144 



BIOLOGICAL EFFECTS OF RADIATION 



THERMOCOUPLE TECHNIQUE 



The highest sensitivity available in present practice is obtained by- 

 using bismuth for one lead, and bismuth with 4.5 to 5 per cent of tin 

 alloy for the other. In the most sensitive forms, the thermocouple is 

 placed in an evacuated space. It is then possible to measure rises in 

 temperature of the order of 10"^ degree. With such small rises in 

 temperature, the influence of surrounding bodies is of paramount impor- 



Rece/ver 



Replacement 

 um'f 



^'/ fn/arged section of 

 receiver assembly 



Fig. 5. — Housing for vacuum thermocouple. 



tance. It is necessary, therefore, that the thermocouple be protected 

 from radiation disturbances, as well as from convection. This may be 

 accomplished in the following way: The blackened receiver is placed at 

 the center of a surrounding shield whose temperature is that of the cold 

 junction. An example of such a thermocouple housing is illustrated in 

 Fig. 5. The thermocouple is constructed upon a convenient replacement 

 vmit C. Two thin pieces of copper are joined by a glass bead, extensions 

 projecting toward the center of each unit. Upon these projections are 

 welded the fine thermocouple leads. The wires are so selected as to 

 yield a resistance of from 10 to 30 ohms. For spectroscopic purposes, the 

 receiver is about 0.1 mm. wide and 4 mm. long. One side is blackened 

 to receive the radiation and the other left bright to reduce the loss by 



