372 Dr. Willows on the Action of a Magnetic 



Volts. 



Mao-net off. Magnet on. 



1 



Magnet off. 



Magnet on. 



290 

 262 

 252 

 208 

 190 

 184 

 186 

 195 



305 



272 



260 



198 



197 



191 



187 (?) 



190 



218 

 248 

 268 

 270 

 285 

 295 

 305 



200 

 218 



235 

 242 

 265 

 275 

 285 



The pressure was gradually reduced from the beginning 

 to the end of the series, the minimum voltage is 184-186 : 

 before this is arrived at the magnet causes an increase, after 

 it is passed a decrease in the volts,, but it never causes the 

 voltage to fall below 186. The field therefore does not 

 reduce the absolute minimum voltage causing the discharge. 

 This is worthy of note, as Carr * has shown that the minimum 

 sparking potential is a constant for the gas, and there is 

 doubtless a close connexion between the minimum sparking 

 jDotential and the minimum voltage required to maintain the 

 discharge. 



With tubes much shorter than 10 cms., the pressure had to 

 be lower than the minimum voltage pressure before the 

 magnet caused a decrease. This is no doubt due to the fact 

 that the field could never be sufficiently localized to act at 

 the cathode only, and where it acts at other points of the 

 discharge it brings about a rise in voltage. 



It would appear, therefore, from my experiments, that the 

 transverse magnetic effect is closely connected with other 

 features of the discharge, and hence is probably not due to a 

 concentration of the luminous portion into a brush or arc. 



I tried next the effect of a longitudinal field on several of 

 the tubes, but, down to a pressure "01 mm., a certain decrease 

 in the terminal voltage was never obtained. The iron core 

 of the magnet employed to produce the field was of greater 

 diameter than the largest cathode used ; the field had a 

 maximum value of 900, it was therefore more intense than that 

 used by Almy or Birk eland. I am unable to account for the 

 discrepancy between my results and those of these physicists. 



One tube was about 2 cms. long, so that the longitudinal 

 field could act along the whole length of the discharge. Still 

 no effect could be found, but if the same field was applied 

 transversely at the cathode it produced a fall of 30 per cent., 

 and this although it was not specially concentrated. The 



* Phil. Trans, cci. p. 403 (1903). 



