CONTRIBUTIONS TO SCIENCE. 553 



being constantly accelerated, it will presently revolve with 

 a velocity half that of P, and then D will roll round B, 

 which, with S, will remain at rest. The piece B S will 

 then continue at rest as long as the rotation of P remains 

 constant corresponding to the cessation of the current in the 

 secondary circuit, while that in the primary remains un- 

 changed, but if P be accelerated, S will revolve in the 

 direction opposite to the motion of P. Now suppose P to 

 be suddenly stopped. The kinetic energy of the cross will 

 cause it to continue to revolve until it has done a correspond- 

 ing amount of work against resistances, and A being at rest, D 

 will roll upon it and compel B, with S, to revolve in the same 

 direction as the cross, that is, in the same direction in which 

 P formerly revolved, and whatever be the resistance to the 

 motion of S, it will be overcome, and S will revolve till the 

 work done against resistance is equal to the kinetic energy 

 originally possessed by the cross. This corresponds to a 

 current induced in the secondary coil on stopping the current 

 in the primary, which current is in the same direction as 

 the primary current, and continues until the energy of 

 rotation of the molecular vortices has been used up in work 

 done against electrical resistance. 



If one operator lay hold of the wheel S, and endeavour 

 to keep it at rest while another applies a steady force to P, 

 the motion of P will be accelerated much less rapidly than 

 if the same force had been applied to it, and S had been 

 free, because P can only move by setting in motion the 

 cross with its great moment of inertia. If the operator who 

 is turning P now suddenly stops it, a great shock will be 

 experienced by the machinery, and the wheel S will slip 

 from the grip of the other operator however firmly he may 

 hold it. The force applied to S may correspond to an air- 

 break in the secondary coil, and this is sufficient to prevent 

 a spark when the battery current is started in the primary, 

 but by suddenly stopping the primary current, as in 

 KuhmkorfFs coil, a disruptive discharge or spark passes 

 through the air between the terminations of the secondary 

 wire. (If the operator who endeavours to keep the wheel 



