NA rURE 



241 



THURSDAY, JULY 14, 1887. 



I 



ELEMENTARY PRACTICAL PHYSICS. 

 Lessons in Elementary Practical Physics. By Balfour 

 Stewart, M.A.', LL.D., F.R.S., Professor of Physics, 

 Victoria University, the Owens College, Manchester, 

 and W. W. Haldane Gee, B.Sc, Demonstrator and 

 Assistant Lecturer in Physics, the Owens College. 

 Vol. IL Electricity and Magnetism (London: Mac- 

 millan ancj Co., 1887.) 



THE second volume of the now familiar " Stewart and 

 Gee " has at length appeared, and it is satisfactory 

 to find that the hopes and expectations to which a just 

 appreciation of the first has given rise have not been 

 formed in vain, but that the same store of exact 

 information down to the minutest details is to be found in 

 " Electricity and Magnetism " as in " General Physical 

 Processes." 



While in this the second volume the authors " have 

 adhered to the plan of subdivision into a series of lessons 

 each descriptive of something to be done by a definite 

 method with definite apparatus," they have treated the 

 subject, to a certain extent, twice over -in the first three 

 chapters generally, with simple and easily extemporized 

 apparatus, and in the remaining chapters as exactly as 

 possible, with all that care and attention to the details of 

 standard patterns of instruments which are essential to 

 the claim, in the title, of" practical." Thus in the second 

 chapter instructions are given for measuring M and H by 

 the method of Gauss without any bought apparatus, while 

 in the sixth chapter is to be found a most complete 

 description of the Kew unifilar magnetometer with figures 

 of the instrument in its two positions and of its parts. 

 There is here a fully worked example, from which a 

 student who has not the advantage of using such an 

 instrument may get an idea of the accuracy obtainable, 

 and from which he may realize the relative importance of 

 the numerous corrections which are applied. To facilitate 

 the calculation of these corrections tables are supplied 

 which will be of value to those using the magnetometer. 



Though all will agree that a little practical introduction 

 to the subject generally is of advantage to the student, 

 and that therefore these three introductory chapters serve 

 a really useful purpose, many will question the wisdom of 

 devoting space in an essentially practical book to an 

 explanation of such terms as electromotive force, con- 

 ductivity, resistance, or of the theory of the battery or the 

 meaning of Ohm's law. These are text-book matters for 

 which a student does not depend on a practical book, nor 

 is he expected to do so. The ten pages devoted to these 

 points are in fact ten pages wasted. 



The fourth chapter deals with the measurement of re- 

 sistance. As this extends over 108 pages, and is divided 

 into sixteen " lessons," it is clear that this very important 

 branch of the subject receives its fair share of attention. 

 The measurement of the resistance of every kind of 

 material, from thick copper bars to insulators, is fully 

 set forth. It is in this chapter that an explanation is 

 given of the method of putting up a reflecting galvano- 

 meter and its several adjustments. The directions for 

 fixing the instrument in the most perfect way are excel- 

 VOL. XXXVL— No. 924, 



lent. Not a word is said, however, to show that for many 

 purposes such a galvanometer set up in any azimuth 

 and almost anyhow is not less serviceable than wher 

 arranged as described. No trouble should be spared, anc 

 none is spared by the authors, in showing how to do any- 

 thing which shall improve the accuracy of work done 

 but in some cases a mass of elaborate adjustment serve; 

 no purpose whatever, and then it should be pointed ou 

 that, though useful for this or that purpose, such adjust 

 ment may be dispensed with. 



In the chapter on the tangent galvanometer the con 

 struction of standard and ordinary instruments is o 

 course explained, and the method of using them. Theii 

 application to the determination of electro-chemica 

 equivalents and of Joule's equivalent is given here. Ii 

 the account of the method of finding the quantity of hea 

 developed there is, apparently, a slight oversight. Th( 

 authors speak of the mean deflection during the time th( 

 experiment lasts. If the current is practically constant 

 the mean deflection may be taken without appreciabli 

 error ; but if it varies, then the square root of the meai 

 square truly represents the heating effect of the current 

 It is true that it should be the aim of the experimenter t( 

 avoid such variations ; but if for some reason they shouh 

 occur, the student should be told how to make the bes 

 of his experiment. 



In the sixth chapter, already referred to in part, w 

 find a most admirable description of the methods of de 

 termining the magnetic elements. This chapter leave 

 nothing to be desired. 



The chapter on electro-magnetic induction contains ai 

 account of a great many experiments on induction 

 magnetism by currents, of currents by currents, and c 

 currents by magnets ; but, as in the third chapter, spac 

 is devoted to matters which might with advantage be lei 

 to the ordinary text-books. For instance, there is m 

 necessity to prove the expressions for the ballistic gal 

 vanometer, or to explain the theory of damping an 

 logarithmic decrement. The determinations of the cc 

 efficients of self and mutual induction form the subjec 

 of one lesson only. This part may now with advantag 

 be greatly extended, since, lately. Prof. Foster ha 

 brought the subject before the Physical Society, anc 

 many others have followed suit. 



In the last two chapters the condenser and electromete 

 are treated. 



A good deal of useful matter is to be found in some c 

 the appendixes. In the first we find the Wheatstone ne 

 and Kirchoff' s laws ; in the second and third, the theor 

 of the electrical units. The fourth will be found themos 

 valuable, in the laboratory, as there is here much addi 

 tional information on the comparison of electromotiv 

 forces and the construction of standard cells. That oi 

 additional practical details is useful as far as it goes. 



There is nothing about electro-capillarity, or about in 

 struments depending upon any action of the'kind ; ther 

 is practically nothing about the electro-dynamometer, am 

 there is no index. 



Though a few faults have been found, they ar 

 mostly unimportant ; and it is a matter of opinioi 

 whether some, especially the introduction of theoretica 

 explanations into a book intended for use in the laboratory 

 are faults at all. The book will be found to be of th. 



M 



