154 LAWRENCE R. PROUTY AND JAMES D. HARDY 



of which contains the animal under study (see Fig. 8) . The thermo- 

 couple arrangement for measuring the temperature difference be- 

 tween the cylinders, temperature of the room, the skin and rectal 

 temperature of the experimental animal, and air temperature within 

 the chamber is shown in Figure 4. This tj^pe of calorimeter, because 

 of the small heat capacity of thermocouple arrangements, permits 

 measurements of heat loss within periods of ten minutes. A very 

 recent modification of this technique described by Benzinger (31) 

 attempts to achieve a total response time of less than one minute. 



5. Limitations of Thermocouples 



Thermocouples yield less precise temperature measurements than 

 resistance thermometers. This is not serious for most biophysical 

 work because the lack of precision is insignificant within the range of 

 most biological temperatures. 



Relative to the liquid-in-glass thermometers, the thermocouple 

 thermometers are more expensive and more difficult to manipulate. 

 For the extremes in sensitivity, a delicate galvanometer system is re- 

 quired. The apparatus is not portable and, for great precision, 

 the best potentiometers and experienced observers are required (3). 

 Base metal thermopiles, which are commonly used for biophysical 

 work, have a tendency to change slightly with age. This requires 

 troublesome recalibration. 



D. RESISTANCE THERMOMETERS 



Resistance varies with temperature according to the approximate 

 experimental law : 



R, = /?,(! + mt) (4) 



where R[ and R^ are resistance at temperatures t and 0°C., respec- 

 tively, and ao is the temperature coefficient of resistance referred to 

 0°C. The resistance of insulators, electrolytes, and carbon decreases 

 as temperature rises and is therefore negative. INIetals have a posi- 

 tive coefficient, the value of which is approximately 0.0038 per degree 

 Centigrade for a large number of pure metals. The relationship be- 

 tween temperature and resistance can be applied effectively for three 

 types of resistance thermometers useful in biophysics. These are: 

 {1) noble metal resistance thermometers, (2) base metal resistance 



