512 COBLENTZ: MODIFICATIONS OF THERMOPILES 



reduced and the time to attain temperature equilibrium., after 

 exposing the receiver to radiation, was shortened so that a 

 maximum galvanometer deflection is attained in three to live 

 seconds. 



It is shown that the radiation sensitivity of a composite re- 

 ceiving surface is proportional to the square root of the area of 

 the exposed surface. However, this relation does not hold true 

 for the single receivers attached to the individual thermo-j unc- 

 tions. In the latter there is an optimum size of the receiver 

 required to compensate for the loss of heat by conduction along 

 the wires, and by radiation from the surface. Using thermo- 

 elements of bismuth wire 0.1 mm. in diameter, and silver wire 

 0.036 mm. in diameter, this optimum size of receiver is of the 

 order of 1.5 by 1.5 mm. 



In the tests on the relation of external resistance to the internal 

 resistance of the gahanometer it is shown that there may be a 

 considerable departure from equality of the two resistances (the 

 galvonometer resistai.ce may be two to three times the internal 

 (thermopile) resistance) without seriously affecting the efficiency 

 of the galvanometer. 



Tests on various samples of bismuth wdre, show that the 

 thermoelectric power, against silver, varies from 75 to 80 micro- 

 volts. 



The construction and test of sensitivity of a thermopile of 

 bismuth alloy is given. The alloy was bismuth + 5 per cent tin. 

 Wires of alloy when joined with pure bismuth wire had a thermo- 

 electric power of 127 microvolts. In spite of this high intrinsic 

 thermoelectric power, the radiation sensitivity of the completed 

 thermopile was not any higher than that of the thermopiles 

 constructed of bismuth and silver, which elements have a 55 

 per cent lower therm.oelectric power. This is due to the high 

 resistance of the alloy. 



A thermopile constructed of bismuth and iron, which, like 

 the above bismuth tin alloy has a higher thermoelectric power, 

 was no more sensitive than the thermopiles constructed of bis- 

 muth and silver. The conclusion arrived at is that the pro- 

 duction of a thermopile having a high radiation sensitivity is 



