October 15, 1S91] 



NATURE 



567 



{(i), we shall find, as Lord Rayleigh has shown, little or 

 no residual magnetism ; if we make it at any point in the 

 second stage (J), we shall find very much residual mag- 

 netism ; and if we make it at any point in the third stage 

 (r), we shall find only a little more residual magnetism 

 than we should have found by making the experiment 

 at the end of stage b. That part of the turning of the 

 molecules which goes on in stage a contributes nothing 

 to the residual magnetism. That part which goes on in 

 stage c contributes little. But that part of the turning 

 which goes on in stage b contributes very much. 



In some specimens of magnetic metal we find a much 



sharper separation of the three stages than in others. By 

 applying strain in certain ways it is possible to get the 

 stages very clearly separated. Fig. 2, a beautiful in- 

 stance of that, is taken from a paper by Mr. Nagaoka 

 — one of an able band of Japanese workers who are 

 bidding fair to repay the debt that Japan owes for its 

 learning to the West. It shows how a piece of nickel 

 which is under the joint action of pull and twist becomes 

 magnetized in a growing magnetic field. There the first 

 stage is exceptionally prolonged, and the second stage 

 is extraordinarily abrupt. 



The bearing of all this on the molecular theory will be 

 evident when we turn to these models, consisting of an 

 assemblage of little pivoted magnets, which may be 

 taken to represent, no doubt in a very crude way, the 

 molecular structure of a magnetizable metal. I have 

 here some large models, where the pivoted magnets are 

 pieces of sheet steel, some cut into short flu bars, others 

 into diamond shapes with pointed ends, others into 

 shapes resembling mushrooms or umbrellas, and in these 

 the magnetic field is produced by means of a coil of in- 

 sulated wire wound on a large wooden frame below the 

 magnets. Some of these are arranged with the pivots on 



NO. I 146, VOL. 44] 



a gridiron or lazy-tongs of jointed wooden bars, so that 

 we may readily distort them, and vary the distances of 

 the pivots from one another, to imitate some of the effects 

 of strain in the actual solid. But to display the experi- 

 ments to a large audience a lantern model will serve best. 

 In this one the magnets are got by taking to pieces 

 numbers of little pocket compasses. The pivots are 

 cemented to a glass plate, through which the light passes 

 in such a way as to project the shadows of the magnets 

 on the screen. The magnetic force is applied by means 

 of two coils, one on either side of the assemblage of 

 magnets and out of the way of the light, which together 

 produce a nearly uniform magnetic field throughout the 

 whole group. You see this when 1 make manifest the 



field in a well-known fashion, by dropping iron filings on 

 the plate. 



We shall first put a single pivoted magnet on the plate. 

 So long as no field acts it is free to point anyhow— there 

 is no direction it prefers to any other. As soon as I 

 apply even a very weak field it responds, turning at once 

 into the exact direction of the applied force, for there was 

 nothing (beyond a trifling friction at the pivot) to prevent 

 it from turning. 



Now try two magnets. I have cut off the current, so 

 that there is at present no field, but you see at once that 

 the pair has, so to speak, a will of its own. I may shake 

 or disturb them as I please, but they insist on taking up 

 a position (Fig. 3) with the north end of one as close as 



Fig. 4. 



possible to the south end of the other. If disturbed they 

 return to it : this configuration is highly stable. Watch 

 what happens when the magnetic field acts with gradually 

 growing strength. At first, so long as the field is weak 

 (Fig. 4), there i^ but little deflection ; but as the deflection 

 increases it is evident that the stability is being lost, the 

 state is getting more and more critical, until (Fig. 5) the 

 tie that holds them together seems to break, and they 

 suddenly turn, with violent swinging, into almost perfect 

 alignment with the magnetic force H. Now I gradually 

 remove the force, and you see that they are slow to 

 return, but a stage comes when they swing back, and a 



