on Discharge in a Vacuum Tube. 475 



after the discharge stopped, were 505, 504, 504, but these were 

 always three or four volts lower while the discharge was passing. 

 The same " weakening" of the gas, noticed above, was found here, 

 for a later reading gave 503 volts. It is hoped that this point 

 may be investigated before long, but at present all that can be 

 stated with certainty is the following — : 



(1) The "negative drop" was found to be the same, whether 

 the radiation was falling on the kathode or not. 



(2) No change was detected in the magnitude of the current 

 when the kathode was illuminated or when the light was shut off. 

 The change — for probably a real change does occur — was at least 

 small in comparison with one of one-half per cent. 



(3) A marked change in the potential required to start the 

 discharge was produced by the action of the ultra violet light : 

 the effect being a lowering of the required potential. In the 

 case under examination the reduction was from 558 volts to 

 510 volts. 



(4) By gradually lowering the potential difference after the 

 discharge had been started and then all rays cut off, it was found 

 possible to carry the potential to a point at least three or four 

 volts lower than the minimum potential required to start the 

 discharge by the action of the ultra violet light. 



(5) The effect is due to the action of the radiation at the 

 kathode and not — at least not to any appreciable extent — to 

 a volume ionization by the rays as they pass down the path of 

 the discharge. 



This last point is shown by making the copper rod the kathode 

 and the zinc electrode positive. A metal screen completely 

 shielded the copper from the light. The following numbers were 

 obtained : 



Copper negative, Zinc positive. 



291 cells will start the discharge in the dark ; 

 290 cells will not start the discharge in the dark, nor when 

 the rays are passing down the tube. 



Zinc negative, Copper positive, 

 (a) In the dark, 



268 cells start the discharge ; 

 267 cells do not start discharge. 



