by Induction-currents. 255 



the two wires. When the internal coil is in equilibrium in this 

 position, its axis is at right angles with that of the external coil. 

 If now the same current is sent through both coils, the moveable 

 one must experience a deviation in virtue of their mutual action. 

 This is determined in the usual manner by means of a mirror, a 

 telescope, and a graduated scale. If i denote the intensity of 

 the current at a given time, and dt the element of time, the devia- 

 tion must be proportional to xfidt, the limits of the integral 

 being the time comprised between the commencement and the 

 end of the current. The heat developed by this induction-cur- 

 rent must be proportional to this quantity, in case the supposition 

 we have made is correct. 



A coil consisting of several turns of silk-covered copper wire 

 was connected with a battery ; the current could be opened or 

 closed by means of a toothed wheel worked by a handle held in 

 the hand. The toothed wheel had fifty teeth, so that at each 

 complete turn the current was broken or closed one hundred times 

 by means of a spring on the side of the wheel. In this induc- 

 tion-coil another similar one was introduced, its external diameter 

 being just sufficient to permit this. The two ends of this latter 

 induction-wire could be united at will either to the dynamometer 

 or to two small perpendicular brass rods, between which was 

 stretched a thin platinum wire which the induced current could 

 traverse. To estimate the heating effect produced by the induced 

 current in the platinum wire, a small thermoelectric pile was 

 adapted to the latter, the current of which passed by means 

 of two copper wires to a mirror-galvanometer at a suitable dis- 

 tance, and which had served in previous researches. The deflec- 

 tions of the galvanometer could be read in the ordinary manner, 

 by means of a graduated scale and a telescope. The thermoelec- 

 tric pile was the same as that previously employed by the author 

 in his investigations on the development of heat produced by the 

 change of volume of solid bodies * ; in some experiments an ana- 

 logous pile was used, in which certain modifications were intro- 

 duced. The platinum wire and the pile were placed under a bell- 

 jar to preserve them from currents of air. 



In order that the induced current may produce a heating of 

 the platinum wire capable of developing an appreciable thermo- 

 electric current, the toothed wheel must be turned a great number 

 of times. When the development of heat due to the induced cur- 

 rent was to be measured, the wheel was turned with a uniform 

 velocity until the needle was at rest and exhibited a constant de- 

 flection. This was observed. It is clear that, when this state 

 has set in, the wire loses in a given time, by radiation and con- 



* Phil. Mag. S. 4. vol. xxiv. p. 329. 



