The Estimation of Acidity. 



245 



In practice the potentiometer is constructed in such a way that 

 a variable resistance is introduced between the accumulator C, 

 and AB, the wire AB (or what corresponds to it) being placed 

 against a scale which is graduated from zero to 1.5 volt. Instead 

 of an unknown e.m.f. at D, a standard cell such as the Weston 



O, 



mm 1 1 1 1 ) 1 1 i ) ! i ijj 11 n 1 1 1 1 1 



tr 



Fig. 5. 



• element is introduced, the movable contact F being placed on 

 AB, at the known voltage of the cell used. When this is done, 

 the variable resistance is adjusted until no deflection of the gal- 

 vanometer E takes place. If now the source of current C be 

 developing a steady e.m.f., the potential drop along the wire AB 

 will correspond to the divisions on the scale, and the instrument 

 will be ready for the determination of the unknown e.m.f. re- 

 quired. 



To do this, the standard cell is cut out by means of suitable 

 switches, and the source of the unknown e.m.f. (which may be 

 a cell, or in this instance, the hydrogen electrode-calomel elec- 

 trode used in these investigations) is thrown into the circuit. The 

 sliding contact is now moved along AB until no deflection of the 

 galvanometer occurs, and the required e.m.f. may be then read 

 off on the scale. 



