Mr Whiddington, On a mechanical vacuum tube regulator. 253 
account of a few experiments with the regulator which suggest 
one explanation and then to point out the complications which 
stand in the way of its complete acceptance. 
The tube with which the following experiments were carried 
out was coil driven with a Lodge rectifier in series to prevent 
reversal of the current. 
(1) When a vacuum tube is hardened by diminishing the gas 
pressure the hollow cylindrical cathode beam becomes more cun- 
centrated about its axis, which in the case of a circular plane 
disc cathode passes normally through the centre; at the same 
time the dark space lengthens out receding from the cathode. 
This further is exactly what happens when the sheath is drawn 
out over the cathode at a constant gas pressure, the cathode 
beam narrows down—a smaller part of the cathode functions— 
and the dark space lengthens out. If the pressure be not too 
low the sheath as it is caused to travel out may ultimately over- 
take the boundary of the dark space which is also travelling but 
at a slower pace. The interesting point to notice is that when 
once the sheath has overtaken the dark space boundary and is 
projecting into it no additional hardening effect can be produced 
by further drawing out the sheath. 
(2) If the sheath is not too heavy and is running easily it 
will travel out of itself when the discharge is passing—ain fact it 
may be made to rise against gravity if the tube be tilted so that 
the cathode points slightly upwards. If now the Lodge rectifier 
be cut out the coil may partly reverse and produce a spot of 
positive column on the cathode, if this happen the sheath will 
commence to slide back again to its original position the tube 
at the same time becoming softer. On switching in the rectifier 
again the sheath once more slides out and the tube hardens. It 
is sometimes necessary to tap the tube to get these mechanical 
effects but usually the vibration set up by the coil is sufficient 
to ensure easy movement. 
(3) The effects described in both the above experiments can 
be obtained if the inside of the sheath is lightly silvered, not 
heavily enough to introduce the complications of an additional 
discharge but heavily enough to ensure that the sheath is at the 
same or nearly the same potential as the cathode itself. 
(4) If the sheath be cut longitudinally into halves and only 
one half is used, the cathode beamn is apparently repelled from 
the sheath, the rays pursuing a curved path when in the neigh- 
bourhood of the glass and proceeding in straight lines when 
beyond its influence. 
These experiments taken alone without further evidence lead 
to the very natural conclusion that the sheath being negatively 
charged on the inside repels the cathode rays towards the centre 
VOL. XVII. PT. III. IL4 
