NATURE 



241 



THURSDAY, JULY 12, ii 



ELECTRICITY AND MAGNETISM. 

 A Treatise on Electricity ci7id Magnetism. By E. 

 Mascart and J. Joubert. Translated by E. Atkinson. 

 Vol. II. (London: De La Rue and Co., 1888.) 



THE English translation of the second volume of the 

 valuable work of MM. Mascart and Joubert is a 

 welcome addition to the class, none too large, of really 

 substantial English books on electricity. We have already 

 directed the attention of the readers of Nature to the 

 first volume of this work ; and we took occasion to point 

 out that in their exposition of the subject the authors 

 follow very closely the general methods of Clerk-Maxwell. 

 That they do so is a great advantage for the English 

 student ; because it enables him, without breach in the 

 continuity of his studies, to use Mascart and Joubert as a 

 commentary upon Maxwell, who is often by no means 

 easy reading. The French work is also supple- 

 mentary to Maxwell, for writers avoid as much as 

 possible the purely theoretical side of electrical science, 

 and treat electrical phenomena, more especially in 

 their second volume, as subjects of observation, and 

 above all, of measurement. This volume, which is 

 now before us, is, in fact, an epitome of all the wisdom 

 in exact electrical measurement which has been gained 

 during a period of extraordinary activity in that field. 

 This period began with the researches of Gauss and 

 Weber ; and may perhaps be said to have culminated in 

 the great series of determinations of the absolute units 

 which were made about the time of the Congress of 

 Electricians at Paris in 1884. The prominent part taken 

 by MM. Mascart and Joubert in this work has well fitted 

 them to record with precision the details of the leading 

 methods by which it was accomplished, and it would be 

 hard to refer the student of electrical science to an 

 authority on electrical measurement at once so clear and 

 precise in detail, and, with a few small and evidently 

 accidental exceptions, so manifestly candid and fair, as the 

 second volume of the treatise of Mascart and Joubert. 



The space at our disposal in the pages of Nature 

 allows us to give but a brief summary of the contents of 

 this volume. Part I. deals with the auxiliaries of elec- 

 trical measurement, such as the measurement of angles, 

 of the periods and amplitudes of oscillations, of couples, 

 and of such properties of circular currents as are im- 

 portant in the construction of galvanometers and other 

 electrical instruments. As an example of the care with 

 which the subject is treated, we may refer to the discus- 

 sion, in §§ 659, 660, of the power of a telescope, and of the 

 relation that ought to subsist between the dimensions of a 

 graduated circle and of the telescope with which it is 

 associated. The conclusion of this discussion is marred 

 in the English version by inadequate translation. Thus, 

 for example, " un cercle de ce diametre [80 cm.] devra 

 done etre associe" a une lunette de 16 centimetres d'ouver- 

 ture," does not mean "a circle of this diameter is 

 therefore comparable with a telescope of 16 cm. aperture." 

 The meaning is, that, to get the full use of the circle, a 

 telescope having an objective lens of 80 cm. aperture is 

 required ; and that a more powerful one is unnecessary. 



Part II., which is the kernel of the volume, describes 

 Vol. xxxviii. — No. 976. 



the various electrical measurements as they are carried 

 out in practice. There are chapters on electrometry, and 

 on measurement of current, resistance, electromotive 

 force, capacity, constants of coils, absolute resistance, 

 and the fundamental velocity, v. The methods are 

 described in great variety and with great detail. They 

 are illustrated by giving not only the old classical results, 

 but also by means of the most recent examples. Nothing 

 is attempted like the exhaustive catalogue of results, good, 

 bad, and indifferent, which makes Wiedemann such an 

 invaluable book of reference. Experimental results are 

 given simply as part of the exposition of the methods by 

 which they are obtained. It is probably for this reason 

 that the authors make no mention of the valuable 

 experiments on dieletric strength recently made by their 

 fellow-countryman, Bailie. 



Part III. is devoted to magnetic measurements, and is 

 excellent so far as it goes. It is by no means so exhaustive 

 as the purely electrical part ; and, probably for that very 

 reason, will be found to be lighter reading for the tyro in 

 electricity and magnetism ; to such we commend more 

 especially the parts relating to the determination of 

 so-called magnetic poles and to the magnetism of feebly 

 magnetic and diamagnetic bodies, subjects which are 

 very frequently imperfectly understood or inadequately 

 expounded in current text-books. 



Part IV., which is called a complement, deals with 

 industrial applications, and contains a table of numerical 

 constants. The table of constants gives full references 

 to the sources of such information as it contains, and will 

 be found most useful. The part that deals with industrial 

 applications is to our thinking the least satisfactory part 

 of the book ; not because there is any want of clearness or 

 soundness in it, but because it is too short and too scantily 

 illustrated by references to practical cases to give the 

 student any real idea of the problems that surround the 

 electrical engineer. 



In describing the various methods of electrical measure- 

 ment the authors are, on the whole, very sparing of 

 criticism. They seem to assume that they are addressing 

 an audience fitted to draw their own conclusions from 

 the facts put before them. Occasionally the weak points 

 of the methods used by various experimenters are pointed 

 out, but the authors never indulge in that species of 

 criticism which consists in treating a fellow-labourer and 

 all his productions with indiscriminate scorn because 

 the critic has discovered some microscopic oversight, or 

 believes that he has wrung one more decimal place from 

 reluctant Nature. 



Thereareone or two little points which mightbe amended 

 in a future edition. For example, the elegant method of 

 discussing resisted motion by means of the equiangular 

 spiral, given in § 682, should be attributed to its author, 

 Prof. Tait. The use of the fish-back galvanometer-needle 

 {i.e. a needle made up of a number of separate parallel 

 needles) was not an invention of M. Deprez, at least not 

 an original invention ; for the writer used, more than 

 twelve years ago, a galvanometer fitted with a needle of 

 this sort, which had been constructed for the B.A. Com- 

 mittee of 1867. Who the inventor was, is doubtful ; but 

 probably he took his idea from the laminated magnets 

 constructed by Jamin and others. Perhaps the most 

 serious historical oversight is made in § 1274, where, in 



M 



