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LXVII. Oji the Electro-inotive Power of Heat. By John 

 W. Draper, M.D., Professor ofChemistri/ i?i the Uiiiversity 

 of New York.* 



[Illustrated by Plate V[.] 



Tj^ROM the memoir of M. Melloni, on the Polarization of 

 Heat, inserted in the second part of the first volume of 

 the Scientific Memoirs, we learn, that M. Becquerel, as well 

 as himself, has made experiments to determine the quantities 

 of electricity set in motion by known increments of heat. 

 From these experiments they conclude, that through the whole 

 range of the thermometric scale, those quantities are directly 

 proportional to each other. 



But as thermo-electric currents are now employed in a 

 variety of delicate physical investigations, and as there appears 

 to be much misconception as to their character, I propose in 

 this memoir to show^, 



1st. That equal increments of heat do not set in motion 

 equal quantities of electricity. 



2ndly. That the tension undergoes a slight increase with in- 

 crease of temperature, a phasnomenon due to the increased 

 resistance to conduction of metals, when their temperature 

 rises. 



Srdly. That the quantity of electricity evolved at any given 

 temperature, is independent of the amount of heated surface; 

 a mere point being just as efficacious as an indefinitely ex- 

 tended surface. 



4'thly. That the quantities of electricity evolved in a pile of 

 pairs, are directly proportional to the number of the elements. 



First, then, as to the comparative march of electric deve- 

 lopment, with the rise of temperature, in the case of pairs of 

 different metals. 



The experimental arrangement which I have employed, is 

 represented in fig. 1. (Plate VI.) A A is a glass vessel, about 

 three inches in diameter, with a wide neck, through which 

 can be inserted a mercurial thermometer h, and one extre- 

 mity of a pair of electro-motoric wires. The wires I have 

 employed have generally been a foot long, and xV^h of an 

 inch in diameter. The extremity s of the wires thus intro- 

 duced into the vessel, ought to be soldered with hard solder : 

 their free extremities dip into the glass cups d d, filled with 

 mercury, and immersed in a trough e containing water 

 and pounded ice. By means of the copper wires jf/^ ^ih of 

 an inch thick, communication is established with the mercury 



• Communicated by the Author. 

 2 H2 



