CHAP, viii.] ELECTRICITY AND MAGNETISM. 345 



+ Bismuth . . . . 89 to 97 



German-silver . . . n*75 



Lead . . . o 



Platinum . . . 0*9 



Zinc . . . 37 



Copper . . . - 3-8 



Iron . . . . 17*5 



Antimony . . . . 22 '6 to 26*4 

 A very small amount of impurity may make a great 

 difference in the thermo-electric power of a metal, and 

 some alloys, and some of the metallic sulphides, as 

 galena, exhibit extreme thermo-electric power. 



The electromotive -forces due to heating single pairs 

 of metals are very small indeed. If the junction of a 

 copper- iron pair be raised iC above the rest of the 

 circuit its electromotive-force is only 13-7 millionths of a 

 volt (i.e. 13-7 microvolts). That of the more powerful 

 bismuth-antimony pair is for iC, about 117 microvolts. 



382. Thermo-electric Inversion. Gumming dis- 

 covered that in the case of iron and other metals an 

 inversion of their thermo-electric properties may take 

 place at a high temperature. In the case of the copper- 

 iron pair the temperature of 280 is a neutral point ; 

 below that temperature the current flows through the 

 hotter junction from the copper to the iron ; but when 

 the circuit is above that temperature iron is thermo- 

 electrically positive to copper. 



383. Thermo-electric Diagram. The facts of 

 thermo-electricity are best studied by means of the 

 diagrams suggested by Sir W. Thomson and constructed 

 by Professor Tait. In that given in Fig. 143, the 

 horizontal divisions represent the temperatures, the 

 vertical distances indicating the differences of potential 

 on a scale of millionths of volts. These differences are 

 measured with respect to the metal lead, which is 

 taken as the standard of zero at all temperatures, because, 

 while with other metals there appears to be a difference 

 of potentials between the metal hot and the same metal 



