Potential at the Electrodes in Vacuum-tube Discharae, 



beyond a doubt that the effect of a magnetic field on the 

 cathode drop increases without limit as the gas-pressure is 

 reduced. 



At the Anode. — The effect of the magnetic field on the 

 •anode drop seems, as in other cases, to be controlled by its 

 effect on the gas-gradient. The results given in Table IV. 



Table IV. — Effect of Magnetic Field on Anode Drop. 

 Gas-pressure 0*5 mm. Field-intensity 600. Current 3 m.a. 



7 ^ , P.D. between 



D /°P, at Anode and 

 Anode ' Neg. Glow. 



P.D. in 

 Gas. 



Mag. on 50-6 



Mag. off 370 



Mag. on 55 



Mag. off 



Mag. on 



185 

 265 



148 

 212 



are typical. They are intended to show the relation between 

 -anode drop and the gradient in the gas, but on account of 

 the form of the tube being unsuitable for conclusive results 

 on this point, only a few observations were taken. With the 

 current passing from A as anode, the anode drop was obtained 

 with and without magnetic field in the sequence recorded. 

 The current was then reversed, and the difference of potential 

 between the anode B and the negative glow at W measured. 

 The difference between this value and the corresponding 

 anode drop, with and without field, gives the total drop 

 in the gas in each case. Without making a gross error we 

 may assume this to be proportional to the gradient near the 

 anode which drives the negative ion into the anode field, in 

 that this is proportional to the drop in the positive column. 

 Referring to the Table it is observed that the values of the 

 anode drop, 52*8 (mean value) and 37, with and without field 

 respectively, bear exactty the same ratio to each other as the 

 corresponding values of the P.D. 212 and 118 in the gas. 

 The accuracy of this ratio is considered accidental. 



The very large effect of the magnetic field on the P.D. in 

 gases at low pressures, as observed by Almy*, is most likely 

 to be attributed to the effect on the cathode drop greatly 

 overshadowing that on the rest of the path. 



It is probable that the direct effect of the field on the 



* J. E. Almy, Proc. Camb. Phil. Soc. vol. xi. pt. iii. 



