566 



NATURE 



[October 15, 1891 



worms in the lawns. The crane-fly, which usually swarms in 

 the' fields of the Mansfield estate in September, has been very 

 rare, too, this season. The dragon-fly visited us this summer 

 for the first time. 



Apropos to the records of the " rare phenomenon," such a 

 summer aurora was observed at Rothbury, Northumberland, in 

 the latter half of August 1880. 



To conclude this farrago of notes : for " non pas travailles " in 

 Mr. Sclater's quotation of the Prince of Canino's words (xliv. 

 p. 518), read '' n'ont ..." J. J. Walker. 



Hampstead, N.W., Octobers. 



THE MOLECULAR PROCESS IN MAGNETIC 

 INDUCTION. 1 



TV/TAGNETIC induction is the name given by Faraday 

 ^^ to the act of becoming magnetized, which certain 

 substances perform when they are placed in a magnetic 

 field. A magnetic field is the region near a magnet, or 

 near a conductor conveying an electric current. Through- 

 out such a region there is what is called magnetic force, and 

 when certain substances are placed in the magnetic field 

 the magnetic force causes them to become magnetized by 

 magnetic induction. An effective way of producing a mag- 

 netic field is to wind a conducting wire into a coil, and pass 

 a current through the wire. Within the coil we have a 

 region of comparatively strong magnetic force, and when 

 a piece of iron is placed there it may be strongly mag- 

 netized. Not all substances possess this property. Put a 

 piece of wood or stone or copper or silver into the field, and 

 nothing noteworthy happens ; but put a piece of iron or 

 nickel or cobalt and at once you find that the piece has 

 become a magnet. These three metals, with some of their 

 alloys and compounds, stand out from all other substances 

 in this respect. Not only are they capable of magnetic 

 induction — of becoming magnets while exposed to the 

 action of the magnetic field — but when withdrawn from the 

 field they are found to retain a part of the magnetism they 

 acquired. They all show this property of retentiveness, 

 more or less. In some of them this residual magnetism 

 is feebly held, and may be shaken out or otherwise 

 removed without difficulty. In others, notably in some 

 steels, it is very persistent, and the fact is taken advantage 

 of in the manufacture of permanent magnets, which are 

 simply bars of steel, of proper quality, which have been 

 subjected to the action of a strong magnetic field. Of all 

 substances, soft iron is the most susceptible to the action 

 of the field. It can also, under favourable conditions, 

 retain, when taken out of the field, a very large fraction 

 of the magnetism that has been induced— more than nine- 

 tenths— more, indeed, than is retained by steel ; but its 

 hold of this residual magnetism is not firm, and for that 

 reason it will not serve as a material for permanent 

 magnets. My purpose to-night is to give some account of 

 the molecular process through which we may conceive 

 magnetic induction to take place, and of the structure which 

 makes residual magnetism possible. 



\Vhen a piece of iron or nickel or cobalt is magnetized 

 by induction, the magnetic state permeates the whole 

 piece. _ It is not a superficial change of state. Break the 

 piece into as many fragments as you please, and you 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 substance— we must go to 

 the molecules. 



Now, in a molecular theory of magnetism there are 

 two possible 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 



' Abstract of a Friday tvening Discourse delivered at the Royal Institution, 

 on May 22, 1891, by J. A. Ewing, M.A., F.R.S., Professor of Applied 

 Mechanics and Mechanism in the University of Cambridge. 



NO. 1 146, VOL. 44] 



that they face more 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-piggledy as regards direc- 

 tion 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 prepon- 

 derating 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 may mention at once— it is almost conclusive in itself. 

 When the molecular magnets are all turned to face one 

 way, the piece has clearly received as much magnetization 

 as it is capable of. Accordingly, if Weber's theory be 

 true, we must expect to find that in a very strong mag- 

 netic field a piece of iron or other magnetizable metal 

 becomes saturated, so that it cannot take up any more 

 magnetism, however much the field be strengthened. 

 This is just what happens : experiments were published 

 a few years ago which put 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, we 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 shows that the 

 molecules do not turn with perfect readiness in response 

 to the deflecting magnetic force of the field. Their 

 turning is in some way resisted, and this resistance is 

 overcome as the field is strengthened, so that the mag- 

 netism of the piece increases step by step. What is the 

 directing force which prevents the molecules from at 

 once yielding to the deflecting influence of the field, and 

 to what is that force due ? And again, how comes it 

 that after they have been deflected they return partially, 

 but by no means wholly, to their original places when the 

 field ceases to act .? 



I think these questions receive a complete and satis- 

 factory answer when we take account of the forces which 

 the molecules necessarily exert on one another in con- 

 sequence of the fact that they are magnets. We shall 

 study the matter by examining the behaviour of groups 

 of little magnets, pivoted like compass needles, so that 

 each is free to turn except for the constraint which each 

 one suffers on account of the presence of its neighbours. 



But first let us see more particularly what happens 

 when a piece of iron or steel or nickel or cobalt is mag- 

 netized by means of a field the strength of which is 

 gradually augmented from nothing We may make the 

 experiment by placing a piece of iron in a coil, and 

 making a current flow in the coil with gradually increased 

 strength, noting at e^ch stage the relation of the induced 

 magnetism to the strength of the field. This relation is 

 observed to be by no means a simple one : it may be 

 represented by a curve (Fig. i), and an inspection of the 

 curve will show that the process is divisible, broadly, into 

 three tolerably distinct stages. In the first stage {a) the 

 magnetism is being acquired but slowly : the molecules, 

 if we accept Weber's theory, are not responding readily — 

 they are rather hard to turn. In the second stage {b) 

 their resistance to turning has to a great extent broken 

 down, and the piece is gaining magnetism fast. In the 

 third stage {c) the rate of increment of magnetism falls 

 off: we are there approaching the condition of satura- 

 tion, though the process is still a good way from being 

 completed. 



P urther, if we stop at any point of the process, such as 

 P, and gradually reduce the current in the coil until 

 there is no current, and therefore no magnetic field, we 

 shall get a curve like the dotted line PQ, the height of Q 

 showing the amount of the residual magnetism. 



If we make this experiment at a point in the first stage 



