MKMOIH XXIV. J T1IK KLECTRO-MOTIVE POWER OF HEAT. 305 



given temperature is independent of the amount of In-at 

 ed surface-, a mere point being just as efficacious as an 

 indefinitely extended surface. 



4th. That the quantity of electricity evolved in a pile 

 of pairs is directly proportional to the number of the 

 elements. 



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

 velopment with the rise of temperature in the case of 

 pairs of different metals. 



The experimental arrangement which I have em- 

 ployed is represented 

 in Fig. 52. A A is a 

 glass vessel, about 

 three inches in diame- 

 ter, with a wide neck, 

 through which can be 



O 



inserted a mercurial 

 thermometer, />, and 

 one extremity of a pair 

 of electro-motoric wires. The wires I have employed 

 have generally been a foot long and one sixteenth of an 

 inch in diameter. The extremities, S, of the wires thus 

 introduced into the vessel ought to be soldered with hard 

 solder; their free extremities dip into the glass cups r/, 

 (I, filled with mercury, and immersed in a trough, e t con- 

 taining water and pounded ice. By means of the cop- 

 per wires//, one sixth of an inch thick, communication 

 is established with the mercury-cups of the galvanom- 

 eter. The coil of this galvanometer is of copper wire 

 one eighth of an inch thick, and making twelve turns 

 only round the needles, which are astatic. The devi- 

 ations were determined by the torsion of a glass thread. 

 It is surprising to those who have never before seen 

 the experiment with what promptitude and accuracy a 



Pig. 62. 



