278 ELECTRICAL MEASUREMENTS 



is imperative that the work be done with great speed, combined 

 with the accuracy necessary in engineering work. 



The Brooks potentiometer was designed with this in mind. 

 By it, results may be obtained even though the P.D. under 

 measurement is not perfectly steady. In this instrument no 



FIG. 158. Brooks deflectional potentiometer. 



attempt is made to obtain an exact balance; the slides are set 

 so near to the null point that the galvanometer deflection is 

 small. The galvanometer is so graduated that it give 8 the 



amount that must be added 

 to the reading of the slides in 

 order to obtain the unknown 

 P.D. 



That certain conditions 

 must be fulfilled may be seen 

 from the following discussion. 

 In general the potentiom- 

 eter is used: 



FIG - *%SSS*&*L~ I- T determine potential 



differences which are within 

 the normal range of the instrument. 



II. To determine, by the use of a volt box, potential differences 

 which are above the normal range of the instrument. 



III. To measure currents by the use of shunts. 

 CASE I. DIRECT MEASUREMENT OF P.D. 



A storage cell is used at e (Fig. 159) so its resistance may be 

 neglected. 



