260 On the Heat disengaged by Induction-currents. 



the wire of the moveable coil were in metallic connexion with 

 the ends of the axis of rotation. These were provided with 

 springs that could be connected with wires through which passed 

 the induced current produced by the rotation. The prin- 

 cipal inducing current, whose intensity was constant, always 

 passed in these experiments through the fixed external coil. 

 When the internal coil was rotating, it produced an induced 

 current in the fixed coil. It is readily understood that the 

 induced current is always in a direction such that the mu- 

 tual action of this current and of the principal inducing current 

 exerts a resistance to the rotatory motion, and that this resist- 

 ance must be overcome by the mechanical force which makes 

 the coil rotate. An expenditure of mechanical force therefore 

 accompanies the induction ; and this expenditure of mechanical 

 force, as can easily be seen, is proportional to the square of the 

 intensity of the principal current so long as the velocity of rota- 

 tion is constant. 



The experiments were made in the following way. The ends 

 of the inner coil were in the first place connected with the 

 platinum wire on which the thermoelectric pile was placed. After 

 that the interior coil was made to rotate, and when the velocity 

 of rotation was constant (forty-five turns in a second), the exterior 

 coil was connected with the voltaic battery, the rotation being 

 continued for thirty seconds ; the battery was disconnected, and 

 after a lapse of twenty seconds the deflection produced in the 

 galvanometer by the thermoelectric current was read off. This 

 deflection was a measure of the quantity of heat disengaged by 

 the induced current during thirty seconds. That being done, 

 two similar experiments were made, with the simple difference 

 that in the principal current was inserted the platinum wire 

 with its thermoelectric pile. In one of two series the internal 

 bobbin was rotating ; in the other it *was at rest ; so that in 

 one case there was induction, and in the other not. By means 

 of the two latter series a measure was obtained of the quan- 

 tity of heat disengaged in thirty seconds by the principal cur- 

 rent in the case in which there was induction, and in that in 

 which there was not. Now it was found that the principal 

 current produced the same quantity of heat whether it did or 

 did not act inductively. In one series of observations a deflec- 

 tion of 166*6 was obtained without, and of 166*9 with induc- 

 tion. The disengagement of heat in the induced circuit in 

 the same series was 49*3. Hence induction of the kind in ques- 

 tion produces an excess of heat, and this excess is exactly equal 

 to the heat which the induced current disengages. But the 

 heat disengaged by the induced current is proportional to the 

 square of the intensity of the induced current ; and this, in 



