and on the Mechanical Value of Heat. 437 



count of the sparks* at the commutator, we have a total of 

 2°*46. Now in order to refer this to the capacity of a lb. of 

 water, I found — 



lbs. lbs. 



Weight of glass tube = 1*65 ss capacity for heat of 0*300 of water. 



Weight of water = 0-61= 0610 



Weight of electro-magnet = 1-67 = ... ... 0-204 



Total weight ... = 3-93= T\\i ... 



2°*46 x 1*114 = 2°*74; and this has been obtained by the 

 power which can raise 4 lbs. 12 oz. to the perpendicular height 

 of 517 feet. 



1° of heat per lb. of water is therefore equivalent to a me- 

 chanical force capable of raising a weight of 896 lbs. to the 

 perpendicular height of one foot. 



Two other experiments, conducted precisely in the same 

 manner, gave a degree of heat to mechanical forces repre- 

 sented respectively by 1001 lbs. and 1040 lbs. 



I now made an experiment similar to those of series No. 10. 

 Eight cells in a series of four double pairs were connected with 

 the large electro-magnet, and two in series with the small re- 

 volving electro-magnet. The velocity of revolution was at the 

 rate of 640 per minute, contrary to the direction of the attrac- 

 tive forces, causing the needle to be deflected to 37° 20', which 

 indicates a current of 1*955. 



A weight of 6 lbs. 4 oz. placed in each scale was just able 

 to maintain the above velocity when the circuits were com- 

 plete ; but when they were broken, and friction alone opposed 

 the motion, a weight of 2 lbs. 8 oz. only was required, which 

 is less than the former by 3 lbs. 12 oz. The fall of the weights 

 was in this instance 551 feet per 15 minutes. 



According to series 10, Table II., the heat due to the cur- 



(T955\ 2 

 ■ ) x 5 0, 88 



= 6°'6. But I had found by calculations, based as usual 

 upon the laws of Ohm, that, in the present experiment, the 

 resistance of the coil of the revolving electro-magnet was to 

 that of the whole circuit, including the two cells, as 1 : 1*303. 

 Therefore the heat evolved by the whole circuit, including 

 0°*18 on account of the iron of the revolving electro-magnet, 

 and 0°*12 on account of sparks at the commutator, was 8°*9, 

 or 9°*92 per capacity of a lb. of water. 



Now when the revolving electro-magnet was stationary, the 

 two cells could pass through it an uniform current of 1*483. 



* The heat evolved by sparks in the above and subsequent instances had 

 been determined by previous experiments. 



