399 



Portuble Capillary Electrometer. 



rented in the figure, is commonly used with the instrument, 

 and keeps the platinum terminals P a and P 2 at the same 

 potential unless the lever S is depressed. When the lever 



Fie. 2. 



Ficr. l. 



E, E 2 



is depressed the potentials of the terminals become E x and 

 E 2 , which may, for example, be the potentials of two points 

 in a potentiometer circuit. It is the function of the instru- 

 ment to determine whether these potentials are the same or 

 different. 



The nature of the modifications made in the present electro- 

 meter is shown in the second figure. 



To prevent evaporation of the sulphuric acid solution, 

 without preventing free motion of the liquids within the 

 apparatus, the wide tubes are closed at the top ; but are 

 joined across by another tube opening into them as shown. 

 With this arrangement the apparatus is made air-tight, and 

 can, if desired, be made air-free by exhaustion of the appa- 

 ratus before sealing. It is obvious also that the apparatus 

 can be upset without spilling of the liquid. The two limbs 



