the Absurdity of Diamagnetic Polarity. 257 



But, if we retain only the most important terms, 



-(—+—) - - $ , l l v + T~T \\ 



■I VPB 3 + YA°) 2 \(r-~ cos 6) (r + 1 cos 6) ) 



mli 



/ t \ 3 



(r 2 -jcos a 0) 



w 



P0 (A " raO = "" {(-|co S ^) 3 (-+£»*)'} 



3ml?' 



H«rf«) 



cos #. 



Thus if we can neglect — ^, as we certainly can when AB is 

 comparable to the size of a molecule, the action on P reduces 



7 Q 7 



to — parallel to BA and — r cos 6 along OP. These forces 



are exactly the same as would have been produced by another 

 short magnet similar to AB, placed along AB with its centre 

 at 0, provided that m'V = ml. Defining ml to be the magnetic 

 moment of the elementary magnet AB, we see that two ele- 

 mentary magnets placed at the same point with their axes 

 coincident are equivalent to each other if their moments are 

 equal. 



If we take three rectangular axes Ox, Oy, Oz through 

 and denote the coordinates of P by (x, y, z) and the angles 

 AB makes with the axes by (ex., j3, 7), the forces exerted on P 

 by three small magnets placed at : — 



712 1 



ml cos ol along Ox, are 3 cos a parallel to Ox and 



3ml cos a. x , ^-p, 

 5 - along UP ; 



r r ° 



17 il 



ml cos /3 along Oy, are w cos /S parallel to Oy and 



3ml cos 8 y , ~^ 

 ^— - J - along OP ; 



