THE ELECTRIC LIGHT 451 



other, and between their poles a Siemens armature. The 

 two ends of the wire which surrounds the armature are 

 now disconnected. In turning the handle and causing 

 the armature to rotate, I simply overcome ordinary me- 

 chanical friction. But the two ends of the armature coil 

 can be united in a moment, and when this is done I imme- 

 diately experience a greatly increased resistance to rota- 

 tion. Something over and above the ordinary friction of 

 the machine is now to be overcome, and by the expendi- 

 ture of an additional amount of muscular force I am able 

 to overcome it. The excess of labor thus thrown upon 

 my arm has its exact equivalent in the electric currents 

 generated, and the heat produced by their subsidence in 

 the coil of the armature. A portion of this heat may be 

 rendered visible by connecting the two ends of the coil 

 with a thin platinum wire. When the handle of the ma- 

 chine is rapidly turned the wire glows, first with a red 

 heat, then with a white heat, and finally with the heat of 

 fusion. The moment the wire melts, the circuit round the 

 armature is broken, an instant relief from the labor thrown 

 upon the arm being the consequence. Clearly realise the 

 equivalent of the heat here developed. During the period 

 of turning the machine a certain amount of combustible 

 substance was oxidized or burned in the muscles of my 

 arm. Had it done no external work, the matter consumed 

 would have produced a definite amount of heat. Now, the 

 muscular heat actually developed during the rotation of 

 the machine fell short of this definite amount, the missing 

 heat being reproduced to the last fraction in the glowing 

 platinum wire and the other parts of the machine. Here, 

 then, the electric current intervenes between my muscles 

 and the generated heat, exactly as it did a moment ago 



