MEASUREMENT OF POTENTIAL DIFFERENCE 277 



The function of the upper sets of coils in the groups marked 

 XlO, Xl and XO.l, which are connected in series with those in 

 the lower or measuring sets, is to maintain the potentiometer 

 current at a fixed value irrespective of the position of the con- 

 tacts D, E } F. The contacts on the upper and lower sets of 

 these resistances are rigidly connected so that if a coil is removed 

 from the lower set an equal coil is added in the upper set. 



When the switch K is on X, the derived circuit containing the 

 unknown P.D. is connected between A -and C via the galva- 

 nometer. By manipulating the switches the resistance between 

 A and C may be varied from to 18,999.9 so if the current be kept 

 constant at 0.0001 amp., any P.D. between and 1.89999 volts 

 may be balanced. 



The standard-cell circuit is connected between the eighth and 

 ninth coils in group XI, 000 and the contact at H ; by moving this 

 contact the resistance between the terminals of the standard- 

 cell circuit may be varied from 10,190 ohms to 10,200 ohms in 

 1-ohm steps. The instrument may thus be adjusted so that 

 standard cells having e.m.fs. from 1.0190 to 1.0200 volts may be 

 employed. To check the potentiometer current it is necessary 

 merely to throw the switch K to the position marked N and 

 to depress the key. 



The instrument is also made in a low-resistance form (14J-^ 

 ohms) which is suitable for thermo-electromotive force determi- 

 nations such as are necessary in pyrometry. 



The Brooks Deflection Potentiometer 9 . The potentiometers 

 thus far described are read by the null method, an exact balance 

 being obtained between the potential difference in the instru- 

 ment, due to the potentiometer current, and the potential 

 difference to be measured. The objection to this method, is 

 that while it gives results of the highest precision, the P.D. 

 to be measured must be steady, and repeated trials have to be 

 made before the balance point is obtained. When the unknown 

 potential difference is not steady, many trials must be made 

 before the null point is hit upon by mere chance, and the ex- 

 penditure of time and patience becomes so great as to be almost 

 prohibitive in much commercial work. 



In the case of a large electrical engineering laboratory, where 

 many instruments must be checked and kept in adjustment, it 



