CHAP. VIIL] ELECTRICITY AND MAGNETISM. 347 



the electromotive-force of thermo-electric pairs it is usual 

 to join a number of pairs of metals (preferably bismuth 

 and antimony) in series, but so bent that the alternate 

 junctions can be heated as shown in Fig. 144. at B B B, 



Fig. 144- 



whilst the other set A A A are kept cool. The various 

 electromotive-forces then all act in the same direction, 

 and the current is increased in proportion to the number 

 of pairs of junctions. Powerful thermo-electric batteries 

 have been made by Clamond, an iron-galena battery 

 of 1 20 pairs affording a strong current; but it is 

 extremely difficult to maintain them in effective action 

 for long, as they fail after continued use, probably 

 owing to a permanent molecular change at the junctions. 

 In the hands of Melloni the thermo-electric pile or 

 thermopile, constructed of many small pairs of anti- 

 mony and bismuth united in a compact form, proved an 

 excellent electrical thermometer when used in conjunction 

 with a sensitive short-coil astatic galvanometer like that 

 of Fig. 88. For the detection of excessively small 

 differences of temperature the thermopile is an invaluable 

 instrument, the currents being proportional to the differ- 



