THERMO-DYNAMICS. 



41 



of copper wire to rotate between the poles of a large mag- 

 net, and the consequence is that electricity is generated in 

 these coils, and the electricity can be conducted along these 

 wires, and can be exhibited by heat in a piece of platinum 

 wire. You see that there is a considerable quantity of 

 electricity generated by the performance of mechanical 

 work. But if I increase the rapidity of rotation so as 



to melt the platinum wire, as soon as the wire is melted 

 the machine almost runs away, because then I have taken 

 away the resistance which was producing this heat ; I am 

 no longer producing an electrical current, and consequently 

 1 have not to spend so much force in turning the machine. 

 That difference in the mechanical exertion which I was 

 using was indicated by the amount of heat which was 



