netization or demagnetization. If the m a g n e t s b e not v e r y close together, it takes careful h a n d l i n g not to t h r o w t h e m into a n e w position of equilibrium. (IV) Allow the m a g n e t s to come to perfect rest and then turn one of t h e m w i t h the h a n d t h r o u g h an angle of 4 °0 or 50% T h e d i s t u r b a n c e will be p r o p a g a t e d along the line at a c o m p a r a t i v e l y slow rate. In this way, E w i n g explains " t i m e lag." 2"
~Zoad'= 0 , Zo~d= I&~Zo.
bl FIG. 6 . - - I l l u s t r a t i n g the effect o f a c o n s t a n t | o a d on the _T_,~ curve.
T h e g r e a t while occupied b y t h e m a g n e t s in c o m i n g to rest after t h e y h a v e once b e e n d i s t u r b e d b y a c h a n g e of current, continually increasing or conti, nually decreasing, their external m o m e n t , chimes in b e a u t i f u l l y w i t h the w o r k of R a y l e i g h m e n t i o n e d in " I V " above. -IV (V) T h e effect of stress on m a g n e t i z a t i o n is s t a t e d as follows in terms of t h e model b y the a u t h o r of t h e t h e o r y (-/)/dL ~ag-., S e p t e m b e r , 189o): " W h e n pulling stress is applied, those rows of m o l e c u l a r m a g n e t s which lie m o r e or less along the direction of t h e stress have their s t a b i l i t y
156
Electrical Sectzon.
[ J. v. I.,
reduced by the lengthening of the lines of centres ; similarly, rows which lie more or less normal to the stress have their stability increased. The resulting effect on the general ,susceptibility of the material will depend on which of these conflicting influences preponderates. Let pull be applied before magnetization begins, while the metal is still in a neutral state. The stretching of longitudinal lines and the contraction of transverse lines will not only alter the stability of those molecules which continue to lie in their original rows. but will tend to make the memb'ers of those rows which are much lengthened swing round, and form transverse lines in which they will be more stable than before. W e may, therefore, reasonably expect that the permeability with regard to strong fields will be reduced by pull, as it actually is both in iron and in nickel, though with regard to weak fields the permeability m a y be increased, as it is in iron. " Again, the theory explains well why the effects of stress are by no means the same ( i ) w h e n the stress is applied first and the magnetic force after, and (2) when the magnetic force is applied first and the stress after. " L e t a moderate magnetizing force be applied and then begin to apply stress. The first effects are in general large for the strain precipitates into instability those molecular magnets which were already on the verge of instability. T h i s is ~.~eautifully apparent in iron (see Phil. Trans., I885, part ii, plates 63 and 64); and the theory shows why t h e first effects are not reversible, why they do not disappear when the stress is removed, and why it is only in subse
Aug., I891.]
Electrical Sectwn.
I57
it m a y produce r o t a t i o n of the molecules and t h u s give t h e effect of zero e x t e r n a l m o m e n t . This m u c h at least m a y be said, t h a t n e a r l y the whole group of m a g n e t s m a y be placed in rapid r o t a t i o n by quickly reversing a s t r o n g c u r r e n t t h r o u g h the helix. Indeed, m o s t a n y violent disturbance, such as a shock, will set a large n u m b e r in rotation, w h i c h state t h e y seem to maintain for a long while. Some facts in the d y n a m i c a l t h e o r y of h e a t lead one to t h i n k t h a t this supposition of r o t a t i n g molecules m a y not be so wild as at first it m i g h t appear. T h e f o r e g o i n g are some of the facts which fall into line, on the supposition of no c o n s t r a i n t a m o n g the molecules except their own m a g n e t i c a t t r a c t i o n and repulsion. Prof. E w i n g has g i v e n m a n y more, and everyone should read his paper. But there are a n u m b e r of o u t s t a n d i n g facts w h i c h d o not s u b m i t so easily to explanation, e. g., the difference of m a g n e t i c b e h a v i o r w i t h reference to t e m p e r a t u r e in t h e cases of nickel and cobalt. [Ency., Brit., art. " M a g n e t i s m , " P. 257.] It m u s t not be f o r g o t t e n t h a t this model r e p r e s e n t s only one single phase of molecular a r r a n g e m e n t . Chemical a n d spectroscopic p h e n o m e n a i n d i c a t e a v a s t l y more complicated structure for the molecule t h a n has y e t been d r e a m e d of. However, it is i n t e r e s t i n g to note t h a t Prof. E w i n g ' s hypothesis leads to conclusions w h i c h have a l r e a d y been derived from other lines of work. l?or instance, the distinction b e t w e e n h a r d a n d soft iron in terms of the model is t h a t in the former case the molecules are g r o u p e d more i n t i m a t e l y t h a n in the l a t t e r ; while Barus and S t r o u h a l (Bulletin of U. S. Geol. Survey, No 14, p. 96), from a s t u d y of the viscous and electrical properties of h a r d and soft iron, h a v e been led to t h i n k t h a t hardness is due to h e t e r o g e n e i t y of i n t e r i o r a r r a n g e m e n t ; hard t e m p e r b e i n g a condition in which the molecules are !ocked up into a new- position of equilibrium, c o r r e s p o n d i n g to a g r e a t e r storage of p o t e n t i a l e n e r g y t h a n in the soft condition.