of Gases exposed to Rontgen Rays. 245 



the dust-charged air from the room were sent directly through 

 the apparatus. 



Electrification from Charged Insulators. 



If the central electrode through which the air was blown 

 was coated with paraffin or sealing-wax, it was found that 

 the amount of electrification obtained was at first about equal 

 to the amount with the bare electrode. If the bulb was kept 

 working the amount of electrification diminished after a time. 

 The central electrode was then connected to earth, and when 

 the A'-rays were acting electrification could still be obtained, 

 but of opposite sign to that obtained before. If the wire 

 with the coating of dielectric on its surface was kept charged 

 to a high potential and the rays continued for some time, on 

 applying a smaller E.M.F. to the wire in the same direction 

 the sign of the electrification is generally changed. 



The explanation of these and similar phenomena is simple 

 if we consider that the conducting particles of the gas either 

 give up their charge to the surface of the insulator, or adhere 

 to the surface which becomes charged opposite in sign to the 

 wire itself. If the bulb is kept working, the electromotive 

 intensity acting on the gas is diminished, owing to the effect of 

 the oppositely charged insulator. The amount of electrifica- 

 tion obtained therefore diminishes if the E.M.F. is not well 

 above the saturation -value. If the central electrode be then 

 connected to earth, the charged insulator causes a current 

 through the gas in the opposite direction, and thus changes 

 the sign of the charge in the gas blown out. If the charge 

 on the insulator is large, as is the case if the central wire has 

 been raised, for example, to a potential of 200 volts and 

 exposed to the rays for some time, on applying an E.M.F. of 

 30 volts, say, in the same direction the electrification changes 

 sign. In this case, the electromotive intensity due to the 

 charged insulator is greater and opposite in sign to that due 

 to the 30 volts, and so the current through the gas is reversed. 



mi ■ ... © & 



The sign of the electrification obtained when the wire is 

 covered with insulating material is thus dependent on the 

 amount and sign of the charge on the surface of the 

 dielectric. 



From a charged wire coated with paraffin or sealing-wax 

 which had been exposed to the Rontgen rays for several 

 minutes, it was found possible to obtain electrified air, by 

 directing a current of air along its surface, several hours after 

 the central electrode had been connected to earth. 



