;*64 



Prof. J. J. Thomson on easily Absorbed 



metal tube, with the window, enclosing the charged disk 

 C can be moved up and down by means of an indiarubber 

 tube between the tube and the discharge-tube ; A and B are 

 aluminium disks used as electrodes. When A is made cathode, 

 it is surrounded by a dark space which is followed by the 

 luminous negative glow. The distance the glow extends up 

 the vertical tube depends on the pressure of the gas and the 

 magnitude of the electric current flowing through the tube. 

 The current through the tube was adjusted by means of a 

 liquid resistance, introduced in series with the tube. With 



Fig. 2. 



Co Electrometer 



this adjustment very considerable variations were obtained in 

 the length of the negative glow without altering the pressure 

 of the gas. W T ith this apparatus no leak occurred when the 

 electrode A was positive; indeed, one could hardly be expected, 

 as very little of the current went up the tube towards the 

 window. When, however, A was made cathode, and a large 

 enough current sent through the tube to make the negative 

 glow reach the window, the charge leaked from the disk 

 behind the window. When the current was varied, the rate 



