5,70 



NATURE 



[October 15, i 



lequire much more coal to be burnt, for every unit of 

 electricity sold, than direct-current systems require. 



The molecular theory shows how this waste of energy 

 occurs. When the molecule becomes unstable and 

 tumbles violently over, it oscillates and sets its neigh- 

 bours oscillating, until the oscillations are damped out 

 by the eddy currents of electricity which they generate in 

 the surrounding conducting mass. The useful work that 

 can be got from the molecule as it falls over is less 

 than the work that is done in replacing it during the 

 return portion of the cycle. This is a simple mechanical 

 deduction from the fact that the movement has unstable 

 phases. 



I cannot attempt, in a single lecture, to do more than 

 glance at several places where the molecular theory seems 

 to throw a flood of light on obscure and complicated 

 facts, as soon as we recognize that the constraint of 

 the molecules is due to their mutual action as magnets. 



It has been known since the time of Gilbert that vibra- 

 tion greatly facilitates the process of magnetic induction. 

 Let a piece of iron be briskly tapped while it lies in the 

 magnetic field, and it is found to take up a large addition 

 to its induced magnetism. Indeed, if we examine the 

 successive stages of the process while the iron is kept vi- 

 brating by being tapped, we find that the first stage {a) has 

 practically disappeared, and there is a steady and rapid 

 growth of magnetism almost from the very first. This is 

 intelligible enough. Vibration sets the molecular mag- 

 nets oscillating, and allows them to break their primi- 

 tive mutual ties and to respond to weak deflecting forces. 

 For a similar reason, vibration should tend to reduce the 

 residue of magnetism which is left when the magnetizing 

 force is removed, and this, too, agrees with the results of 

 observation. 



Perhaps the most effective way to show the influence of 

 vibration is to apply a weak magnetizing force first, before 

 tapping. If the force is adjusted so that it nearly but hot 

 quite reaches the limit of stage («), a great number of the 

 molecular magnets are, so to speak, hovering on the 

 verge of instability, and when the piece is tapped they go 

 over like a house of cards, and magnetism is acquired 

 with a rush. Tapping always has some effect of the same 

 kind, even though there has been no special adjustment 

 of the field. 



And other things besides vibration will act in a similar 

 way, precipitating the break-up of molecular groups when 

 the ties are already strained. Change of temperature 

 will sornetimes do it, or the application or change of 

 mechanical strain. Suppose, for instance, that we apply 

 pull to an iron wire while it hangs in a weak magnetic field, 

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

 t)\& weight, the magnetism of the wire at once increases, 

 often very greatly, in consequence 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. Remove 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. Reapply 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 application of the 

 weight, yield now, because they have lost the support ' 

 they then obtained from neighbours that have now en- 

 tered into new combinations. Indeed, this kind of action 

 may often be traced, always diminishing in amount, 

 during several successive applications and removals of the 

 load (see Fig. 13), and it is only when the process of 

 ioading has been many times repeated that the magnetic 

 <:hange brought about by loading is just opposite to the 

 magnetic change brought about by unloading. 



Whenever, indeed, we are observing the effects of an 

 NO. 1 146, VOL. 44] 



alteration of physical condition on the magnetism of 

 iron, we have to distinguish between the primitive effect, 

 which is often very great and is not reversible, and the 

 ultimate effect, which is seen only after the molecular 

 structure has become somewhat settled through many 

 repetitions of the process. Experiments on the effects of 

 temperature, of strain, and so forth, have long ago shown 

 this distinction to be exceedingly important : the mole- 

 cular theory makes it perfectly intelligible. 



Further, the theory makes plain another curious result 

 of experiment. When we have loaded and unloaded the 

 iron wire many times over, so that the effect is no longer 

 complicated by the primitive action 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 off. 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 intermediate value 

 of the load. The changes of magnetism lag behind the 

 changes of load : in other words, there is hysteresis in the 



Fig. 13. — Effects of loading a soft iron wire in a constant field. 



relation of the magnetism to the load (Fig. 14). This is 

 because some of the molecular groups are every time 

 being broken up during the loading, and re-established 

 during the unloading, and that, as we saw already, in- 

 volves hysteresis. Consequently, too, each loading and 

 unloading requires the expenditure of a small quantity of 

 energy, which goes to heat the metal. 



Moreover, a remarkably interesting conclusion follows. 

 This hysteresis, and consequent dissipation of energy, 

 will also happen though there be no magnetization of the 

 piece as a whole : it depends on the fact that the mole- 

 cules are magnets. Accordingly, we should expect to 

 find, and experiment confirms this (see Phil. Trans., 1885, 

 p. 614), that if the wire is loaded and unloaded, even 

 when no magnetic field acts and there is no magnetism, 

 its physical qualities which are changed by the load will 

 change in a manner involving hysteresis. In particular, 

 the length will be less for the same load during loading 

 than during unloading, so that work may be wasted m 

 every cycle of loads. There can be no such thing as per- 

 fect elasticity in a magnetizable metal, unless, indeed, the 

 range of the strain is so very narrow that none of the 



