810 



SCIENCE. 



[N. S. Vol. II. No. 50. 



matical treatment. How then has Prof. Thomson 

 managed to strip off the mathematical dress 

 and to present the naked facts? First, by a 

 thorough familiarity with the mathematical 

 treatment which has enabled him to seize the 

 essential, in spite of disguise, and secondly by 

 an unusual gift of exposition. It is only the 

 thorough knowledge of mathematics that enables 

 one to express mathematical truths in plain 

 language. It is a very common opinion that a 

 great talent for research is incompatible with 

 excellence as a teacher. Unfortunately many 

 instances may be cited in support of this propo- 

 sition, but we contend that it is by no means 

 necessarily true. The same faculties that make 

 one eminent in research should also go to make 

 him successful as a teacher. For either is 

 necessary first enthusiasm, then a thorough 

 acquaintance with the subject, while the teacher 

 needs in addition only the power of saying what 

 he has to say. Given a good style, and some- 

 thing to say, with a wish to teach, and we can- 

 not see what more the teacher needs. Our 

 present author is an example in support of our 

 contention. 



Where all is so good it is difficult to select 

 special portions for commendation. We will, 

 however, mention a few matters not usually 

 well treated in elementa>'y books. The parallel 

 treatment of dielectrics and magnetizable bodies 

 is clearly carried out, and the distribution of 

 the energy in the medium, of fundamental im- 

 portance in the modern theory, is careftilly de- 

 duced. We are gratified to notice that the 

 author uses in quantitative statements the ex- 

 pression (unit) tubes of force, rather than the 

 usual lines of force. The inappropriateness of 

 denoting the flux of induction in a dynamo by 

 so many lines is illustrated by a recent letter to 

 one of the technical journals, in which the 

 writer makes the luminous remark j that the 

 dimensions of the unit of induction cannot be 

 as usually and correctly given, because the 

 dimensions of a line are the same as of a length! 

 It seems to us to be regretted that Prof. Thom- 

 son has here, as in his large volume, made use 

 of the term ' Faraday tubes ' to denote tubes of 

 electrical induction. Magnetic tubes are cer- 

 tainly as much due to Faraday as electric. The 

 chapters on fields of force are illustrated by 



numerous diagrams, some of them new. The 

 confusing matter of magnetic force and mag- 

 netic induction is made plain, and the uniform 

 magnetization of a sphere and of an ellipsoid 

 worked out. The statement may be noticed 

 that a long ellipsoid tends to place itself along 

 the lines of force in a uniform field, whether 

 magnetic or diamagnetic. It is so frequently 

 stated in text-books that a diamagnetic body 

 tends to set itself across the field that this will 

 surprise many. The setting across usually ob- 

 served comes from the lack of uniformity in the 

 field, diamagnetic bodies being repelled from 

 stronger to weaker regions. The correct state- 

 ment was made forty years ago by Lord Kelvin, 

 who stated, however, that the force tending to 

 make diamagnetic bodies set along the field was 

 probablj^ too small to be observed. It has been 

 observed by the present writer, and the method 

 of observing its influence upon the time of swing 

 of an ellipsoid has been suggested as a means of 

 determining the permeability of diamagnetic 

 bodies, and is now being carried out by one of 

 his students, Mr. A. P. Wills. ' 



Prof. Thomson gives a good treatment of 

 electrolysis and of the electromotive force of 

 batteries, but we think that the fact that the 

 electromotive force can be calculated from the 

 chemical work in the manner stated, only when 

 the cell has no temperature coefficient, should 

 not have been omitted. In the chapter on in- 

 duction the similarity of the system of currents 

 to a mechanical system is well brought out, and 

 a new and very simple model described. It 

 consists of three weights, hung from carriages 

 rolling on three parallel rails, and kept in line 

 by a straight bar passing through swivels on the 

 carriages. The velocities of the outside car- 

 riages being independent, the sj'stem may be 

 assimilated to two currents. On account of the 

 third mass, the kinetic energy contains a term 

 in the product of the velocities of the outside 

 carriages, and this term gives rise to the phe- 

 nomena resembling mutual induction. For in- 

 stance, if one carriage is started the other goes 

 backward, and when both are moving with con- 

 stant velocities, if one is arrested the other goes 

 faster. The disadvantage of the model comes 

 from its simplicity, in that the coefficients of in- 

 duction are constant, so that electromagnetic 



