256 THE MOLECULAR PROCESS IN MA(iNETIC INDUCTION. 



When a piece of iron or nickel or cobalt is magnetized by induction, 

 the magnetic state j^ermeates the whole jjiece. It is not a suijerficial 

 change of state. Break the piece into as many fragments as you please, 

 and yon will find that every one of these is a magnet. In seeking an 

 explanation of magnetic quality we must penetrate the innermost 

 framework of the sul)stance — we must go to the molecules. 



Now, in a molecular theory of magnetism there are two i)ossibh', 

 beginnings. We might suppose, with Poisson, that each molecule 

 becomes magnetized when the field begins to act. Or we may adopt 

 the theory of Weber, which says that the molecules of iron are 

 always magnets and that what the field does is to turn them so that 

 they face morc^ or less one way. According to this view, a virgin 

 piece of iron shows no magnetic polarity, not because its molecules are 

 not magnets, but because they lie so thoroughly '^higgledy-]»iggledy" as 

 regards direction that no greater number point one way than another. 

 But when the magnetic force of the field begins to act, the molecules 

 turn in response to it, and so a preponderating number come to face in 

 the direction in which the magnetic force is applied, the result of which 

 is that the piece as a whole shows magnetic polarity. All the facts go 

 to confirm Weber's view. One fact in particular I nsay mention at 

 once — it is almost conclusive in itself. When the molecular magnets 

 are all turned to face one way, the piece lias clearly received as much 

 magnetization as it is cai)able of. Accordingly, if Weber's theory be 

 true, we must expect to find that in a very strong magnetic field a piece 

 of iron or other magnetizable metal heronwi^ S(ifvr(( fol, so thut it can 

 not take up any more magnetism, however much the field be strength- 

 ened. This is just what happens. Experiments were published a few 

 years ago which ])ut the fact of saturation beyond a doubt, and gave 

 values of the limit to which the intensity of magnetization may be 

 forced. 



When a piece of iron is put in a magnetic field, wt' do not find that 

 it becomes saturated unless the field is exceedingly strong. A weak 

 field induces but little magnetism; and if the field be strengthened, 

 more and more magnetism is acquired. This shoAA s that the molecules 

 do not turn with perfect readiness in response to the deflecting mag- 

 netic fin'ce of the field. Their turning is in some way resisted, and this 

 resistance is overcome as the field is strengthened, so that the magnet- 

 ism of the piece increases stej) by step. What is the directing force 

 which })revents the molecules from at once yielding to the deflecting 

 influence of the field, and to what is that force due? And ;igain, how 

 comes it after they have been deflected they return partially, but by no 

 means wholly, to their original i»laces when the field ceases to act? 



I think these questions receive a. complete and satisfactory answer 

 when we take account of the forces which the molecules necessarily 

 exert on one another in consequence of the fact that they are magnets. 

 We shall study the matter by examining the behavior of groui)S of 



