456 Dr. Draper on the Electro-motive Povoer 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 w’as 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 p (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 p 
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, how^ever, 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; 2nd, copper and 
iron. 
