V. TEMPERATURE DETERMINATIONS 141 



thermal capacity to the system. A sensitive thermometer is usually 

 slow, requiring several seconds or even minutes to reach e(iuilibrium. 

 This delay is known as thermometric lag and is due to the fact that 

 heat is exchanged between the thermometer and the surrounding 

 medium at a rate proportional to the ratio of the thermal capacity of 

 the thermometer to the area of the bulb exposed. Formulas derived 

 from differential equations are available for determining thermometric 

 lag time. If the thermal capacity of the medium is large compared 

 to that of a thermometer and the rate of stirring is adequate, errors 

 due to thermometric lag can be ignored. However, lag time is often 

 important in measurements of air temperature, particularly if there 

 are rapid changes. A third important limitation is that liquid-in- 

 glass thermometers often cannot be read in situ so that artifices must 

 be resorted to, such as constricting the bore of a clinical thermometer. 

 A fourth limitation is the difficulty in making precise measurements 

 of temperatures between the fixed points at which the thermometer 

 has been calibrated and under circumstances that dift'er significantly 

 from those prevailing during the calibration. According to Busse (3) 

 this type of thermometer reached its highest development for precise 

 work some fifty years ago, when electrical thermometers were not 

 readily available. In that period, individual portions of the scale 

 were calibrated with mercury threads. Corrections were made for 

 internal and external pressure, effects of changes in bulb volume (i.e., 

 shift in zero reading), and other factors, so that a reproducible scale 

 from to 100°C. could be obtained. Months of fine workmanship 

 were spent in making and calibrating a single thermometer of uniform 

 bore. 



C. THERMOELECTRIC THERMOMETERS 



Seebeck, in 1882, discovered that in a closed circuit, a small elec- 

 tromotive force is set up between a pair of bimetallic junctions at 

 different temperatures. Thermal electromotive force values are 

 small, of the order of 40 microvolts per degree Centigrade, and are 

 proportional to the temperature difference between hot and cold 

 junctions. Bimetallic junctions are called thermocouples and are 

 extremely useful in determining tem])eratiu-es in biological work. 

 Frecjuently, several thermocouples are connected in series and are 

 then called thermopiles. A third dissimilar metal may be used in the 

 circuit without aft'ecting the thermoelectric force, provided both ends 



