258 THE MOLECULAR PROCESS IN MAGNETIC INDUCTION. 



In some specimens of magnetic metal we find a much sharper separa- 

 tion of the three stages than in others. By applying strain in certain 

 ways it is jiossible to get tlie stages very clearly separated. Fig. 2, a 

 beautiful instance of that, is taken from a paper by Mr. Nagaoka — one 

 of an able band of Japanese workers wlio are bidding fair to repay the 

 debt that Japan owes for its learning to the West. It shows how a 

 piece of nickel whicli is under the joint action of pull and twist becomes 

 magnetized in a growing magnetic field. There the first stage is ex- 

 ceptionally prolonged, and the second stage is extraordinarily abrupt. 



The bearing of all this on the uK^lecular 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 flat bars, others into diamond shapes with i)ointed 

 ends, others into shapes resembling mushrooms or umbrellas, and in 

 these the magnetic field is produced by means of a coil of insulated 

 wire wound on a large wooden frame below the magnets. Some ot 

 these are arranged with the pivots on a gridiron or lazy- tongs of jointed 

 wooden bars, so that we may readily distort them, and vary the dis- 

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

 strain in the actual solid. But to display the experiments 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 mag- 

 netic force is ai)[)lied 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 wliole group. 

 You see this when I 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 ])oiut anyhow— there is no direction it prefers 

 to any other, As soon as I apply even a very weak field it responds, 



