140 LAWRENCE R. PROUTY AND JAMES D. HARDY 



C. at 41 °C. This means that the readings of two thermometers with 

 the maximum allowable variation may differ from each other by as 

 much as 0.2°C. at 38°C. and 0.4°C. at 41 °C., and shows the impor- 

 tance of using the same thermometer for a patient throughout the 

 course of an illness. Tolerances for other mercurial thermometers 

 may be found in the National Bureau of Standards Circular #8, cur- 

 rent edition, on testing of thermometers. 



3. Applications of Liquid-in-Glass Thermometers 



Within the range of precision that can be obtained with liquid-in- 

 glass thermometers — which is generally sufficient for most biophysical 

 work — this type has the specific advantages of low cost and ease of 

 manipulation beyond that of any other type of thermometer. They 

 are easily calibrated and show little deterioration with age and fre- 

 quent use. If proper consideration is given to size, sensitivity, and 

 accuracy factors, liquid-in-glass thermometers may be used to 

 measure (1) environmental temperature and humidity, {B) internal 

 temperature of animals and plants, (3) temperature of solutions in- 

 volved in chemical and metabolic processes, (4) temperatures of 

 water baths, autoclaves, incubators, and low temperature furnaces, 

 (6) surface temperatures of interfaces, plants, animals, and man 

 (measurement often being facilitated by rolling the thermometer 

 over the surface), (6) maximum and minimum temperatures with 

 thermometers of the self-registering type, and (7) differential tempera- 

 ture increments with the Beckmann thermometer. The liquid-in- 

 glass thermometers also serve as laboratory standards for calibrating 

 electrical thermometers. . 



4. Limitations of Liquid-in-Glass Thermometers 



From the point of view of biophysical research methods, the most 

 serious limitation of the liciuid-in-glass thermometer is its lack of sen- 

 sitivity. Temperature changes of 0.001 °C\ can be measured with 

 the Beckmann type of thermometer, but it is often desired to measure 

 temperatures one-hundredth or less of this value. This limitation is 

 due to the fact that the ratio of bulb volume to bore cannot be in- 

 creased indefinitely and also to the fact that a sensitive thermometer 

 requires a considerable amount of heat to produce temperature 

 changes. A second important limitation is that it is often desirable 

 to measure temperature changes in tissues without adding appreciable 



