436 Mr. Joule on the Calorific Effects of Magneto-Electricity > 



easily ascertain the force necessary to move the apparatus at any 

 given velocity ; for, having given in the first instance the re- 

 quired velocity with the hand, it was easily observed, in the 

 course of about 40 revolutions of the axle, corresponding to 



Fig. 8. 



r 



about 270 revolutions of the revolving piece,whether the weights 

 placed in the scales were just able to maintain that velocity. 



The experiments selected for repetition first were those of 

 series No. 2. Ten cells, in a series of five double pairs, were 

 connected with the large electro-magnet ; and the small com- 

 pound electro-magnet (restored to its place in the centre of 

 the revolving tube) was connected, through the commutator, 

 with the galvanometer. Under these circumstances a velocity 

 of 600 revolutions per minute was found to produce a steady 

 deflection of the needle to 24° 15', indicating 0*983 of current 

 magneto-electricity. 



To maintain the velocity of 600 per minute, 5 lbs. 3 oz. had 

 to be placed in each scale; but when the battery was thrown 

 out of communication with the electro-magnet, and the motion 

 was opposed solely by friction and the resistance of the air, 

 only 2 lbs. 13 oz. were required for the same purpose. The 

 difference, 2 lbs. 6 oz., represents the force spent during the 

 connexion of the battery with the electro-magnet in over- 

 coming magnetic attractions and repulsions. The perpendi- 

 cular descent of the weights was at the rate of 517 feet per 

 15 minutes. 



According to series No. 2, Table I., the heat due to 0*983 



(983\ 2 

 ) X 1 0, 56 = 1°*85. 



But as the resistance of the coil of the revolving electro-mag- 

 net was to that of the whole circuit as 1 : 1*13, the heat 

 evolved by the whole conducting circuit was 1°*85 x 1*13 

 = 2 o, 09. Adding to this 0°*33 on account of the heat evolved 

 by the iron of the revolving electro-magnet, and o, 04 on ac- 



