74 THE ROYAL SOCIETY OF CANADA 



with re-distilled water. The tubes were dried by a current of air, 

 drawn through a drying train to ensure freedom from dust. Too 

 great care in washing cannot be exercised if smooth action is desired. 



The mercury was washed in dilute nitric acid and distilled. After 

 the first distillation, the still was carefully cleaned and the mercury 

 re-distilled into the electrometer tubes as needed. The electrolyte for 

 the major part of the work was 25 per cent sulphuric acid. The 

 platinum wires for connecting to the mercury in the electrometer were 

 carefully cleaned before inserting and all points in the connecting 

 tubing were either welded or they were sealed with wax in the case of 

 glass-rubber connections. If carefully prepared, there is no sign of any 

 leakage of air from the apparatus, practically no leakage of charge, no 

 trace of local emf. when on closed circuit, or creeping on open circuit. 



For charging the electrometer, a simple potentiometer consisting 

 of a stretched nichrome wire provided with sharp-edged moveable 

 contacts, was used. This potentiometer wire was connected in series 

 with a rheostat, a milammeter and a storage cell. The wire itself was 

 carefully tested and found to be uniform and the contacts were without 

 appreciable resistance. The latter point is very important for low 

 potentials, as any irregularity in contact leads to incorrect values of the 

 capacity. 



The excursions of the electrometer were measured with a micro- 

 meter microscope mounted on a micrometer slide. 



Capacity Measurements 



Since the capacity of any electrometer depends on the position of 

 the meniscus in the tube, all observations of the capacity for purposes 

 of comparison must be made at the same point in the tube. The 

 capacity can be measured by the method of mixtures using the electro- 

 meter itself instead of a galvanometer, but a better method is to charge 

 a standard condenser to a known potential and allow it to share its 

 charge with the uncharged electrometer. The condenser is charged to 

 the desired P.D. which is known from the potentiometer to which it is 

 connected. Next, the condenser is insulated from the source of supply 

 and connected to the electrometer. The excursion of the meniscus is 

 noted and then a second connection to the potentiometer gives the 

 P.D. after the mixture. When the condenser is charged to a new P.D. 

 the position of the meniscus is adjusted by changing the pressure so 

 that the meniscus always comes to rest at the same point in the tube 

 throughout the series. 



The method of charging the electrometer and allowing it to share 

 its charge with a standard condenser is not satisfactory owing to the 



