Measures for Electric and Magnetic Quantities. 435 



(— ii) Jacoby's force-function. If we introduce for m another 

 measuring unit, and measuring with it obtain m l7 Q l5 J l 

 instead of m, fl, J, then must, according to Maxwell's defi- 

 nition, 



and therefore 



77lJ = 77l 1 J 1 . 



The unit of current therefore increases in the inverse propor- 

 tion of the newly chosen unit of magnetism to the old one ; 

 but the force which the unit of current exerts upon the unit 

 of magnetism remains constant. With Prof. Clausius, on the 

 contrary, both m and J increase in the same, and not in the 

 inverse ratio, and the force increases as m 2 or J 2 . According 

 to his determinations, the feigned surface of Ampere always 

 produces one more leap, equal to the momentum of unit of 

 surface, in the function %{mjr)', but this function has then 

 no longer the signification of the force-function for the newly 

 chosen magnetic units. 



In all this I cannot perceive any mistake of Maxwell's; and 

 his equations, derived from the formulation chosen by him of 

 the fundamental phenomenon, are altogether as consistent with 

 each other and as correct, if understood in the sense of their 

 author, as those of Prof. Clausius. Bather, this case shows 

 that, if we abandon Gauss's determination of the magnetic 

 unit, we again fall into at least two different and equally jus- 

 tifiable systems of measuring-units; and, for my part, I could, 

 with respect to both Maxwell and Clausius, draw from this 

 the practical conclusion that we ought by no means to forsake 

 the above-mentioned method of Grauss until we have particu- 

 larly important reasons and a definite purpose for such a pro- 

 ceeding, when the choice between the systems of Maxwell and 

 Clausius would probably be decided on positive grounds. 



Kow, although I must vindicate Maxwell from the charge 

 of having, in consequence of an oversight, set up incorrect 

 equations, yet it should be mentioned that in the wording of 

 the text of § 623 of his work, where he reduces the dimensions 

 of all the rest of the electric and magnetic quantities to the 

 dimensions of any one chosen from among them, an omission 

 occurs which might easily lead the reader into error, if he 

 does not closely examine the connexion of the somewhat com- 

 plicated systems of equations of § § 622 and 623, and which 

 seems to give to the propositions of § 623 a greater extension 

 than Maxwell himself would probably have given them. For 

 at the beginning of Chapter X., in § 620, he speaks of electro- 

 static and electromagnetic but not of magnetic units, and 



