254 



Dr. R. 8. Willows on the Effect of a 



corresponding to fig. 1 B cuts the axis of pressure ; they 

 also show that the pressure at which the potential required 

 to maintain the discharge is a minimum is lower when the 

 magnet is on. This is shown in the curves of fig. 3. {Cf* 

 fig.4B.) 



Fig. 3. 







\ 

























v\ 

























w 

























\ 









A MAG 



V£T OS 











CO 





> 



V\ 







B MAG 



MET OF 



f 









vl 







A 1 

























\ 



\\ 













A^ 





N 









V 



V 























\ 



^ 



-*■" 



^ 





;L _J_- 





80 I0Q 



PRESSURE 



The pressure at which the magnet has no effect on the 

 current depends on the field and also on the current through 

 the tube before the field is put on. With the same current 

 through the tube an increase of the magnetic field increases 

 the pressure at which the current is not affected by it and 

 the curve corresponding to fig. 1 is steeper. If the current 

 through the tube be decreased and the magnetic field kept 

 constant, the critical pressure is rapidly lowered. This 

 explains the result noted above, viz. : that for very small 

 currents the magnetic field may produce a further decrease, 

 although the pressure is as low as '1 mm. 



The effects described can be obtained in different tubes 

 and with the current or field reversed, so that they are not 

 due to the magnet deflecting the ions in the tube and 

 causing them to focus better on the other electrode. 



The slightly different results obtained in some cases 

 according as the cathode was a point or a disk may have 

 some relation to the distribution of electricity on the internal 

 walls of the tube. These differences are only observed when 

 the conditions of the experiment are such that the negative 

 dark space reaches the walls of the tube. 



