56 



NATURE 



[September 17, 1914 



ELECTROSTATICS AND MAGNETISM. 



A Text-book of Physics. By Prof. J. H. 

 Poynting and Sir J. J. Thomson. Electricity 

 and Magnetism. Parts i. and ii. Static Elec- 

 tricity and Magnetism. Pp. xiv + 345. (Lon- 

 don: Charles Griffin and Co., Ltd., f9i4.) 

 Price 10s. 6d. 



THE late Prof. Poyntingf's qualities as a phy- 

 sicist are reflected in the volume on 

 electrostatics and magnetism now before us, 

 for he was always distinguished by soundness and 

 clearness of thought, and care and accuracy of 

 experiment rather than by showy brilliance. The 

 book is very clearly written, and evidently no 

 trouble was spared in the attempt to make it as 

 easily understood as possible. A large part of it 

 is, in fact, a model of exposition, and could 

 scarcely be improved upon. The chief value of 

 the book, however, lies in the fact that it goes a 

 little farther than the ordinary elementary text- 

 book has done up to the present. Most English 

 text-books deal in a very cursory way with both 

 electrostatics and magnetism, and it is extremely 

 useful to have a book which gives a somewhat 

 more adequate treatment without at the same time 

 becoming too difficult and exhaustive. 



The chapters on "The Dielectric," "Specific 

 Inductive Capacity," "Stresses in the Dielectric," 

 ''Alterations in the Dielectric," and "Pyro-elec- 

 tricity and Piezo-electricity," are particularly good 

 in the " Electrostatics " ; and those on " Forces on 

 Magnetised Bodies," "Paramagnetic and Dia- 

 magnetic Substances," and "Magnetism and 

 Light" in the "Magnetism." The simple state- 

 ment of the titles of these chapters will give a 

 clue to the difference between this text-book and 

 others. All the extra matter is quite important, 

 and in no way abstruse, yet a considerable part of 

 it is not to be found in any other Enghsh ele- 

 mentary text-book. The great value of the book 

 to students is therefore obvious, and is scarcely 

 lessened by the few defects which exist. 



The first and most serious of the faults of the 

 book is the inconsistency with which previous 

 knowledge in the student is assumed. The first 

 chapters in both the electrostatics and the mag- 

 netism are written as if for students with abso- 

 lutely no previous knowledge of the subjects, 

 while to understand the chapters on magnetic in- 

 duction a student would require a reasonable 

 knowledge of the magnetic field due to a current. 

 In a book of this character it would seem that 

 the most reasonable assumption to make is that 

 the knowledge of the student is of about the inter- 

 mediate science standard. The extremely ele- 

 mentary general accounts of the common pheno- 

 NO. 2342, VOL. 94] 



mena could thus be omitted, and one could start 

 without any preamble on the more important part 

 of the book. 



The order in which the various divisions of the 

 subject are taken is rather strange in places. It 

 seems the reversal of the proper order to give a 

 number of the consequences of an inverse square 

 law of force in one chapter and to give the experi- 

 mental proof of the law in the following chapter, 

 but the most flagrant case is the attempt to give 

 a general account of susceptibility, permeability, 

 and the hysteresis loop before any of the magnetic 

 quantities have been defined. To an elementary 

 student these things would be very confusing, 

 but as there is no doubt that all students who 

 come to this book will bring with them a fair 

 working knowledge of the most elementary parts 

 of the subject, very little harm will be done. 



A few not unimportant matters of detail might 

 also be improved. There are two totally different 

 definitions of quantity of electricity, one on p. 29 

 and one on p. 67. It is true that these two are 

 consistent with one another, but this is not proved, 

 and the first definition is quite unnecessary and 

 is nowhere used. The definition on p. 42 of the 



electric potential at a point as - seems to be 



r 



converting potential into a mathematical symbol 



with no direct physical meaning. If we define the 



difference of potential between two points as V 



when the work done in taking an indefinitely small 



quantity dq from one point to the other is V.dq, 



we not only have the physical conception of work 



per unit quantity but we can also define any other 



kind of potential in an exactly similar way. 



It is also desirable that Franklin's jar with its 

 movable coatings should be given a long-deserved 

 rest. It does not prove, what it was originally 

 supposed to prove, that the seat of electrical action 

 is in the dielectric ; it merely proves that glass is 

 a very bad substance to use as an insulator, be- 

 cause it so readily forms a conducting layer on its 

 surface by the absorption of moisture from the 

 atmosphere. A Franklin's jar made with a good 

 insulator does not work, and therefore there seem 

 to be no reasons for introducing it. 



Most of the diagrams are very clear and suit- 

 able ; they are of good size and are simplified as 

 much as possible. But Fig. 194 on p. 250 offends 

 the eye of a physicist just as keenly as a note 

 badly out of tune offends the ear of a musician. 

 One scarcely expects all the diagrams of lines of 

 force and equipotential surfaces to be drawn 

 accurately to scale, but one does demand that 

 lines of force and equipotential surfaces shall not 

 be palpably inclined to one another as they are 



