NATURE 



72> 



THURSDAY, NOVEMBER 26, 1891. 



EL ECTRO-MA GNE TISM. 

 The Ehctro-magnct and Electro-magnetic Mechanism. 

 By S. P. Thompson, D.Sc, F.R.S. (London: E. and 

 F. N. Spon, 1891.) 



THIS is a reprint, with additions, of a course of Cantor 

 Lectures which the author delivered during 1890 

 before the Society of Arts. The book, it may be stated 

 at the outset, is an excellent account of electro-magnetic 

 mechanism, and abounds in information at once of his- 

 torical, scientific, and practical value. Evidently the 

 author is willing to spare no pains to give completeness 

 to any work of this kind he undertakes, and the present 

 book, like his treatise on dynamo-electric machinery, will 

 no doubt be widely read and appreciated. 



In the preface, and elsewhere in the body of the book 

 itself. Dr. Thompson indulges in some statements which 

 we think are a blemish on an excellent and well-written 

 treatise. It serves no good purpose to distribute in a 

 scientific book praise or blame to certain classes of 

 scientific investigators, and while glorifying a certain 

 section of workers, to pour what seems little short of con- 

 tempt and derision on the labours of another. We refer 

 here to the statements (chiefly in the preface) regarding 

 the earlier mathematical theories of magnetism, and their 

 alleged influence in retarding electro-magnetic discovery. 



It is quite true that some of the older theorists, concen- 

 trating their attention on the magnetic field of a system 

 of permanent magnets, gave only a secondary attention 

 to the problem of the internal constitution of a magnet. 

 But it is hardly fair to put down against them, by inference, 

 the errors of the persons who persisted in assuming that, 

 because it had been proved that a magnet produces a field 

 which can also be accounted for by a distribution of 

 imaginary magnetic matter over the surface of the magnet, 

 the magnetization of the magnet did consist in such a distri- 

 bution. Such an equivalent distribution is a conception 

 helpful in itself, inasmuch as it can be experimentally 

 determined, and expresses exactly the manner in which 

 the lines (unit tubes) of magnetic induction leave the 

 surface. But to accuse it of misleading those who mis- 

 understood it, is to put on the shoulders of Gauss and 

 others who considered surface distributions a responsi- 

 bility which is not theirs, and a blame which properly 

 belongs to the perverse experimentalists who have insisted 

 on determining the actual positions of "poles" in bar 

 magnets, and on measuring other things equally indeter- 

 minate or non-existent. 



But is the following statement quite free from possibility 

 of misconstruction ? " Gradually, however, new light 

 dawned. It became customary, in spite of the mathe- 

 maticians, to regard the magnetism of a magnet as 

 something that traverses or circulates around a definite 

 path, flowing more freely through such substances as iron 

 than through other relatively non-magnetic materials." 

 If any student of the subject does get into his head the 

 idea that an actual material something flows round a 

 magnetic circuit, he might quite as justly hold Dr. Thomp- 

 son responsible for this false notion as blame the mathe- 

 NO. II 52, VOL. 45] 



maticians for causing him to suppose a magnet to be a 

 body plastered over with imaginary magnetic matter. 



In tracing the evolution of the notion of magnetic 

 permeability. Dr. Thompson might have made some 

 mention of the contributions to molecular theory given 

 by Poisson and others ; for after all, these men really knew 

 as well as we do " that magnetism, so far from residing 

 in the end or surface of the magnet, is a property resident 

 throughout the mass." But, as has already been stated, 

 their attention was chiefly directed to the fields of per- 

 manent magnets ; and to find a doctrine which " will 

 afford a basis of calculation such as is required by the 

 electrical engineer" is the business of the electrical 

 engineer himself, and others interested in the proble m 

 Hitherto, indeed, so far as such a doctrine has been 

 found, it has been discovered in great measure by the 

 physical mathematician ! The mathematical doctrine o 

 " magnetic permeability," or— to use Faraday's phrase — 

 " conductivity for magnetic lines of force," was given so 

 long ago as 1872 by Sir William Thomson, and fully 

 illustrated by analogies and applications ; and it is 

 certainly curious that so accurate an historian as Dr. 

 Thompson should, in his historical resiimd, have made 

 no mention of this very important paper. It is also the 

 fact that our knowledge of the properties of iron, which, 

 with the " simple law of the magnetic circuit," consti- 

 tutes, according to Dr. Thompson, the stock-in-trade of 

 the designers of dynamos, has come in no small degree 

 from the same source. The most eminent investigator 

 and improver of the dynamo is also a mathematician ; 

 but perhaps, as Sir William Hamilton (of Edinburgh, not 

 of Dublin !) said of a certain mathematician who, he 

 wasi forced to admit, reasoned correctly, he did it in 

 spite of his mathematics, not because of them. 



All honour to the great mathematicians who first at- 

 tacked the difficult subject of magnetic action. But even 

 if they had sinned so grievously by their unpracticality 

 as it seems now the fashion to try to make out, their 

 successors, entering into their labours, have been able to 

 do much to atone by helping the engineer in the diffi- 

 culties which beset his advance into a region, viewed in- 

 deed from afar, but hitherto untrodden. In this great 

 work the engineer and mathematician are necessarily 

 companions, and for either to reproach the other, as to 

 what happened or did not happen in the past, is only to 

 provoke useless recrimination, and delay their onward 

 progress. 



It is time now to come to the subject-matter proper of 

 the book, and for this we have, on the whole, nothing but 

 commendation. There are some things which might, 

 perhaps, have been expressed differently with advant- 

 age, but this, of course, is only a matter of opinion. 

 In the first chapter a very interesting account is 

 given of the inventions and researches carried out by 

 Sturgeon, Henry, and Joule, and ends with a description 

 of notable electro-magnets. The sketch of Sturgeon's 

 career, given in an appendix to this chapter, is of re- 

 markable interest as a plain unvarnished record of heroic 

 toil in the pursuit of knowledge, carried on amid un- 

 congenial surroundings, and in spite of the hard pinch of 

 poverty. In happier circumstances, this experimental 

 genius would no doubt have done much for science. 

 Still, to us there is some compensation, for, had it not 



