1897.] 



on Cathode Bays. 



427 



leak from the disc when it is charged still continues, and is now 

 equally rapid, whether the original charge on the disc is positive or 

 negative. When the pressure falls so low that cathode rays begin to 

 fall on the end of the cylinder, then the disc acquires a negative 

 charge, and the leak from the disc is more rapid when it is charged 

 positively than when it is charged negatively. If the cathode rays 

 are pulled off the end of the cylinder by a magnet, then the negative 

 charge on the disc and the rate of leak from the disc when it is posi- 

 tively charged is very much diminished. A very interesting point 

 is that these effects, due to the cathode rays, are observed behind 

 comparatively thick walls. I have 

 here a cylinder whose base is 

 brass about 1 mm. thick, and yet 

 when this is exposed to the 

 cathode rays the disc behind it 

 gets a negative charge, and leaks 

 if charged positively. The effect 

 is small compared with that in 

 the cylinder with the thin alu- 

 minium base, but is quite appre- 

 ciable. With the cylinder with 

 the thick end I have never been 

 able to observe any effect at the 

 higher pressures when no cathode 

 rays were coming off. The effect 

 with the cylinder with the thin 

 end was observed when the dis- 

 charge was produced by a large 

 number of small storage cells, as 

 well as when it was produced by 

 an induction coil. 



Eiirlfi 



Electrometer 



Fig. 10. 



It would seem from this experiment that the incidence of the 

 cathode rays on a brass plate as much as 1 mm. thick, and connected 

 with the earth, can put a rarefied gas shielded by the plate into a con- 

 dition in which it can conduct electricity, and that a body placed 

 behind this screen gets a negative charge, so that the side of the 

 brass away from the cathode rays acts itself like a cathode though 

 kept permanently to earth. In the case of the thick brass the effect 

 seems much more likely to be due to a sudden change in the potential 

 of the outer cylinder at the places where the rays strike, rather than 

 to the penetration of any kinds of waves or rays. If the discharge in 

 the tube was perfectly continuous the potential of the outer cylinder 

 would be constant, and since it is connected to earth by a wire through 

 which no considerable current flows, the potential must be approxi- 

 mately that of the earth. The discharge there cannot be continuous ; 

 the negative charge must come in gusts against the ends of the 

 cylinder, coming so suddenly that the electricity has no time to dis- 

 tribute itself over the cylinder so as to shield off the inside from the 



