EXPERIMENT IN MAGNETO-ELECTRIC INDUCTION. 



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

 place by buffers fore and oft 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 

 tin- 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 depending on the displacement than when 

 the body is perfectly free. 



The electricity in the primary coil when it is closed corresponds 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 primaiy coil is interrupted 

 by a condenser, the electricity is resisted with a force proportional to the 

 accumulation, 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 mathe- 

 matical 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 machine, N, IS 

 the poles of the magnets, x the current led through the coil of the electro- 

 magnet R, and interrupted by the condenser C. Let the plates of the condenser 

 be connected by the additional conductor y. 



