Pole in the Electrostatic System of Units. 363 



These relations must all hold in any consistent system of 

 units, since they express physical truths ; but of course they 

 are not all independent. The number of independent relations 

 must be limited by the number of fundamental experiments, 

 viz. three — Coulomb, Coulomb, and Oersted ; and the short- 

 est way of "writing the independent relations is this : — 



[^ 2 ] = [Km 2 ]~[ML] ) 

 and J- . . . . (5) 



The electrostatic convention makes [K] = l; the electromag- 

 netic convention makes [/&] = !. 



So far every thing being pretty clear and straightforward, 

 we have now to ask how it was that Prof. Clausius should 

 have jumped to the conclusion that Maxwell had fallen into 

 error*, or else that he held a theory of magnetism different 

 from (and not merely an amplification of) Ampere's and 

 Weber'sf. With this latter horn of the dilemma, by the way, 

 he is half allowed by Mr. J. J. Thomson (September) to have 

 succeeded in transfixing Maxwell; and Dr. Everett (June) is 

 not extremely energetic in his repudiation even of the alterna- 

 tive of the first. 



Now it is perfectly true that Maxwell, in stating the current 

 theory of magnetism, says without any kind of retraction or 

 hesitation that " the magnetic action of a small plane circuit 

 at distances which are great compared with the dimensions of 

 the circuit is the same as that of a magnet whose axis is normal 

 to the plane of the circuit, and whose magnetic moment is 

 equal to the area of the circuit multiplied by the strength of 

 the current and if a magnetic shell .... &c. be substi- 

 tuted .... then the magnetic action of the shell on all distant 

 points will be identical with that of the current/' And in dis- 

 cussing Ampere's theory, he ignores the existence of magnetic 

 media whose /u, does not equal 1 as completely as Prof. Clausius 

 could wish. 



But then, according to Weber's extension of Ampere's 

 theory (an extension I suppose universally accepted), the pro- 

 perties of magnetic substances of all kinds are explained by 

 molecular electric currents, and no magnets or magnetic sub- 

 stances other than those consisting of current-conveying 

 molecules exist. [And with reference to a remark of Mr. 



* Phil. Mag. June, pp. 387 & 392. t Ibid. August, p. 126. 



2B 2 



