5s elie’ 
102 Mr Watson, Some Experiments on the 
voltmeter being disconnected, the pressure was reduced until the 
dark space just reached the anode. If then the pressure was 
very slightly further reduced or the current density lowered by 
locreasing the external resistance, the current ceased to pass. 
continuously, and passed in flashes. By careful adjustment these 
could be made to succeed each other with great rapidity, or at 
regular intervals as far apart as quarter of a minute. This dis-_ 
charge was previously noted in the experiments carried out in 
conjunction with F. W. Aston (loc. cit.), and affords a beautiful 
illustration of the well known although rarely concisely stated 
fact, that if the anode be brought inside the Crookes dark space, 
no current will pass unless an enormously increased voltage ig) 
applied. In the present case matters are so adjusted that if the © 
current were to pass continuously, it would be so weak that the 
dark space would be longer than the distance between the elec-— 
trodes. Consequently this cannot occur. Increasing the current — 
density, however, shortens the dark space, and a large current can 
pass. To effect this, the energy seems to store itself up until 
sufficient is accumulated to pass over in a burst. As the con-_ 
denser capacity of the vacuum tube must be quite small, it is 
difficult to tell how this storage takes place, and it is remarkable 
that the flashes can be obtained at such long intervals as those 
already mentioned. A further investigation of this point would — 
most certainly prove of great interest, and might throw valuable — 
light on the phenomenon of initial discharge in gases at low 
pressure. : 
Another form of discharge was observed which was probably — 
similar to that from a Wehnelt cathode. Its characteristic was — 
that it occurred from a single point on the cathode, and it was 
induced by traces of impurity on the surface. It only appeared 
once in the carefully cleaned tube used for spark potentials, and — 
was exceedingly persistent in tube no. 7, with magnesium elec- 
trodes. Its appearance in the case of neon was extraordinarily © 
beautiful, and resembled nothing so much as the sun just setting — 
in a perfectly clear atmosphere. Its brilliance was dazzling as 
may well be imagined from the fact that currents of the order of 
50 milliamperes frequently passed from this one point. 
The voltage when this discharge was passing was very low, 
some figures being: with helium in spark potential tube 154 volts, 
neon in tube no. 3, 17-5 mm. pressure, aluminium wire cathode, © 
109 volts; neon in tube no. 7, magnesium electrodes, 75 volts at 
18 mm. pressure, and 110 at 3 mm.; neon in tube no. 2 aluminium 
wire cathode 1 mm. pressure, 85 volts. These values did not 
vary greatly with the current and seemed to bear no relation to 
one another. As with the other abnormal forms of discharge, it 
was impossible to tell when this form would make its appearance. — 
Nine Ce AO ATI 
ak 
