212 Prof. Thomson, Some experiments on Canal- Strahlen. 



Some experiments on Canal- Strahlen. By J. J. Thomson, 

 M.A., F.E.S. 



[Mead 27 November 1905.] 



Goldstein discovered that when the electric discharge passes 

 through an exhausted tube having a perforated cathode, luminous 

 rays streaming through the perforations can be observed behind 

 the cathode. These rays are deflected by a magnetic field to a 

 small, but only very small, extent and have been shown by W. Wien 

 to consist of positively charged ions whose mass is never less than 

 that of a hydrogen atom and is often considerably greater. 

 Similar streams of ions impinge against the cathode itself, and 

 in modern theories of electric discharge through gases play an 

 important part in the mechanism of the electric discharge, thus 

 in my book on the Electric Discharge through Gases, I have 

 supposed that it is by the impact of these positive ions on the 

 cathode, that the cathode rays originate. Again, we know that 

 the cathode disintegrates and the question arises whether this 

 is not due to the impact of the positive ions. The question of 

 ionization by the impact of positive ions against the molecules 

 of a gas, and the nature of the spectra produced by such impacts 

 can also be conveniently studied by means of canal-strahlen : for 

 these reasons I have made a series of experiments on these rays 

 using a discharge tube of the kind represented in Fig. 1. 



The cathode C is a large disc nearly filling the tube, with a 

 hole in the middle opening on to a metal tube ; the canal-strahlen 

 pass through this tube on their way to strike against the metal 

 plate If at the end of the discharge tube. A wire soldered to this 

 plate passes out from the discharge tube and allows the plate to 

 be charged positively or negatively. 



The discharge was produced by a large induction coil, and to 

 prevent cathode rays coming down the tube from the reversal of 

 the discharge, a strong transverse magnetic field was applied to 

 the portion B of the tube, so as to drive any cathode rays against 

 the walls of the tube and prevent them from reaching the metal 

 plate. The experiments were also repeated with a large 

 Wimshurst machine in place of the induction-coil, the results 

 were the same in the two cases. 



When the canal-strahlen strike against a solid they cause it to 

 emit cathode rays moving with small velocities. This can easily 

 be shown by charging the metal plate, against which the canal- 

 strahlen impinge negatively, to a potential of say 80 volts, then 

 from the part of the plate struck by the canal-strahlen a pencil 

 of feebly luminous rays can be seen, the appearance being like 

 that shown in Fig. 2 ; these rays are easily deflected by a magnet 

 in the direction indicating a negative change. That their initial 



