OF THALLIUM, INDIUM, AND TIN 



THE POTENTIOMETER. 



Considerable time was spent in the elaboration of a suitable potentiom- 

 eter for use in this work. The arrangement used by Richards and 

 Forbes, while probably accurate to 0.000005 of a volt, was complicated 

 and involved troublesome calibrations. Moreover, it seemed desirable to 

 dispense with the one-volt element and compare the drop of potential 

 directly with a standard Weston cell. The arrangement finally adopted is 

 shown in fig. 5. It was elaborated with the help of R. N. Garrod-Thomas, 

 and was used also for his work, to be described later. 



H 



roo o o ] 



IP O O O I 

 IG Q O O i 



E 



V 



X 



Fig. 5. The Potentiometer. 



A large Daniel cell F was used as the source of the fall of potential. 

 When in use, it was found best to keep it short-circuited through a resist- 

 ance of about 300 ohms in the box E. The rough box D was so adjusted 

 that the fall of potential between the points U and V was equal to 1.0184 

 volts, as measured against the normal cell H. C was a constant resistance 

 of 9000 ohms. A and B were resistance boxes of mi ohms each, and 

 MN was a manganin wire of 1.063 ohms resistance. At the commence- 



