456 Dr. Draper on the Electro-motive Fawer of Heat. 



But this peculiarity of thermo-electric currents depends 

 on the conducting resistance of the system that generates 

 them. It is possible to give a current a higher or a lower 

 tension, by simply making use of thin or thick wires to ge- 

 nerate or to carry it. In the foregoing table the current from 

 platina and palladium had a high tension,because slender wires 

 of those metals happened to be used to generate it ; and the 

 current from antimony and bismuth had a low tension, be- 

 cause thick bars of those substances were employed. In the 

 former case, the conducting resistance was greater than in 

 the latter, and hence the tension of the current was higher. 



That this is strictly true, will appear on examining the cur- 

 rent evolved by any number of systems, under the same con- 

 dition of resistance to conduction. I took a pair of copper 

 and iron, and soldered it to a similar pair of platina and cop- 

 per, as is shown in fig. 4, so as to form one continuous me- 

 tallic line. The point of junction formed by the wires i (iron) 

 and ip (platina), was kept carefully at 63° Fahr., by immersion 

 in a water-bath : the point of junction p (platina) and c (cop- 

 per) was treated in like manner ; but that of e and i was raised 

 to 212° Fahr. Under these circumstances, it was found that 

 181 degrees of electricity were evolved, of which 50 went 

 through a given secondary wire. Then raising the junction jp 

 and c to 212° Fahr., and bringing e and i to 63° Fahr., 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 currents from different sources does not 

 differ. 



These results inform us how much the tension of a current 

 depends on the resistance 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. 



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 employing, however, a small black-lead furnace, to which 

 was adapted a covered sand-bath, into which the wires could 

 be plunged, I succeeded at last ; for with this arrangement a 

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



The experiment was made with care in the case of two 

 systems of metals: 1st, copper and platina; 2ndj copper and 

 iron. 



