DEPENDS ON INCREASED RESISTANCE TO CONDUCTION. 95 



secondary wire. Then, raising the junction p and c to 212* Fah., and bringing e and i 

 to 63 Fah., there passed at the galvanometer 71 degrees, of which 19 could traverse the 

 same secondary wire, but 



As 181 : 50 :: 71 : 19-6; 



and hence I infer that, where the conducting resistance is the same, the tension of cur- 

 rents from different sources does not differ. 



357. These results inform us how much the tension of a current depends on the re- 

 sistance to conduction of the system which it traverses, as well as on the dimensions 

 of the system itself; an observation the value of which we shall presently see. 



358. In a great number of trials which I made, I failed in getting any trustworthy 

 results as respects tension of currents at high temperatures, on account of the difficulty 

 of maintaining the thermo-electric pair at the same degree without variation. By em- 

 ploying, however, a small blacklead furnace, to which was adapted a covered sand- 

 bath, into which the wires could be plunged, I succeeded at last ; for with this arrange- 

 ment a regulated temperature could be kept up for a length of time. 



359. The experiment was made with care in the case of two systems of metals : 

 1st, copper and platina ; 2d, copper and iron. 



1st. At the boiling point of water, a pair of copper and platina, the unexcited ex- 

 tremity of which was carefully maintained at 67 Fah., evolved as a mean of four trials, 

 three of which were absolutely identical, 123 degrees of electricity, of which 23 could 

 pass a secondary wire. 



Then, by the aid of the furnace and sand-bath, the temperature was raised until the pair 

 evolved 783 degrees as a mean of four trials ; of these 163 could pass the secondary wire. 

 Now, As 783 : 163 : : 123 : 25| instead of 23, 



showing, therefore, a slight rise of tension. 



360. 2d. The pair of copper and iron gave, at the boiling point of water, 300 degrees, 

 of which 57 passed the secondary wire. The temperature was now raised, with the 

 following results : 



490 degrees passing the primary, 95 the secondary wire. 

 553 " " " 113 " " 



545 " " " 112 " " 



493 " " " 110 " " 



It will be understood, that although the quantities of electricity indicated in the first 

 column do not regularly increase, the temperatures were, notwithstanding, going regu- 

 larly upward ; to this peculiarity of the systems into which iron enters I have already 

 alluded (348). Let us now compare these measures with those obtained for the boiling 

 point of water : 



As 490 : 95 : : 300 : 58 instead of 57. 

 553 : 113 : : 300 : 61 

 545 : 112 : : 300 : 61 

 493 : 110 : : 300 : 67 



We find, therefore, that in the case of both these systems of metals, the tension slowly 

 rises with increase of temperature, being much better marked in the latter than in the 

 former instance. 



