produced by chatiges of Temperature. 313 



From the preceding tables, it appears that the iron is always 

 positive with platina, copper, gold, silver, &c. and, consequent- 

 ly, the rise of temperature exalts the electric effects produced 

 by its contact with the metals. For if it had been otherwise, 

 the strongest positive electricity would be furnished by the 

 joint whose temperature is the lowest, and then the current 

 would change its direction. With the copper the following ef- 

 fects take place. From 0° to 140° of temperature, the inten- 

 sity of the electro-dynamic force increases, and the same quan- 

 tity for each equal increment of temperature. From 140° this 

 increase diminishes with considerable rapidity, and at 300° it is 

 hardly sensible. This very remarkable effect leads me to sup- 

 pose that the current changes its direction. I plunged, indeed, 

 the points of junction in a flame, in order to give it a high 



