Discharge in a Transverse Magnetic Field. 57 



19. When the pressure is very low (about —^ of a 

 millimetre) the negative glow reaches the sides of the vessel 

 and the whole tube becomes phosphorescent, owing evidently 

 to the impact of corpuscles on the sides of the tube. 



On the excitation of the magnetic field, the phosphorescent 

 effect becomes concentrated around the cathode, while the 

 anode is surrounded by a red luminosity, the line of de- 

 marcation between the two regions being clearly marked. 



20. A characteristic variation of the potential difference 

 with pressure during these changes is (as was noted in a 

 previous paper) the most marked feature of these experiments. 

 In order to make a more detailed study of this variation than 

 was attempted previously — through all ranges of pressure — 

 it was necessary, in the first place, to standardize the E.M.F. 

 available with the induction-coils A and B. For this, we 

 observed the spark-distance between two brass spheres, each 

 of radius 3 cm., when the voltage of the primary of the 

 induction-coil was gradually changed, the spring of* the 

 interrupter being kept at a 'constant tension. The result is 

 plotted in curve III., fig. 18, in which ordinates represent 

 spark-lengths in millimetres and the abscissa the voltage of 

 the primaries of the induction-coil A. 



Fig. 18. Curve III. 



3 



r» 14 



5 10 





















1 













! 



1 

















#* 



s* 











1 









a 



^p 

























,4 



























/ 



■ : x 



























<* 



$/ 



















































f 



*x 



! 









































































































1 

 1 















\ i 



i 















Jl 



i 







20 40 

 >■ 'Spark length] in. m.m 



21. Comparing these with the result given in page 461 

 of J. J. Thomson's ' Discharge of Electricity through Gases/ 



