V. TEMPERATURE DETERMINATIONS 167 



peratiirc. Unless specially designed, however, their accuracy usually 

 is less than 0.1°C. Their use is limited to panel, wall, remote posi- 

 tion, or other indicating thermometers of the dial type. A combina- 

 tion of a metal with a high temperature coefficient, such as brass, 

 and one with a low coefficient, such as Invar, gives the best result. 



Bimetallic strips find their widest application as thermostats for 

 control of constant temperature cells, baths, or rooms. The metals 

 selected should have low mechanical hysteresis and high tensile 

 strength so that, when cooled or heated repeatedly, they will warp 

 to the same degree consistently. Such strips may be used to open or 

 close electrical contacts or a mechanical valve, or to move indicators. 

 If more exact temperature changes are required, the strip is used un- 

 loaded and its movements are followed by means of a wire strain gage 

 or a change in capacitance between the bending strip and a fixed 

 plate, or the movements can be followed by means of a phototube 

 optical system. 



6. Thermos copes 



The thermoscopic method is the simplest for measuring tempera- 

 tures and is employed in the laboratory to designate a certain arbi- 

 trary temperature, usually for calibration of a calorimeter or electric 

 furnace. One form of thermoscope employs a small piece of lead, 

 paraffin, or sulfur placed so that the melting point of any one of these 

 or similar substances indicates the temperature of another mass 

 under similar thermal conditions. A hollow metal sphere that would 

 pass through a given aperture at a certain temperature but not at any 

 other is another form of thermoscope. If the substance used as a 

 thermoscope undergoes a change of state at some known "fixed point" 

 (such as the melting or boiling point of sulfur) this fixed point can be 

 related to a known temperature scale. Although thermoscopes have 

 the advantage of simplicity, they have the disadvantage of indicating 

 only one temperature point and this only momentarily. 



G. SPECIAL TEMPERATURE PROBLEMS IN BIOPHYSICS 



Temperature is probably the most important universal factor 

 affecting all life processes. Biophysical research is concerned not onlj'' 

 with temperature determinations on man but on the organisms shar- 

 ing his environment and temperature factors in the environment it- 

 self. The general methods available have been described in the pre- 



