1897.] 



on Contact Electricity of Metals 



535 



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 Yolta-difference between two metallic sur- 

 faces hitherto measured is, I believe, 3 • 56 volts, which Murray, in 

 another part of his work, found as the Volta-diflference 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 iodised 

 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 conditions 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 



K 



Fig. 9. 



represented in Fig. 9, with its hot junction at 25° and its cold at 15°, 

 the 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 tempe- 

 ratures, 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 connection with a delicate quadrant elec- 

 trometer. But the test would be difficult, because of the difficulty of 

 preparing the opposed surfaces of two equal and similar discs, so as 

 to make them equal in their surface- Volta-potentials within one 

 one-thousandth of a volt, or even to make their difference of potentials 

 constant during the time of experiment within one one-thousandth of 

 a volt. There would, however, be no interest in making the experi- 

 ment in this way, because by the electromagnetic method we can 

 with ease exhibit and measure with great accuracy the difference of 

 potentials between A and B, by keeping them exactly at one tempe- 



2 N 2 



