the Ions in Gases at Loio Pressures. 555 



Ann. xxxviii. p. 27) showed that the rate of leak from a 

 negatively electrified filament was at low pressures diminished 

 by the action of the magnetic field. On the theory of charged 

 ions, the effect of the magnet in diminishing the rate of leak 

 could be explained in the same way as the effect on the con- 

 vection due to ultra-violet light. A series of experiments 

 were made which showed that the effects due to the mag- 

 netic field were consistent with this explanation, and led to 

 a determination of e/m for the carriers of the negative 

 electricity. 



The apparatus was of the same type as that used in the 

 preceding experiments. The wire gauze and the zinc plate 

 were replaced by two parallel aluminium disks about 1*75 

 centim. in diameter ; between these disks, and quite close to 

 the upper disk, there was a small semicircular carbon 

 filament which was raised to a red heat by the current from 

 four storage-cells. The carbon filament was placed close to 

 the axis of the disks ; the object of the upper disk was to 

 make the electric field between the disks more uniform. The 

 lower plate was connected with the electrometer. The plates 

 and filaments were enclosed in a glass tube which was con- 

 nected with a mercury-pump, by means of which the pressure, 

 after the vessel had been repeatedly filled with hydrogen, was 

 reduced to *01 millim. of mercury. Great difficulty was 

 found at first in getting any consistent results with the 

 incandescent carbon filament : sometimes the filament would 

 discharge positive as well as negative electricity ; indeed 

 sometimes it would discharge positive and not negative. Most 

 of these irregularities were traced to gas given out by the 

 incandescent filament ; and it was found that by keeping the 

 filament almost white-hot for several hours, and continually 

 pumping and refilling with hydrogen, and then using the 

 filament at a much lower temperature than that to which it 

 had been raised in this preliminary heating, the irregularities 

 were nearly eliminated, and nothing but negative electrifica- 

 tion was discharged from the filament. When this state was 

 attained, the effect of magnetic force showed the same charac- 

 teristics as in the case of ultra-violet light. When the 

 difference of potential between the filament and the lower 

 plate was small, the effect of the magnetic force was very 

 great, so much so as almost to destroy the leak entirely ; when, 

 however, the potential-difference exceeded a certain value, 

 the magnetic force produced little or no effect upon the leak. 

 An example of this is shown by the results of the following 

 experiment : — 



