358 COBLENTZ: BISMUTH THERMOPILES 



radiations. If the unexposed junctions are joined directly to the 

 metal pins used in supporting the elements, the galvanometer 

 mirror will ''drift/' so that it is practically impossible to use such 

 an " unsymmetrical" thermopile with a sensitive galvanometer. 



The novelty in the thermopiles as constructed at the Bureau 

 of Standards is the receiver, which, like a bolometer, can be made 

 entirely opaque and can be built up so as to present a large sur- 

 face. ^ The square root law, cannot of course hold for a very 

 large receiver attached to a thermo junction. With bismuth 

 wire 0.1 mm. diameter it was found that the optimum length 

 of receiver was 2.5 to 3 mm. which, however, is sufficiently long 

 (loc. cit) to enable the construction of a large receiver by using 

 many junctions. The importance of submitting the theoretical 

 conclusions to experiment is further illustrated in connection 

 with the square root law. For example: Four receivers each 

 having exposed areas of 1 X 1 mm! are twice as sensitive as a 

 single receiver 1x4 i^j^\ Experimentally however, with bis- 

 muth wire, 0.1 mm. diameter, there is but little gain in sensi- 

 tivity by placing more than two junctions per millimeter length 

 of the composite receiver. 



In order to keep the resistance low, it was shown (loc. cit.) that 

 the best diameter of bismuth wire is 0.1 mm. The proper diam- 

 eter of silver or copper wire is 0.036 to 0.038 mm. In spite of 

 theory which would have required a wire having a larger diameter, 

 it was found that an iron wire, 0.036 mm. in diameter, joined 

 to bismuth (in these tests the receivers are the same size, loc. cit. 

 figure 2, and they are exposed alternately, or simultaneously for 

 the differential deflection) was about 9 per cent more sensitive 

 radiometrically than a similar sample 0.0418 mm. in diameter. 

 In order to still further reduce the resistance, two junctions are 

 joined in parallel, by joining them to a single receiver. These 

 pairs of junctions are then joined in series. 



It is found that the thermal emf . of various samples of bismuth 

 varies from 75 to 82 mv. per degree with but little variation in 



2 Bull. Bur. Standards 9: 7. 1912. In further references this' will be cited 

 "loc. cit." 



