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LAWRENCE R. PROUTY AND JAMES D. HARDY 



equals arm 4. This condition is obtained when the variable resist- 

 ance oi AB equals the varying resistance of the detector coil, BT. 

 Direct temperature readings at T (rather than ohms) are possible 

 with proper calibration of the slide wire, which constitutes the vari- 

 able section of the resistance of arm 3. 



Although the above circuit is the most generally satisfactory for 

 ordinary biophysical work, precision resistance thermometry em- 

 ploying a platinum \vire thermometer requires somewhat different 

 circuits. The basic operation of precision resistance thermometry 

 consists in the measurement of the potential drop across the resistance 

 thermometer as compared to that across a standard resistor. Either 

 a potentiometer or a bridge circuit can be used. The potentiometer 



Arm 3 



Arm A 



Fig. 11. Commonly used Wheat- 

 stone bridge circuit for three lead re- 

 sistance thermometer (modeled after 

 a Leeds and Northrup circuit). 



is a more versatile instrument capable of measuring temperatures 

 over a greater range than a bridge circuit with equal sensitivity, but 

 is somewhat more difficult to use. In the potentiometric method 

 the voltage drop across two resistors carrying the same current is 

 measured. One of these resistors is a standard of the four terminal 

 type and the other is the resistance thermometer. Both are con- 

 nected in series with a battery. Resistance thermometers for use in 

 the biophysical temperature range usually have a potential drop of 

 0.1 V. or less and are best used with low voltage potentiometers. 

 Mueller (^4) states that for accuracy of 0.001 °C. in temperature de- 

 termination, resistance measurements must be made with an accu- 

 racy of two to four parts in a million. 



