October 26, 1900.] 



SCIENCE. 



647 



cleared up by subsequent statements given by 

 the authors, but on the whole the introduction 

 seems very unsatisfactory. 



Length, angle, mass and time are called 

 measurable quantities because these attributes 

 (to speak of them briefly) may be divided into 

 parts, which by means of one or another kind 

 of congruence, are judged to be equal or like 

 parts, and these parts may then be counted. 

 This fundamental notion which is due, we be- 

 lieve, to Helmholtz, is no doubt the real basis 

 of quantitative relations in physics ; and it 

 should be remembei-ed that, although we fre- 

 quently speak of the measurement of an elec- 

 tric current, of a magnetic field and what not, 

 we never do actually measure anything but 

 lengths, angles, masses and time intervals. 



In the first chapter, on electrical units and 

 quantities, Vigneron and Letheule make a dis- 

 tinction between electromotive force and poten- 

 tial difference, which distinction, being largely 

 in vogue among electricians and not being 

 based upon the fundamental conception of 

 potential, it is a disservice to perpetuate. A 

 distinction, however, there certainly is between 

 the two, and it is, according to Maxwell, as 

 follows : 



When electric charge is transferred from one 

 point to another work is usually done. The 

 amount of work done depends In general upon 

 the path along which the charge is carried. 

 The work done in carrying unit charge along a 

 given path is called the electromotive force along 

 that path. 



In special cases the electromotive force is the 

 same along any two coterminus paths. In such 

 a case the common value of the electromotive 

 force is called the potential difference between 

 the terminal points. 



Now it seems to us that no author should at- 

 tempt to make any other distinction between 

 electromotive force and potential difference than 

 the above. In particular the distinction between 

 the total electromotive force of an electric gener- 

 ator and the electromotive force between the ter- 

 minals of the generator should not be confused 

 with the distinction between electromotive force 

 and potential difference. One may answer, in- 

 deed, that the practical electrician is concerned 

 with the distinction between total and external 



electromotive forces of electric generators, and 

 not at all concerned with the fine distinction, , 

 according to Maxwell, between electromotive 

 force and potential difference. This is too true, 

 but this is no reason why electricians should be 

 permitted to misuse these terms without pro- 

 test, for very certainly the distinction between 

 total and external electromotive force of a gen- 

 erator has nothing essentially in common with 

 the distinction between electromotive force and 

 potential difference in the sense in which Max- 

 well uses these terms. 



There Is one thing in which we know of only 

 one person (Heaviside) who agrees with us, 

 namely, that the notion of electric potential 

 might best be dropped in the subject of electro- 

 dynamics, and we are convinced that the pref- 

 erence of most electricians for the term poten- 

 tial to the term electromotive force is in their 

 tongues, not in their heads. 



W. S. Franklin. 



BOOKS KECEIVKD. 



Text-book of Physiology. Edited by E. A. Schaper. 



Edinburgh and London, Young J. Pentland. New 



York, The Macmillan Company. 1900. Vol. 11., 



pp. xxiv + 1365. $10.00. 

 The Theory and Practice of Hygiene. J. LANE NOTTEB 



and W. H. HoRROCKS. Philadelphia, P. Blakis- 



ton's Sons & Co. 1900. Second Edition. Pp. 



xvii -I- 1085. S7.00. 

 A Treatise on Zoology. Edited by E. Ray Lankes- 



TER. Part II., The Porifera and Caltntera. E. A. 



MiNCHiN, G. Herbert Fowler and Gilbert C. 



Bourne. London, Adam and Charles Black. New 



York, The Macmillan Company. 1900. $5.50. 

 Free-hand Perspective. VICTOR T. WILSON. New 



York, John Wiley & Sons. Loudon, Chapman & 



Hall, Limited. 1900. Pp. xii + 268. $2.50. 

 Dynamo Electric Machinery. SAMUEL SHELDON. 



New York, D. Van Nostrand Company. 1900. 



Pp. 281. $2.50. 

 Die Lehre von SIcelet des Ifenschen. F. Frenkel 



Jena, Gustav Fischer. 1900. Pp. vl -f- 176. M. 



4.50. 

 Among the llushrooms. ELLEN M. DALLAS and 



Caroline A. Burgen. New York, Drexel Biddle. 



1900. Pp. xi + 175. 

 77(6 Principles of Mechanics. FREDERICK Slate. 



New York and London, The Macmillan Company. 



1900. Pp. X -1-299. 



