98 



Lord Kelvin on 



kind as, bnt more permanent than, those which he had found 

 to be produced by violent scratching with emery- paper. 



§ 18. It is interesting to remark that Murray's most 

 highly burnished zinc differed from his emery-polished copper 

 {a) by 1-13 volts. This is considerably greater, I believe, 

 than the highest hitherto recorded Volta-difference between 

 pure metallic surfaces of zinc and copper. 



By far the greatest Volta-difference between two metallic 

 surfaces hitherto measured is, I believe, 3*56* volts, which 

 Murray, in another part of his work, found as the Volta- 

 difference between bright sodium protected by glass and his 

 standard gold. He had previously found a copper surface 

 after exposure to iodine vapour to be —'34 relatively to his 

 standard gold. The difference between this iodized surface 

 and the bright metallic surface of sodium was therefore 3' 90 

 volts : which is the highest dry Volta-electromotive force 

 hitherto known. 



§ 19. Seebeck's great discovery of thermoelectricity (1821) 

 was a very important illustration and extension of the twenty 

 years' earlier discovery of the contact electricity of dry metals 

 by Volta. It proved independently of all disturbing con- 

 ditions that the difference of potentials between two metals in 

 contact varies with the temperature of the junction. Thus, 

 for instance, in the copper-iron arrangement represented in 

 tig. 9, with its hot junction at 25° and its cold at 15°, the 



Fig. 9. 



J 



mo 



i 



15° 



Copper A iB Copper 



K 



electromotive force tends to produce current from copper 

 to iron through hot, and its amount is '00148 volt : that is to 

 say, if the circuit is broken at A B the two opposed faces 

 A, B, at equal temperatures, present a difference of electric 

 potential of *00148 volt, with B positive relatively to A. This 

 is not too small a difference to be tested directly by the Volta- 

 static method, worked by two exactly similar metal discs 

 connected to A and B, when they are at their shortest distance 

 from one another, and then disconnected from A and B, and 

 separated and tested by connexion with a delicate quadrant 



