On the Dimensions of a Magnetic Pole. 427 



during the passage of the current, and observe the loss of 

 moment (AM) of the hollow rod. See whether the change of 

 magnetic force deduced from the oscillations is or is not less 



than — ^ • 

 a° 



II. Prepare two small coils of precisely equal inductive 

 power. Place one inside, and the other outside, the hollow rod 

 at the middle of its length. Connect them both to a ballistic 

 galvanometer or a telephone in such a manner that, if the cur- 

 rents induced in them are equal, there will be no resultant 

 effect. Then pass a current along the rod. If the effects are 

 simply magnetic, the induction will be the same for both coils; 

 but if the passage of the current produces any electromagnetic 

 force in the interior, this equilibrium will be much disturbed. 



This last has the merit of being a null method ; and it has 

 the additional advantage, that a strong longitudinal magneti- 

 zation may be given to the rod by enclosing it in a long sole- 

 noid through which a steady current may be kept circulating 

 during the experiment. 

 Tokio, Japan, 



September 22, 1881. 



LV. On the Dimensions of a Magnetic Pole in the Electrostatic 



System of Units. By J. J. Thomson. 



To the Editors of the Philosophical Magazine and Journal. 



Trinity College, Cambridge, 



Gentlemen, May 16, 1882. 



IN the last number of the Philosophical Magazine there is 

 a paper by Prof. J. D. Everett, on the Dimensions of a 

 Magnetic Pole in the Electrostatic System of Units, in which 

 he supports the view taken by Clausius, that in this system 

 the dimensions of a magnetic pole are M*L*T -2 , and not 

 M*L* as stated by Maxwell. Clausius gets his value from 

 the equation 



Moment of magnet — current x area. 

 It seems to me, however, that the equation should be 



Moment of magnet = /n current x area; 

 where jm is the magnetic permeability of the medium in which 

 the current is placed, and is of dimensions T 2 L~ 2 in the elec- 

 trostatic system : if we use this equation we get the same 

 dimensions for the magnetic pole as Maxwell. The following- 

 are the reasons which seem to me to show that the value given 

 by Maxwell is the correct one. It will be convenient to see 

 how Maxwell's value is obtained. The easiest way of getting 



