in the Discharge at Low Pressures. 189 



Fig. 6 gives the results for a tube 3'5 cm. in diameter, 

 electrodes of aluminium 13*6 cm. apart. To curve A there 

 corresponds a faint negative glow, a broad beam from the 



Fiff. 6. 



1200 



y iooo 

 S eoo 



I 



\ 600 

 4-00 



200 



JlpL 



J. iT 7 



B 6 4 



Distance from cathode in cm. 



cathode, voltage 1700 ; to B, the point when the glow had 

 faded away, voltage 4700 ; C. thin pencil of cathode rays, 

 bright phosphorescence, voltage 7000. 



In spite of the decrease in pressure and the big increase 

 in voltage, from 4700 volts for B to 8000 for 0, there is 

 only a small extension of the region in which there is any 

 appreciable force. The length of this field is about 7"5 cm., 

 being intermediate between the corresponding values for 

 tubes of greater and smaller diameter. 



In one experiment the anode was placed in a side tube 

 20 or 30 cm. away from the cathode. The pressure was very 

 low, 12,000 volts being required to drive the discharge, and 

 the glass "was phosphorescing 40 cm. away from the cathode ; 

 cathode rays were traversing the whole length of the tube. 

 No force could be detected except in the region just in front 

 of the cathode. 



The pressure-gauge and mercury-pump were cut off and 

 evacuation carried out solely with the aid of liquid air and 

 charcoal, thus eliminating mercury vapour from the tube ; 



