284 



ELECTRICAL MEASUREMENTS 



It is obvious that a deflection potentiometer must be used with 

 the particular volt box and shunts for which it was designed. 



FIG. 163. Volt box for Brooks deflectional potentiometer. 



The "Thermokraftfrei" Potentiometer 10 . In the measure- 

 ment, of small potential differences such as those of thermo- 

 couples a low resistance potentiometer is used, and it is necessary 

 to eliminate all thermo-electric disturbances due to contact of 

 dissimiliar materials and inequalities of temperature in the 

 potentiometer itself; therefore the metal employed in the coils 

 and in their terminals must be carefully selected with this 

 result in view. For the same reason, the design should be such 

 that the effect of thermo-electromotive forces introduced by 

 the manipulation of the necessary sliding contacts and switches 

 will be reduced to a minimum. This is accomplished in the 

 instrument under discussion, which includes elements of design 

 due to H. Hausrath, W. P. White and H. Diesselhorst. The 

 potentiometers previously , discussed have been series arrange- 

 ments, that is, the compensating potential difference is the sum 

 of the potential differences existing between the terminals of 

 various groups of coils of which the circuit is composed. Haus- 

 rath suggested the use of a divided circuit in place of coils in 

 series. In that case the compensating potential difference is 

 due to the difference of the potential drops along the two branches 

 measured from the point where the current enters the apparatus. 

 Fig. 164 shows in diagram the arrangement of the coils. 



The potentiometer current is kept at its standard value, 

 0.001 amp., in the usual manner by equating the drop in eg to 



