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LXVI. On the Dimensions of a Magnetic Pole in the Electro- 

 static System of Units. By C. K. Wead*. 

 IN the May number of this Magazine, p. 376, Dr. Everett 

 presents Clausius's deduction of the dimensions of a mag- 

 netic pole in the electrostatic system, finding them to be 

 P e =M*L ! T" 2 , instead of M*L*, as Maxwell gave it without 

 explicit statement of the method of derivation. In Maxwell's 

 treatise this unit is obtained, along with all the rest, from fif- 

 teen simultaneous equations involving twelve unknown quan- 

 tities ; the substitution of the final results in the original 

 equations shows that the suggestion of a misprint may be set 

 aside at once. 



In the smaller books that use the clearer method of deriving 

 the dimensions step by step, the development does not usually 

 include magnetic quantities under the electrostatic system : 

 they are not found in the British-Association Reports on Elec- 

 trical Standards, Reprint, p. 80, nor in Kohlrausch's ' Physical 

 Measurements/ nor in Everett's ' Units and Physical Con- 

 stants.' Only in Herwig's Physikalische Begrijfe, p. 78, do I 

 find any values for intensity of a magnetic field, moment of a 

 magnet (strength of pole is involved in this, but not given 

 separately), and magnetic potential in this system. An ex- 

 amination of his method, which leads to Maxwell's results, 

 shows at once that the whole question turns on the step, or 

 steps, between current-strength and magnetic moment. Using 

 so far as possible Everett's notation, but with the subscript 

 letter e or m to distinguish the two systems when necessary, 

 we shall have to consider Strength of current C, Intensity of 

 a magnetic field I, Moment of a magnet fx or PL, Strength 

 of magnetic pole P, Quantity of electricity Q. 



re 27rr 2 3 



Herwig gives J- = — jj — ; and since C e =M^L 2 T (agree- 

 ing with Everett), I e = ^ =M^L^T" 2 . 



Then, since a magnet whose moment fi in the field I is subject 

 to a couple, I e ^ e = a couple, 



~ or P e L= M 1 L 7~', =M*L*; 

 ' M*L*T" 2 



, r work MLfT" 2 -,, T ,__, 



v e — -p— = ]vi>-ri = = ma S uetlc potential. 



* Communicated bv the Author. 



