188 Mr. T. Harris on Distribution of Electric Force 



constricting the length of the dark space, was examined (fig. 5). 

 Here A and B (two measurements at different times) were 

 taken when the negative glow was faint without a definite 

 boundary, and the discharge was beginning to concentrate 



Fig-. 5. 



\ 



;♦-> *^ 



500 



4 3 2 



Distance from cathode in cm. 



into the centre of the cathode. The voltage required to 

 drive it was 750 volts. C was taken at a lower pressure 

 when the glow had faded away and phosphorescence was 

 beginning to appear round the anode ; D, when the phos- 

 phorescence had extended about half way up the tube and 

 the visible discharge was a beam of cathode rays, voltage 

 6000 ; F, when the pressure was still lower, the tube phos- 

 phorescing brightly between anode and cathode, visible 

 discharge a narrow pencil of cathode rays, voltage 8500. 



In this case the curves all end at about the same point, 

 and show that there is a region in front of the anode where 

 the force is extremelv small, although it is inside the normal 

 dark space, which, as was seen above, is about 8 cm. 



