162 LAWRENCE R. PROUTY AND JAMES D. HARDY 



5. Limitations of Resistance Thermometers 



Resistance thermometers have the disadvantage of being much 

 more difficult to construct than thermocouples for ordinary biophysi- 

 cal measurement. Also, it is not generally feasible to construct 

 resistance thermometers of the small size attainable with thermo- 

 couples. This fact limits their use for the type of measurement re- 

 quiring very small, sensitive elements as, for example, measurements 

 of intravascular temperature. The introduction of the thermistor 

 may permit such measurements although as yet they have not been 

 so used. Resistance thermometers require more elaborate physical 

 apparatus than any of the other temperature measuring methods 

 described. For effective use, they also require a higher degree of 

 manipulative skill. Resistance thermometers of the base metal 

 variety may change in calibration with use. Soderstrom has reported 

 that readings with his nickel resistance thermometers change as much 

 as 0.2°C. during the first year of use and within 0.02°C. during the 

 second. 



E. TEMPERATURE MEASUREMENT BY RADIATION 



1. General Principles 



For high temperature measurements, optical pyrometers can be 

 used. These instruments are essentially photometers employing 

 monochromatic light — usually red — in which the intensity of radia- 

 tion from a standard source is compared to that from the object 

 being studied. The intensity is controlled by means of absorption 

 filters or iris diaphragms interposed into the pathway of the radiation 

 from either the standard source or the radiating object. These in- 

 struments are of interest in biophysics principally in the measure- 

 ment of the heat energy output from furnaces, arc lamps, and other 

 high temperature sources to which biological material may be exposed. 



Of special interest to the biophysicist concerned with determining 

 temperature of surfaces is the radiometer. The radiometer is an in- 

 strument that converts incident radiant energy into thermal energy, 

 which can be measured quantitatively. The detecting element of a 

 radiometer is usually either a thermopile or a resistance element 

 (bolometer) connected to a suitable potentiometric or bridge circuit 

 for measurement. 



A crude radiometer can be devised by blackening the end of a 



