Oil the Electrostatic Dimensions of a Magnetic Pole. 429 



which they are placed ; but, according to Maxwell, the mag- 

 netic force between a current and a magnetic pole does not. 

 Hence the moment of the magnetic shell equivalent to a given 

 current depends on the medium surrounding the shell. If the 

 field be occupied by one medium only, the moment of the 

 magnetic shell =/* . current x area. This equation gives the 

 dimensions M^Li for m. 



The effect of the medium surrounding a current upon the 

 magnetic force produced by the current has, as far as I know, 

 not been investigated. The only experiments I know bearing 

 on this point are some experiments of Faraday on the effect 

 of the surrounding medium on the electric currents induced 

 by a moving magnet. The result of the experiments was, that 

 he could not detect that changing the medium had any effect 

 on the induced currents. If this be so, then Maxwell's state- 

 ment must be right. For, imagine a long magnet in presence 

 of an electric current, and let one pole be in one medium and 

 the other in another ; then, if the forces on the two poles are 

 different in magnitude, we may carry the magnet round the 

 current so as to get work out of it. Now this work must have 

 come from the battery ; but, since the inductive action of the 

 magnet on the current is independent of the medium, there 

 will be on the whole no currents induced by the motion of the 

 magnet : hence no work can come from the battery ; hence 

 the forces on the poles must be equal, or the magnetic force 

 produced by a current must be independent of the surrounding 

 medium. I may mention that Mr. E. B. Sargant, of Trinity 

 College, is at present experimentally investigating this point. 

 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 dimen- 

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

 I am, Gentlemen, 



Your obedient servant, 



J. J. Thomson. 



LVI. On the Electrostatic Dimensions of a Magnetic Pole. 

 By Prof. J. Laemok*. 



CILAUSIUS'S formula to connect the dimensions of cur* 

 1 rent and pole on the static system, viz. 



(A) Moment of Magnet = Current x Area. 



seems really to involve the electromagnetic system of Units, 

 It is plain that, to be able to make and establish practically 

 * Communicated bv the Author. 

 Phil Mag, S. 5. Vol. 13. No." 83. June 1882. 2 K 



