264 



Mr. Joule on the Calorific Effects 



Part I. — On the Calorific Effects of Magneto-Electricity. 



The general plan which I proposed to adopt in my experi- 

 ments under this head, was to revolve a small compound 

 electro-magnet, immersed in a glass vessel containing water, 

 between the poles of a powerful magnet; to measure the elec- 

 tricity thence arising by an accurate galvanometer ; and to 

 ascertain the calorific effect of the coil of the electro-magnet 

 by the change of temperature in the water surrounding it. 



The revolving electro-magnet was constructed in the fol- 

 lowing manner: — Six plates of annealed hoop-iron, each eight 

 inches long, 1| inch broad, and y^th of an inch thick, were 

 insulated from each other by slips of oiled paper, and then 

 bound tightly together by a ribbon of oiled silk. Twenty-one 

 yards of copper wire y^th of an inch thick, well covered with 

 silk, were wound on the bundle of insulated iron plates, from 

 one end of it to the other and back again, so that both of the 

 terminals were at the same end. 



Having next provided a glass tube sealed at one end, the 

 length of which was 8| inches, the exterior diameter 2*33 

 inches, and the thickness 0*2 of an inch, I fastened it in a round 

 hole, cut out of the centre of the wooden revolving piece a, 

 fig. 1. The glass was then covered with tinfoil, excepting a 



narrow slip in the direction of its length, which was left in 

 order to interrupt magneto-electrical currents in the tinfoil 

 during the experiments. Over the tinfoil small cylindrical 

 sticks of wood were placed at intervals of about an inch, and 

 over these again a strip of flannel was tightly bound, so as to 



