"Experiment in Magneto-electric Induction" 361 



of a large boat resists the efforts of a man trying to move it 

 backwards and forwards. 



The condenser resists the accumulation of electricity on its 

 surface, just as a railway-buffer resists the motion of a carriage 

 towards a fixed obstacle. 



Now let us suppose a boat floating in a viscous fluid, and 

 kept in its place by buffers fore and aft abutting against fixed 

 obstacles, or by elastic ropes attached to fixed moorings before 

 and behind. If the buffers were away, the mass of the boat 

 would not prevent a man from pulling the boat along with a long- 

 continued pull ; but if the man were to push and pull in alter- 

 nate seconds of time, he would produce very little motion of the 

 boat. The buffers will effectually prevent the man from moving 

 the boat far from its position by a steady pull ; but if he pushes 

 and pulls alternately, the period of alternation being not very 

 different from that in which the buffers would cause the boat to 

 vibrate about its position of equilibrium, then the force which 

 acts in each vibration is due, partly to the efforts of the man, 

 but chiefly to the resilience of the buffers, and the man will be 

 able to move the boat much further from its mean position than 

 he would if he had pushed and pulled at the same rate at the 

 same boat perfectly free. 



Thus, when an alternating force acts on a massive body, the 

 extent of the displacements may be much greater when the body 

 is attracted towards a position of equilibrium by a force depend- 

 ing on the displacement than when the body is perfectly free. 



The electricity in the primary coil when it is closed corre- 

 sponds to a free body resisted only on account of its motion ; 

 and in this case the current produced by an alternating force is 

 small. When the primary coil is interrupted by a condenser, 

 the electricity is resisted with a force proportional to the accu- 

 mulation, and corresponds to a body whose motion is restrained 

 by a spring ; and in this case the motion produced by a force 

 which alternates with sufficient rapidity may be much greater 

 than in the former case. I enclose the mathematical theory of 

 the experiment, and remain, 



Yours truly, 



J. Clerk Maxwell. 



Mathematical Theory of the Experiment. 



Let M be the revolving armature of the magneto-electric ma- 

 chine, N, S the poles of the magnets, x the current led through 

 the coil of the electromagnet fi, and interrupted by the con- 



