Blectrfc Generators. 75 



great difference of potential are commonly 

 used. The use made of thermal currents is 

 chiefly for determining slight differences of 

 temperature. An apparatus called the thermo- 

 electric pile has been constructed out of a 

 great number of pairs of antimony and bis- 

 muth bars. This instrument in connection 

 with a galvanometer makes a most delicate 

 means of determining slight changes of 

 temperature. If one face of a thermopile is 

 exposed to a temperature greater than its own, 

 the needle will move in one direction; if to 

 a temperature lower than its own, the needle 

 will be deflected in the opposite direction. If 

 both faces of the pile are exposed to the same 

 changes of temperature simultaneously, of 

 course no electrical manifestations will occur. 

 The earth is undoubtedly a great thermal 

 battery that is kept in action by the constant 

 ehanges of temperature going on at the earth's 

 surface, caused by its rotation every twenty- 

 four hours on its axis. The sun, of course, is 

 at some point heating the earth, which at other 

 I if lints is cooling, making a constant change 

 of potential between different points. If we 



.1 metal ring at one point a current of 

 eli Ttriritv will flow around it especially if it 



i'le of two <li <-imilar metals until the 



heat is equally distributed throughout the ring. 



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Company first began operations between New 



