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XXXIV. The Theory of Magnetism and the Absurdity of 

 Diamagnetic Polarity. By J. Parker, M.A., Fellow of 

 St. Johns College, Cambridge. 



[Continued from p. 203.] 



IT appears from experiment that the properties of exerting 

 actions at a distance by a magnet are mainly situated at or 

 near the ends of the magnet. Suppose, then, that we have 

 two long magnets A, B, which may be considered to possess 

 the magnetic properties only in their ends, and let these 

 magnets be so placed that we need only take into account one 

 end of each. Also let these two ends be so far from each 

 other that they may be regarded as mathematical points 

 P, Q. Then the only magnetic forces between the two 

 magnets will be equal forces at the poles P, Q, acting along 

 the line PQ in opposite directions. 



Now let the two magnets be situated in a " vacuum " and be 

 made to undergo a reversible cycle in which the velocities 

 are constantly zero. To do this, they must be held by external 

 forces equal and opposite to gravity and to the magnetic 

 forces between P and Q. But if the equal forces between P 

 and Q be denoted by F, a repulsion being considered positive 

 and an attraction negative, the work done by F in a small 

 change of the distance PQ (=>') will be ¥dr. Hence the 

 work done on the system during the cycle by the external 

 forces is —J F dr, where the two limits are identical. This 

 must be zero, by the principles of thermodynamics, and there- 

 fore F must depend only on r, or F^=/' (r). From experi- 



ment it appears that /(»') is proportional to ~ 2 , so that if the 



force between P and Q when their distance is one centimetre 



be X dynes, the force will be -^ dynes when the distance is 

 t centimetres. r 



Now let there be any number of poles R, R/, R", . . . , 

 which may be treated as mathematical points, acted on simul- 

 taneously by P and Q. Then it is inferred from experiment, 

 supported by theory, that if the two poles P, Q repel each 

 other, the forces they exert on any one of the other poles, 

 R, will be both repulsive or both attractive ; but that if P 

 and Q attract each other, the forces they exert on any one 

 of the other poles will be one repulsive and the other attrac- 

 tive. Conversely, if P and Q both repel or both attract the 

 pole R, they will repel each other ; while if one attract and 



