Air in dosed Vessels and its Causes. 



553 



bodies. Just above the mercury -cup the screen was per- 

 forated to admit an insulated wire, the extreme end of which 

 was bare. This wire was connected to a potentiometer 

 arrangement, and so by inserting it into the mercury-cup it 

 was possible to bring the electrode system to zero, or any 

 other convenient potential. At first a paraffin-key was used 



Fisy. 1. 

 #rf" -'^ ~ E 



A. Vessel. B. Ebonite Insulation , C. Sulphur Insulation. D, Electrode. 

 E. Earth. F. Guard-tube. G. Mercury-cup into which potentio- 

 meter-wire may be inserted. II. Wilson Electroscope. 



to connect the electrode and electroscope, but the paraffin 

 caused endless difficulties by acquiring charges, and was 

 ultimately discarded in favour of the arrangement described. 

 This reduced the insulation to a minimum, and was found to 

 work very satisfactorily. The method of experimenting was 

 as follows: — The potentiometer-wire was inserted in the 

 mercury-cup and the system was brought to zero potential, 

 the position of the gold-leaf being read off on the microscope- 

 scale. The potentiometer was then adjusted till the wire was 

 brought to a potential of one volt, the position of the gold- 

 leaf being read again. The difference between the two readings 

 gave the sensitiveness of the electroscope in divisions per volt. 

 The potentiometer-wire was now once more brought to zero, 

 the reading of the gold-leaf taken and the wire withdrawn 

 from the mercury-cup. This left the electrode connected up 

 to the electroscope, but otherwise insulated. After twenty 

 minutes the reading of the gold-leaf was taken again, the 

 Phil. Mag. S. 6 Vol. 9. No. 52. April L905. 2 



