at Low Pressures. 253 



uniform column can only be obtained with the very smallest 

 current that will maintain the discharge. At any pressure below 

 about 2 mms. as the current is increased from the smallest 

 possible steady value the column is at first uniform, but after a 

 time begins to develope faint striae, which rapidly become more 

 and more distinct as the current rises in value. At pressures of 

 several millimetres the current begins to oscillate when it is 

 increased much, but striae do not usually appear. 



The electrodes E and E' although made as small as possible 

 nearly always produced some disturbance of the luminosity of the 

 column. Usually they appear surrounded by a faint dark space, 

 and followed on the side nearest the anode by a faint stria. This 

 effect was generally greatest at pressures near one millimetre. 

 At higher or lower pressures it was possible to nearly get rid of 

 it by carefully adjusting the current. If this disturbance was 

 very marked the results obtained did not agree very well with 

 those obtained when it was not present, but when it was slight 

 they were in good agreement. It will be seen in some of the 

 tables of results given below that the discrepancies which the 

 observations show from the regular laws which they nearly follow 

 are rather greater at pressures near one millimetre than at higher 

 or lower pressures. This is no doubt due to this disturbance 

 of the discharge by the small electrodes being especially marked 

 at these pressures. When an observation was being taken if this 

 disturbance was slight, then the results were always sure to be in 

 good accord with the others of the same kind. All the obser- 

 vations given in this paper were taken when the disturbance in 

 question was very small. 



The application of the magnetic field always produces a 

 transverse motion of the positive column so that it becomes 

 brighter along one side of the tube than along the other. This 

 shift is in the direction in which a flexible conductor carrying 

 a current moves. The electrodes E and E' were sufficiently near 

 the axis of the discharge to be still well immersed in the bright 

 part of the column after the field was applied. 



This concentration of the discharge on one side by the field is 

 theoretically a necessary consequence of the Hall effect. 



Table I. shows the variation of the P.D. between the electrodes 

 E and E' with the angle between EE' and a plane perpendicular 

 to A B. Evidently if the fall of potential along the discharge 

 is uniform this P.D. should vary as the sine of the angle in 

 question. The results given in Table I. show that this is approxi- 

 mately the case. 



