Duanc — Heat Generated by Radio-active Substances. 263 



which is proportional to the square of the current and to the 

 resistance. The fifth column in the table contains the heat 

 absorbed per hour and per ampere by the tube, and it appears 

 that this quantity is independent of the current in the tube. 

 This means that the absorption of heat is proportional to the 

 intensity of the cooling current, i. e., the resistance of the tube 

 is so small that the heat generated according to Joule's law is 

 inappreciable, if the cooling is no larger than - 6 calorie per 

 hour. 



The mean value of the heat absorbed (or generated) per 

 hour and per ampere in the iron-nickel tube is 8 - 3 gram- 

 calories. I found 8*2 calories for the couple used in the earlier 

 experiments. 



It is interesting to note that the electromotive force in the 



Fig. 2. 



s 



«5 

































































1 



a 





c 



4 







- i i i i i 



-*-— 

































2 



■ a , . ■ ■- 



b 





d 























































































20 



30 

 Minutes 



surface between the iron and the nickel must be about *00055 

 volt to produce this effect. 



In order to determine the sensitiveness of the instrument I 

 sent a very small current through the iron-nickel tube, and 

 observed the change in the velocity of the bubble due to the 

 heat absorbed or generated. The curves in figure 2 represent 

 the displacements of the image of the bubble. The lines ab 

 and cd represent the bubble's natural drift. The abscissas of 

 the points b are the instants at which the electric current 

 commenced to flow through the tube, and the abscissas of the 

 points e are those at which the current was broken. For the 

 first curve the direction of the current was such as to generate 

 heat, and for the second to absorb it. It appears that the dis- 

 placement of the bubble due to the current was about the same 

 in the two cases but in opposite directions. This confirms 

 Peltier's law, and indicates that the Joule effect is negligible. 



