364 Dr. 0. J. Lodge on the Dimensions of a Magnetic 



Thomson's in the September number, p. 225, 1 may say in pass- 

 ing that it seems to me that Maxwell held, though no doubt 

 tentatively and hypothetically, the view that electric currents 

 and small magnets are identical and not only equivalent.] 



The coefficient ja is thus foreign to Ampere's theory applied 

 universally; and this is how it has happened that Prof. Clausius 

 has failed to recognize its existence and has been led into error*. 

 A system dealing with Amperian magnets in media for which 

 fj, does not equal 1 is a mongrel combination which may no 

 doubt be occasionally convenient but which never can be 

 thoroughly satisfactory. 



We may accept then without hesitation Clausius's presen- 

 tation of Maxwell's views, viz. both that a small magnet is an 

 electric current, and that magnetic moment always equals 

 simply integral current x area — remembering, however, that 

 there exist currents in molecules besides the gross and arti- 

 ficial currents in our copper wires, that these are directed by 

 our artificial currents and add to their effects, and that in all 

 cases they are most distinctly to be taken into account. 



In air, so far as it is non-magnetic, these molecular currents 

 are zero, and the magnetic induction and the magnetic force 

 are everywhere equal ; but in media consisting of Amperian 



* Very many errors, I now find; for he lias also ignored K, Faraday's simple 

 old electrostatic constant; and accordingly his equations (1), (2), (4), &c. 

 express mere conventions (if they were any thing more, then tndy m g 

 would have to equar md, and e e =ed, as be begins to perceive in his August 

 letter): while his general equation (3), which is the foundation of his 

 reasoning, is quite wrong, and is indeed at the bottom of the whole con- 

 fusion. In using the term " error" here, I would be understood to mean 

 rather " divergency from opinions commonly held in this country " than 

 absolute incorrectness as to matter of fact. For it would not be becoming 

 to apply the latter term to views held by Prof. Clausius when the experi- 

 mental foundation of opposing views is confessedly incomplete. The 

 views held by Prof. Clausius are no doubt perfectly consistent, and would 

 probably be in accord with fact if only the medium produced no effect 

 such as it is here commonly supposed to produce ; and whether the me- 

 dium does or does not produce such an effect appears to some extent at 

 present a subject of legitimate debate and a matter for experimental 

 investigation. It will be understood therefore, that in stating one side 

 strongly I have been influenced by the wish to be clear, rather than with 

 the desire to dogmatize. 



Since the above letter was in type Dr. Francis has kindly called my 

 attention to a paper by Prof. Helmholtz in "Wiedemann's Annakn, No. 9, 

 1682, to which I might have further referred if I had known of it in 

 time. As far as I can hurriedly understand his position, Prof. Helmholtz 

 in part endeavours to reconcile the views of Maxwell and of Clausius by 

 throwing a doubt upon the Weber-Ampere theory ; and in fact he appeals 

 to pure physicists not to abandon the old elecrro'static for the more cum- 

 brous and less surely founded electromagnetic system. 



[A translation of Professor Helmholtz's paper will appear in our next 

 number. — Ed. Phil. Mag.] 



