46 PROCEEDINGS OF THE AMERICAN ACADEMY 



properties of the metals change, when the metals are under the 

 influence of heat and a current of electricity. 



The thermo-electric current depends essentially on the differences of 

 the two metals. The slightest change in the structure or the composi- 

 tion of the metals makes a perceptible change in the current. There- 

 fore, to determine the variations of the electric current with the 

 temperature, it is necessary to know the variations of the metals with 

 the temperature. A junction of iron and silver might be expected to 

 give twice as strong a current at 100° as at 50°. This might be the 

 case if the iron and silver were exactly the same at 100° as at 50° ; 

 but iron at 100^ is a different metal from iron at 50° : the thermal con- 

 ductivity, the electric conductivity, the specific gravity, and many other 

 properties have changed. It may be due to this fact — that the prop- 

 erties of the metals change with the temperature — that the thermo- 

 electric lines are not straight. 



