[ ?52 ] 



LXXXIII. Rays of Positive Electricity. 

 By Sir J. J. Thomson *. 



TFIND that the investigation of the Positive Rays or 

 Canalstrahlen is made much easier by using very large 

 vessels for the discharge-tube in which the rays are produced. 

 With large vessels the dark space around the cathode has 

 plenty of room to expand before it reaches the walls of the 

 tube ; the pressure may therefore be reduced to very low 

 values before this takes place, and in consequence the potential 

 difference required to force the discharge through the tube 

 at these low pressures is much lower than when the tubes are 

 smaller. It is possible with large tubes to work with much 

 lower pressures than with small ones, and at the lower pres- 

 sures phases of the phenomena of the positive rays come to 

 light which are absent or inconspicuous at higher pressures. 

 With small tubes and therefore comparatively high pressures, 

 when the arrangement used to investigate the rays is that 

 described in my former paper (Phil. Mag. [6] xviii. p. 821, 

 1900), i. e. when the rays passing from a hole in the cathode 

 through a long narrow tube fall on a phosphorescent willemite 

 screen after passing through superposed magnetic and electric 

 fields, the appearance on the screen is as follows. 



Fii?. 1. 



The bright spot A which marks the place where the 

 uncleflected rays strike the screen is drawn out by the magnetic 

 and electric forces, producing respectively vertical and hori- 

 zontal displacements, into a straight band AB (fig. 1) of 



* Communicated by the Author. 

 Association, Sept. 1, 1910. 



Head at the meeting of the British 



