264 THE MOLECULAR PROCESS IN MAGNETIC INDUCTION. 



strained. Change of temperature will sometimes do it, or the applica- 

 tion or change of mechanical strain. Suppose, for instance, that we 

 apply pull to an iron wire while it hangs in a weak magnetic lield, by 

 making it carry a weight. The first time that we put on the weight, 

 the magnetism of the wire at once increases, often very greatly, in con- 

 sequence of the action I have just described (Fig. 13). The molecules 

 have been on the verge of turning, and the slight strain caused by the 

 weight is enough to make them go. Eemove the weight, and there is 

 only a comparatively small change in the magnetism, for the greater 

 part of the molecular turning that was done when the weight was put 

 on is not undone when it is taken off. Re-apply the weight, and you 

 find again but little change;, though there are still traces of the kind of 

 action which the first application brought about. 

 That is to say, there are some groups of molecules 

 which, though they were not broken up in the first 

 api^lication of the weight, yield now, because they 

 have lost the sup])ort they then obtained from neigh- 

 bors that have now entered into new combinations, 

 indeed, this kind of action may often be traced, al- 

 ways diminishing in amount, during several succes- 

 sive applications and removals of the load (see Fig. 

 13), and it is only when the process of loading has 

 been many times repeated that the magnetic change 

 brought about by loading is just opposite to the 

 magnetic change brought about by unloading. 



Whenever indeed we are observing the effects of 

 an alteration of jihysical condition on the magne- 

 tism of iron, we have to distinguish between the 

 I)rimitive effect, which is often very great and is 

 not reversible, and the ultimate effect, which is seen 

 only after the molecular structure has become some- 

 what settled through many repetitions of the proc- 

 ess. Experiments on the effects of temperature, of 

 strain, etc., have long ago shown this distinction 

 to be exceedingly important; the molecular theory 

 makes it perfectly intelligible. 



Further, the theoiy makes i)lain another curious result of experiment. 

 When we have loaded and unloaded the iron wire many times over, so 

 that theefiect is no longer complicated by the primitive acti<m I have 

 just described, we still find that the magnetic changes which occur 

 while the load is being put on are not simply undone, step by step, 

 while the load is being taken ofi". Let the whole load be divided into 

 several parts, and you will see that the magnetism has two different 

 values, in going up and in coming down, for one and the same inter- 

 mediate value of the load. The changes of magnetism lag behind the 

 changes of load; in other words, there is hysteresis in the relation of 



2 160 



"Load 



Fig. 13.— Effects of load- 

 ing a soft iron wire in 

 a cons'anr field. 



