July 28, 1892J 



NATURE 



297 



parts, from the rudimentary, uneconomical, and violently 

 periodic machine of twenty years ago, or to compare 

 the powerful alternator of the present day with the 

 ineffective and wasteful toy instrument, which used to 

 figure in cabinets of apparatus and the older books on 

 electricity. 



Chapter ii. deals with magnetic induction, and chapter 

 iii. with the induction of currents by the motion of con- 

 ductors in a magnetic field. These extend over almost 

 100 pages, or about one-fourth of the whole volume, a 

 space none too large for the subject, but perhaps a little 

 out of proportion to that devoted to dynamo machinery, 

 which is still further restricted by the allocation of fifty 

 pages in chapter iv. to methods of measurement. 



Signor Ferrini's treatment of the theoretical part of his 

 subject seems on the whole marked by completeness and 

 accuracy. He has evidently given careful attention to 

 the late developments of magnetic research, and in his 

 chapter on measurements has included most of the im- 

 provements recently made, such, for example, as the 

 methods of measuring power, &c., in the circuits of 

 alternators and transformers which have been invented 

 by Ayrton and others. No mention i-; made, however, 

 of Blakesley's ingenious " split dynamometer " method for 

 transformers, and determining the difference of phase of 

 two alternating currents. Nor is the method (p. 171) of 

 finding the true mean activity in an alternating current 

 from the apparent activity attributed to its inventor. 

 Prof Ayrton. 



We notice here a few points which have occurred to us in 

 looking over this part of the book as perhaps calling for re- 

 mark. First of all with respect to the definition of a uniform 

 magnetic field given at p. 58, it may be noticed that if 

 the numerical value of the intensity of the magnetic force 

 be the same at all points of a finite space, its direction 

 must be the same at all points of the same space, and 

 that the intensity cannot vary in magnitude from point to 

 point without varying also in direction, and vice versd. 

 This does not seem to be generally understood, at 

 any rate it is common to define a uniform field 

 as one for which the magnitude and tke direction of 

 the magnetic force are the same at every point. That 

 the former implies the latter, and the latter the former, 

 may be seen by considering a closed surface formed by a 

 portion of a tube of force, in the field, intercepted between 

 two equipotential surfaces. The cross-sections at the two 

 ends must have the same area, since the magnetic force 

 at each end is the same. Further, the lines must be 

 straight, for if they be supposed curved, the portion of the 

 tube may be taken so that it is concave on one side and 

 convex on the other. The line-integral of magnetic force 

 round a closed circuit, taken along the convex and con- 

 cave sides and across the ends, vanishes. But nothing is 

 contributed to it by the ends of the tube. Hence the 

 magnetic force along the convex side must be on the 

 whole less than that along the shorter concave side, which 

 contradicts the supposed uniformity of magnitude of the 

 field-intensity. 



At p. 66 difference of potential, Vj-Vq, between two 

 points is defined as the work which must be done against 

 magnetic forces in carrying a unit magnetic pole from the 

 point of lower to that of higher potential ; and at p. 74, 

 where the field of a solenoid is considered,- ^V/</.i- ap- 

 pears as the force on a pole of strength ;«. 



At p. 81 mention might have been made of the influence 

 of mechanical stress and disturbance on the magnet- 

 ization of iron observed by Lord Kelvin and others, and 

 of the fact that very much higher permeabilities than the 

 2COO quoted from Rowland's experiments have been 

 obtained by Ewing for soft iron subjected to molecular 

 vibration produced by tapping. 



The subject of hysteresis is dealt with at p. 91, and again 

 at p. 235 in the chapter on the construction of a con- 

 tinuous-current dynamo. In the latter place a proof is 



NO. 1187, VOL. 46] 



furnished of the well-known formula given by Warburg 

 in 1 88 1 or 1882, and a little later by Ewing, for the energy 

 dissipated in a closed cycle of magnetization. In the 

 course of that proof, to which in itself we take no exception, 

 one or two statements are made which, if we have under- 

 stood the author aright, are erroneous. It is stated that 

 when the integral induction * through each turn of a 

 magnetizing helix of ;; windings, each carrying a current 

 r, is increased by an amount d<b, a quantity of energy 

 = - ncdQ (= - vH^B/47r), where v is the volume of the 

 medium magnetized, H the field intensity and B the in- 

 duction, both supposed uniform) is given out by the spiral 

 and converted into heat. Now (the sign being left 

 out of account) this is certainly the energy sent into 

 the field from the battery or generator, but it is 

 not the case that it is all converted into heat. 

 The amount of energy spent in unit volume of 

 the magnetized medium is H^B/47r, but of this (H</B 

 + B^H)/87r goes to increase the electrokinetic energy, 

 the amount of which per unit volume of the medium is 

 BH/87r. The total amount of energy spent per unit 

 volume in the cycle of magnetization, otherwise than in 

 increasing the electrokinetic energy, is therefore 



U\ 



H^B 



-(H<^B-B^/H) 



2 



the integrals being taken round the cycle. (It is to be 

 noticed that this balance of energy may be negative, and 

 in that case energy is taken from the field to make up the 

 increase of electrokinetic energy.^) 

 But for a closed cycle 



|(H^B + B</H) = o, 



and hence the energy spent is 



'- f Hr/B. 



47rj 



This must have been dissipated, since the medium at 

 the end of the cycle has returned to the same state as at 

 first. 



No affirmation can be made as to what becomes of the 

 balance of energy, except with reference to a closed 



cvcle. TT u 1- • 



' Again, at p. 237 it is stated that if Hi, -Hi, be limits 

 of H corresponding to limits Bi, - Bi of B, 



H«'B 



B/H. 



This is certainly not correct, as may be easily seen by 

 representing the integrals graphically, or by considering 

 that taken round a closed cycle 



WIS. 



H^^B, 



\ (H'^B + B'/H) = \ ^(BH) = o 



for the cycle. 



This error, a mere oversight no doubt, has appeared 

 more than once in connection with this subject, and an 

 erroneous demonstration founded on it and a mistaken 

 identification of the energy dissipated with the electro- 

 kinetic energy, has been used by more than one writer. ^^ 



The chapters on the " Continuous Current Dynamo,^^ 

 the " Dynamo in Action," and " Alternating Dynamos," 

 are excellent in many respects. The subject is well and 

 fairly comprehensively treated, and the very useful notion 

 of the magnetic circuit has been employed throughout 

 with good effect. Some well-known machines do not 

 seem to be described, for example, the Victoria among 



' See a paper by the writer !n the /'////. Mag., December tSgo- 



