126 On tke JHmensions of 'a Cud of Magnetism. 



the words " Mr. W. D. Niven has pointed out to me that the 

 value given by Clausius for the dimensions of a magnetic pole 

 does not make the magnetic force between two such poles of 

 the dimensions of a force, which ought clearly to be the case." 

 I cannot understand this objection. In the eleetrodynamic 

 system of measures the force between two magnetic poles [wd] 

 is represented by the formula of mechanical force. Xow, if 

 in the electrostatic system also the force between two magnetic 

 poles [m s ~] is to be represented by a formula of the same 

 dimensions, then must [m s ] have the same dimensions as [»? rf ], 

 which is no more the case with Maxwell than with me, and 

 also cannot be the case: while if instead of [;«,] the quantity 

 is put which I have denoted by v. d. [m,] (the value of the 

 electrostatic unit of magnetism reduced to eleetrodynamic 

 measure), the expression of the force between two magnetic 

 poles assumes again the dimensions of the mechanical-force 

 formula. Mr. Xiven might have made the same objection 

 against the eleetrodynamic unit of electricity as he has made 

 against the electrostatic unit of magnetism. 



J. J. Thomson raises also an objection of his own. He says 

 that, in determining magnetic force the magnetic permeability 

 /j, of the medium in which the current is placed must also be 

 taken into account ; and then he adds : — " Thus the force be- 

 tween two magnetic poles depends on the medium in which 

 they are placed ; but, according to Maxwell, the magnetic 

 force between a current and a magnetic pole does not." In 

 this sentence a distinction is made between the force of a mag- 

 netic pole and the magnetic force of a current, which is quite 

 foreign to Ampere's theory. According to that theory a small 

 closed current can completely replace a small magnet with 

 respect to the forces exerted by it upon other magnets or 

 closed currents, so that it is no longer necessary, in order to 

 explain the magnetic actions of a body, to assume the presence 

 in it of a special agent to be designated by the name of mag- 

 netism, but instead of this we can suppose that the molecules 

 are encircled by electric currents, and that these exert the 

 actions in question. It is of course contradicting this to say 

 that the magnetic forces proceeding from an electric current 

 act according to other laws than those which govern the forces 

 proceeding from a magnet. 



For all these reasons I must continue to maintain that, if 

 physicists accept Ampere's theory (which is certainly done by 

 most physicists, and, as I believe, was clone also by Maxwell), 

 they must, to be consistent, accept also the formula determined 

 by me for the electrostatic unit of magnetism, instead of the 

 formula set up by Maxwell. 





