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RADIATION BIOLOGY 



flux, or to a thermal sink consisting of a relatively large mass of metal, 

 which may be the housing. Compensating thermocouples (or thermo- 

 piles) are commonly made of two complete couples connected so that the 

 generated voltages oppose one another. In this case the cold junctions 

 are connected to thermal sinks. The thermopile is composed of two or 

 more thermocouples arranged in series or series parallel to increase the 

 receiver area and generated voltage. 



When a circuit is composed of two dissimilar metals, a contact potential 

 is present at each of the two junctions. If the two junctions are of the 

 same temperature, the contact potentials are ecjual and opposite, and no 

 current flows in the circuit. If, however, the junctions are at different 

 temperatures, a current will flow which is proportional to (1) the temper- 

 ature difference AT and (2) the algebraic sum of the thermoelectric powers 

 of the two metals Q and inversely proportional to (3) the resistance of the 

 circuit R. Thus / = KNQ AT/R, where N is the number of pairs of 

 junctions in the circuit (N is 1 for a thermocouple). The ideal pair of 

 thermoelectric materials has a high thermoelectric power Q and a low 

 specific resistance a. Of the many combinations of metals available for 

 radiation thermocouples, copper or manganin with constantan, and bis- 

 muth with bismuth-tin alloys probably have been used most extensively. 

 The construction of fast thermocouples and thermopiles with fine wires 

 has been described by many authors (Harris, 1946; Hornig and O'Keefe, 

 1947; Roess and Dacus, 1945). Certain semiconductors involving alloys 

 of tellurium in combination with bismuth offer the highest thermoelectric 



Table 3-16. Comparative Performance of Thermocouples and Bolometers 



