COBLENTZ: BISMUTH THERMOPILES 359 



V 



resistance. It is therefore necessary to test the material to be 

 used. 



In the following tests one end of the bismuth wire was joined 

 to silver (0.36 mm.) and the other to iron, and both were covered 

 with receivers of the same area. 



A sample of steel wire (0.036 mm. diameter) having an emf. 

 of 9 mv. when joined with bismuth has a radiation sensitivity 

 about 23 per cent higher than the other junction of bismuth sil- 

 ver. A sample of iron wire (0.36 mm. joined to bismuth) hav- 

 ing an emf. of 13.5 mv. was 28 per cent more sensitive than the 

 junction of bismuth and silver. The increase in sensitivity 

 of iron over steel was not so marked as was anticipated. 

 Since, however, it is difficult to place more than two junctions 

 per millimeter length of receiver, it will be a decided advantage 

 to use junctions of bismuth iron, when the instrument is in a 

 vacuum to avoid rusting. The resistance is kept low by joining 

 two junctions in parallel to the same receiver. 



For work in air exposed to sulphur fumes, a thermopile of bis- 

 muth-copper (or pure gold) will probably outlast iron or silver. 

 In all cases, however, the wires are given a thin coat of shellac. 



The bismuth wire cannot be rolled, but it can be easily flat- 

 tened by pressing the wire (cut to the proper length for the 

 thermopile) between plate glass. The samples tested were of 

 0.1 mm. diameter and were pressed to a width of 0.25 to 0.3 mm. 

 and a thickness of 0.04 mm. If pressed thinner the breakage is 

 increased. 



To test the radiation sensitivity a flat bismuth wire 3 mm. long 

 was welded to a similar piece of the round wire. The free ends 

 were joined to silver wires (0.036 mm.) and to these junctions 

 were attached receivers 1.5 x 1 mm. which were painted with 

 lamp black and smoked. When exposed to radiation (either 

 alternately or simultaneously) it was found that the flat wire had 

 a radiation sensitivity about 9 per cent higher than the round 

 wire. It is convenient to handle the flattened wire and therefore 

 desirable to construct the thermopile of flattened wire which 

 reduces the heat conduction. 



