160 LAWRENCE R. PROUTY AND JAMES D. HARDY 



ferences smaller than 0.001 °C., it is unnecessary to make tempera- 

 ture corrections for the bridge coils, the resistance of which varies 

 slightly with room temperature. 



3. Calibration of Resistance Thermometers 



Calibration of the thermometer at fixed points is laborious and for 

 most purposes it is sufficient to compare the resistance thermometer 

 to a calibrated mercury thermometer in a stirred bath, the tempera- 

 ture of which is varied slowly in steps over the desired range. If 

 greater accuracy is desired, a precise platinum wire resistance ther- 

 mometer is used. It is usually desirable to keep one of the latter for 

 use as a laboratory standard and to use base metal resistance ther- 

 mometers or thermistors — both of which are comparatively inexpen- 

 sive — for routine temperature measurements. 



A graph may be made (as in Fig. 9) of ohms of thermometer re- 

 sistance against degrees Centigrade for the working temperature 

 range. It is often feasible to calibrate the slide wire of the bridge or 

 potentiometer to give readings directly in degrees of temperature. 



4. Applications of Resistance Thermometers 



Platinum resistance thermometers in biophysics are used mostly 

 as laboratory standards. Although they may be used over a wider 

 temperature range and are less susceptible to deterioration than base 

 metal thermometers, they require an expensive bridge or potentio- 

 metric circuit and more precise techniques if advantage is to be taken 

 of their greater precision. 



At the present time, base metal resistance thermometers — usually 

 Nichrome — are used for almost all routine biophysical temperature 

 determinations in which resistance thermometers are employed. 

 Beside being inexpensive and easily obtainable, they give satisfactory 

 readings in the moderate temperature range and will hold a calibra- 

 tion within 0.02°C. for several years if suitably mounted and pro- 

 tected. 



Some general applications of resistance thermometers are as fol- 

 lows: (a) Rectal thermometers that are rugged, yet very sensitive to 

 any significant changes in temperature, may be constructed (26) 

 (see Fig. 13). (b) Temperature inside the clothing may be obtained 

 by threading resistance wire into a sewing machine and stitching the 

 resistance wire directly into the clothing [27). (c) Resistance units 



