Dr. Draper on the Electro-motive Potaer of Heat. 455 



increments of temperature throughout the range of the mer- 

 curial thermometer. 



Let us now proceed to the second proposition, " That the 

 tension undergoes a slight increase with increase of tempera- 

 ture, a phaenomenon due to the increased resistance to con- 

 duction of metals when their temperature rises." 



It will be seen, on consulting the following table, that pairs 

 of differejit metals, at the same temperature, have tensions 

 which are apparently very different. 



The currents, the tensions of which are here indicated, were 

 generated by keeping one end of the thermal pair in boiling 

 water, the other ends being maintained at a temperature of 

 32° Fahr. 



Table III. 



A pair of 



Tension. 



A pair of 



Tension. 



Antimony and bismuth 



Copper and iron 



Silver and lead 



•137 

 •183 

 •307 

 •313 

 •380 



Platina and iron 



Copper and platina 

 Platina and palladium 

 Tin and iron 



•470 

 •473 

 •500 

 •518 

 •567 



Lead and palladium... 

 Silver and platina 



Platina and tin 



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 1 000 degrees, and then abstract 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 temperature being still so ad- 

 justed 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. 



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 

 temperatures, 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 transparent to light, and rock salt as trans- 

 parent to heat, so too we might regard a copper wire or any 

 conducting medium as transparent to electricity. 



