Difference between the Electrodes of a Vacuum Tube. 185 
pressures were taken ; this magnetic diminution is at first rela- 
tively large when fluorescence of the glass begins to be produced. 
Afterwards the magnitude of the effect increases very rapidly. 
(Table III.) 
That the “ critical ” magnetic field, and also the magnitude 
of the diminution of potential difference in the tube, were both 
not independent of the size of the electrodes seemed so probable 
that the study of the effect of variation of elec- 
trode area upon these was made. 
To enable variation of the area of an elec- 
trode to be made without variation of the other 
conditions of discharge the tube Fig. 1 was 
devised. 
The fixed electrode being a plane alumin- 
ium disc, 15 mm. in diameter, the lower elec- 
trode was the top of a barometer column which 
ended in a funnel-shaped expansion of the tube. c 
The area of this mercury surface could be varied 
at will by changing the position of the cistern of 
the barometer. The length of the discharge tube 
was made large so that the variations of the dis- 
tance between the electrodes are relatively small. 
The variation of the p.-d. between the elec- 
trodes as the area of the anode varies was observed. 
It was found that, with a given constant current 
passing through the tube, except for very small 
anode areas, the variation of P.-D. is directly pro- 
portional to the change of anode area. And with 
given constant potential difference applied, the 
current obtained varied in like simple proportion. 
The discharge from the Holtz machine, the Kelvin 
voltmeter, and a D’Arsonval galvanometer, as 
ammeter, were used here. 
Using the induction coil and spark gap, with- 
out any measure, or regulation of the current in 
the tube, except that the coil was driven as uni- 
formly as could be, the effect of variation of ca- 
thode area was noted. Also the effect of a mag- 
netic field of 400 lines normal to the tube was 
observed. 
We see here (Table IV) that the p.-d. between the electrodes 
increases, and somewhat abruptly as the cathode area increases 
above 1*8 sq. cm. but the p.-d, with the magnetic field on, 
while it increases with increasing cathode-area, does so at a more 
nearly uniform rate. 
Experiments (Table V) were tried, using a large Wimshurst 
