on Discharge in a Vacuum Tube. 473 



hole, h, gave connection between T and the main tube. The joint 

 between S and T was made with sealing-wax ( W). 



To l\t ?UT 



The experiments were made at a pressure giving a dark space 

 of 6 to 8 mm. The source of ultra violet light was the spark 

 from a 6-inch induction coil — with a leyden jar across the« 

 secondary — taken between aluminium terminals. No condensing 

 lens was used but the spark gap was placed immediately in front 

 of the quartz window. In order to prevent induction effects from 

 the spark gap and its leads, it was found necessary to enclose 

 them in an earthed metal case, except for a small opening through 

 which the rays passed. The effectiveness of the shielding will 

 be seen from the fact that although the discharge would start 

 in the dark for a potential difference of 555 to 557 volts, and 

 while the radiation was falling on the kathode it would start 

 at 511 volts — a bit of ordinary window-glass placed in front of 

 the quartz window prevented the discharge from starting at 

 554 volts. 



When the discharge was started an electrostatic voltmeter, 

 whose terminals were connected to the kathode (K) and the 

 needle (iV), showed a potential difference of 375 volts, and no 

 change could be detected in this when the rays were put on or 

 shut off, though a change of two volts should have been easily 

 seen had it occurred. 



