APPENDIX C. 



Cu 



Bi 



Fig. 21. 



MISCELLANEOUS PHENOMENA. 



18. Thermo-electricity.* Seebeck's discovery. In 1821 See- 

 beck found that an electric current is produced in a circuit of two 

 metals when one of the junctions of the two metals is warmer 

 than the other. Seebeck used the arrangement shown in Fig. 2 1 . 



The ends of a bent bar of copper 

 were soldered to the ends of a 

 rod of bismuth, a magnetic needle 

 was pivoted between the bars as 

 shown in the figure, and one of 

 the junctions was heated by a 

 spirit lamp. The existence of current is indicated by the de- 

 flection of the magnetic needle, and the direction of the current 

 which is produced is shown by the arrows in Fig. 21. An 

 arrangement such as is shown in Fig. 2 1 is called a thermo-element. 



The thermopile. The electromotive force 

 of a single thermo-element seldom exceeds a 

 few thousandths of a volt, even when the two 

 junctions are at widely different temperatures. 

 A number of thermo-elements may, however, 

 be connected in series, as in Fig. 22, in which 

 AAAA are bars of one metal and BBBB are 

 bars of another metal. Junctions I, 3, 5 and 

 7 are heated, while junctions 2, 4 and 6 are kept cool, or vice 

 versa. 



The thermo-element used as a pyrometer. f When one junction 

 of a thermo-element is kept at a constant standard temperature, 



* A very good discussion of Thermo-electricity is given in Magnetism and Elec- 

 tricity for Students by H. E. Hadley, pages 359-382, Macmillan and Company, 1906. 

 | A pyrometer is a thermometer for measuring very high temperatures. 



315 



Fig. 22. 



