170 



NA TURE 



[June 



1904 



;Ln;ilvtic;]l nirthod, the (ithcr proceeding- from n con- 

 sideration of tlic curves of current and E.M.F. If 

 these two lines of arg;ument rested on independent 

 foundations, tlien tlieir mutual atfreement that alter- 

 nators can so run would .Ljivr weifjht. But, imfor- 

 lun.itelw both proofs assume that the machines g^ive 

 smooth E.M.F. waves of the sine form and that the 

 armatures have no iron, or, at any rate, that their 

 self-inductions are constant. Further, the serious 

 difficulty arising from " hunting " of the machines is 

 not referred to. Hence the general conclusion, " Vou 

 may therefore with confidence attempt to run alternate- 

 current machines in parallel circuit for the purpose of 

 producing any e.xternal efl'ect," must be regarded as 

 at any rate a bold one. 



.\s a m.-itterof fact, the two machines he e.xperimented 

 on did run in parallel perfectly, and others have since 

 bi-en built to do the same, but it does not follow, and 

 indeed it is not the fact, that all machines possess this 

 property. Dr. Hopkinson, of course, realised the 

 limitations of his equations; indeed, he expressly men- 

 tions some of them. Hence in making his prediction 

 we must conclude that he was more influenced by his 

 experiment ;il results than by his theoretical reasoning. 

 In the numerator of the expression which occurs at the 

 bottom of p. 149 of this paper 



'Z should be '" . 

 T 1 I 



-\ similar criticism as regards limitation in the 

 reasoning may be passed on the equations used at the 

 commencement of paper No. 10, p. 156, vol. i., on 

 " .Mternate Current Dynamo Electric Machines." 

 Here he starts with the equation : — 

 Ra = E-(L.v)\ 



where the dot (left out by a printer's mistake on p. 

 1(13) signifies differentiation with respect to time. 

 But. as Dr. Hopkinson remarks, " we do not know 

 how L may vary," and so to obtain a solution he 

 .issumes L to be independent of time, which of course 

 is not the fact, and so, as he points out, the ordinary 

 theory does not fully account for the facts. This 

 criticism, however, does not apply to the remainder 

 of this paper, which is occupied with an experimental 

 investigation of the currents induced in the coils and 

 in the cores of t4ie magnets of alternate current 

 machines by the varying currents and by the varying 

 position of the arm.iture. .\ method of determining 

 the efficiency of alternate current machines is also 

 given, and the result used to show that in certain 

 cases of relation of phase of current to phase of elec- 

 tromotive force, the effect of the local currents in the 

 iron cores is to increase instead of to diininish the 

 electromotive force of the machine. 



We next come to a .short but important paper in 

 which the equations which hold in a transformer with 

 a closed magnetic circuit are given and partially dis- 

 cussed ; and in the next paper a method of testing 

 tr.'insformers is described and illustrated, the method 

 being similar to that previously given for continuous 

 current dynamos 



The remaining papers in this volume, consisting 

 NO. 1S08. VOL. 70] 



mostly of addresses, do not call for special remark,, 

 except for one paragraph, on p. 249, vi/., " We know 

 nothing of wh.at light is; we do know that it is a 

 wave." " Is it not infinitely probable that the waves 

 of light are none other than the electrical waves which' 

 we know must exist, ,ind must be propagated with the 

 observed velocity of light? .And, mark, this theory- 

 demands no ether." "Whether the postulate of an 

 all-pervading ether be, or be not, a metaphysical neces- 

 sity, surely it is well for the practical man and the 

 physicist to leave the question to the metaphysician." 

 Here, then, waves are allowed but an ether is denied, 

 or looked on with suspicion, and so we have appar- 

 ently to imagine a wave as independent of and apart 

 from any medium ; a vibration with nothing to vibrate 

 seems a rather dilficull conception. Nor is it easy to 

 see what the metaphysician, in the proper sense of this 

 inuch abused word, has got to do with the question of 

 j an ether ; the physicist in his laboratory rather than 

 the metaphysician in his armchair would seem to be 

 the proper man to deal with it. 



The papers in the second volume may be roughly 

 divided into three groups. The first grou]j contains 

 papers dealing with residual charge and specific induc- 

 tive capacity, the second group papers on the magne- 

 tisation of iron and the effect of temperature, whilst 

 the third consists of papers on miscellaneous subjects. 



The accurate determination of a physical constant 

 of any substance is always a matter of scientific in- 

 terest, but great additional interest and importance 

 were attached to Dr. Hopkinson 's experiments on 

 specific inductive capacity from, the fact that, accord- 

 ing to Maxwell, the specific inductive capacity of a 

 dielectric ought to be equal to the squ.ire ol its inde.x 

 of refraction. To appreciate properly the full import- 

 ance of these experiments we must remember that 

 when Maxwell's treatise appeared in 1873, Maxwell 

 was able to write that there was only one substance, 

 paraffin, the capacity of which was' known with sutfi'- 

 cient accuracy for a comparison. To this solitary example 

 Dr. Hopkinson added some four different kinds of 

 glass and nine different oils, and it is hardly too much 

 to say that if in all these substances the above relation 

 had been fulfilled, then Hopkinson, and not Hertz, 

 might have been regarded as the man who first ex- 

 perimentally verified Maxwell's theories, although 

 Hertz's work would still have had its great value in 

 connection with the actual propagation of electric 

 waves in space. Lnfortunately for Hopkinson, 

 though the relation was found to be true in the hydro- 

 carbon oils, in the vegetable and animal oils and in 

 the glasses it was far from being satisfied. 



So far, however, from jumping to the conclusion 

 that Maxwell was wrong. Dr. Hopkinson, in a paper 

 written in 1878, regards it as sufficient to add the 

 caution, " it should not be inferred that his [Maxwell] 

 theory in its more general character is disproved," 

 w hilst in a paper written in 18S1 he remarks, " It 

 must, however, never be forgotten that the time of 

 disturbance in the actual optical experiment is many 

 thousands of millions of times as short as in the fastest 

 electrical experiment even when the condenser is 

 ch.-irged or discharged for only the 1/20,000 second." 



