146 



LAWRENCE R. PROUTY AND JAMES D. HARDY 



mocouple is determined by adjusting the slide wive contact until the 

 potential drop in resistor BC equals the potential of the thermocouple. 

 The slide wire is provided with a scale divided into suitable units. 

 The galvanometer serves only as a null indicator in this type of poten- 

 tiometer. 



Potentiometers may be used with great accuracy. They may be 

 operated manually or they may be made self-recording. If the stand- 

 ard junction temperature changes, the galvanometer must be ad- 



standard 

 Cell 



Thermocouple 



Fig. 3. Potentiometric circuit for thermocouples with standard cell. 



justed. It is possible to compensate automatically for these tempera- 

 ture changes by installing the standard junction within the potenti- 

 ometer and attaching a bimetallic spring to the control spring of the 

 galvanometer coil. 



A second type of potentiometric circuit for use with thermocouples 

 is shown in Figure 4 (^). Materials for the circuit are available in 

 most laboratories and it may be constructed readily at little expense. 

 It has the advantage of not requiring a standard cell. Basically, the 

 circuit employs a galvanometer as a null instrument to balance a 

 known current supplied by a 1.5 v. dr^^ cell against the thermoelectric 

 current. The current is set at a constant value on the milliammeter 

 by adjustment of a 200 ohm (coarse) and a 20 ohm (fine) variable 

 resistor. The galvanometer is brought to the null point on each 

 temperature determination by means of a 7 ohm slidewire resistance 

 calibrated to read either in divisions or degrees of temperature. As 



