the Absurdity of Diamagnetic Polarity. 199 



forming the experiment described above, the assumed repul- 

 sive force between the steel and the bismuth may be so 

 affected by the rapidity of the motion that the work required 

 to force the bismuth from the position X to the position Y 

 may be > W, and not less, as we supposed. 



Dr. Lodge's objection may be illustrated very simply as 

 follows : — Let a quantity of gas be contained in a cylinder 

 fitted with a smooth air-tight piston, and suppose it to be 

 provided that the cylinder can absorb or give oat heat freely, 

 but only at some constant temperature 0. When the whole 

 is in equilibrium at the uniform temperature 6, let the piston 

 be pushed in so slowly that the system is constantly in equi- 

 librium, and the temperature always uniform and equal to 0. 

 The work required for this purpose may be called W. Then 

 let the piston be drawn out slowly until the original state is 

 attained. The work restored will be W, and, on the whole, 

 there will be neither loss nor gain of work. 



Next, let the piston be pushed in as much as before, but 

 far more rapidly. The work expended on the system in 

 doing this will be >W. Now let the piston remain in its 

 new position until the temperature has everywhere sunk to 

 6. Then let the piston be rapidly drawn out to its original 

 place, and then wait until the original state is brought about. 

 The work restored by the system will be < W, so that there 

 will have been a loss of mechanical work in the cycle. 



Dr. Lodge's objection may be obviated by means of two 

 assumptions : — 



(1) We assume that the magnetization of a body at rest 

 may be in a state of unstable equilibrium. For example, in 

 the experiment with the piece of bismuth, we assume that 

 when the bismuth is at rest in any position X, the repulsive 

 force of the steel magnet may have a value / which does not 

 properly belong to the position X, and that this force may 

 continue unchanged so long as the bismuth is held steadily 

 enough in the position X. We further assume that if the 

 bismuth be moved slowly from the position X, and then 

 slowly brought back again, the repulsive force of the steel 

 magnet will again be /, if sufficient care has been taken to 

 move the bismuth about steadily. When the magnetization 

 of a body at rest is in a state of unstable equilibrium, a slight 

 shake, by which no perceptible amount of mechanical work is 

 done, will be sufficient to cause the magnetization to rise or 

 sink to its proper value. 



(2) We also assume that if two magnetized bodies be 

 moved about with any velocities, the attraction or repulsion 

 between them when they are passing through any given 



