94 



INCREASE OF TENSION WITH INCREASE OF TEMPERATURE. 



352. It will be seen, on consulting the following table, that pairs of different metals, 

 at the same temperature, have tensions which are apparently very different. 



353. The currents, the tensions of which are here indicated, were generated by keep- 

 ing one end of the thermal pair in boiling water, the other ends being maintained at a 

 temperature of 32 Fah. 



TABLE III. 



We perceive, therefore, that there apparently exist specific differences in the qualities 

 of electric currents derived from different sources. If, for example, we take a pair of 

 platina and palladium, and expose it to a temperature which shall generate a current 

 capable of deflecting the torsion balance through 1000 degrees, and then obstruct it by 

 a wire of such dimensions as to stop one half, or only allow 500 degrees to pass, and 

 repeat the experiment with a current generated by bismuth and antimony, the tempera- 

 ture being still so adjusted as to give a deflection of 1000 degrees, on making this pass 

 through the same intercepting wire, perhaps not much more than one eighth of it will 

 go through the galvanometer. 



354. It might be supposed that these characteristic differences of thermal currents, 

 derived from different sources, were due to some modification of the electricity itself, 

 similar to those of radiant heat derived from different sources, or at different tempera- 

 tures, which M. MELLONI has attempted to show are analogous to the colours of light, 

 being, like them, of different degrees of refrangibility, and permeating absorbent media 

 with different degrees of facility. For in the same way that we regard glass as trans- 

 parent to light, and rock salt as transparent to heat, so, too, we might regard a copper 

 wire or any conducting medium as transparent to electricity. 



355. 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 generate 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 cur- 

 rent from antimony and bismuth had a low tension, because thick bars of those sub- 

 stances were employed. In the former case, the conducting resistance was greater than 

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



356. That this is strictly true will appear on examining the current evolved by any 

 number of systems under the same condition of resistance to condition. I took a pair 

 of copper and iron, and soldered it to a simple pair of platina and copper, as is shown 

 in fig. 34, so as to form one continuous metallic line. The point of junction formed 

 by the wires i (iron) and p (platina) was kept carefully at 63 Fah., by immersion in 

 a water-bath ; the point of junction, p (platina) and c (copper), was treated in like 

 manner, but that of e and i was raised to 212 Fah. Under these circumstances, it was 

 found that 181 degrees of electricity were evolved, of which fifty went through a given 



