Mr Whiddington, On a mechanical vacuum tube regulator, 251 
| On a mechanical vacuum tube regulator. By R. WHIDDINGTON, 
M.A., St John’s College. 
[Read 19 May 1913.] 
In several of the writer’s previous papers mention has been 
made of a particular form of vacuum tube regulator. It is the 
object of this short paper to give a few experiments in connection 
with this device which not only serve to indicate the range of its 
usefulness but may perhaps help to throw a little light on its 
mode of action. 
: The usual method of altering the hardness of a vacuum tube 
is to vary the gas pressure within the tube. Under ordinary 
circumstances a diminution of pressure produces a hardening 
effect, or in other words the lower the pressure the faster the 
cathode rays shot off from the cathode—the more difficult it is 
to force current through the tube. This is only true however 
when the gas pressure is below a certain amount depending on 
the dimensions of the tube and its electrodes. 
Winkelmann appears to have been one of the first to recognise 
the fact that the size, form, and position of the cathode in a 
discharge tube exerted a profound effect on the discharge. He 
found that it was possible by using tubes of small dimensions 
with the cathode in a confined space to produce quite fast cathode 
rays even when the pressure within the tube was as high as 
several mms. of mercury. 
Of the many mechanical regulators based on this effect in- 
vestigated by Winkelmann and others, perhaps the most useful 
is the one in which a sliding glass tube tightly fitting the cathode 
slides in and out, the position of this sheath determining largely 
the velocity of the rays. It is possible in this way to produce 
very considerable variations in the speed of cathode rays without 
in any way altering the gas pressure. This device was first 
described by Campbell Swinton and later by Wehnelt in 1903. 
There are many possible methods of varying the position of the 
sheath and one of the most convenient is to attach a small piece 
of iron to the sheath and influence it from without by a magnet. 
In this kind of way the sheath may be instantly adjusted to any 
desired point, the further the sheath is outdrawn the faster the 
cathode rays emitted from the cathode. 
The graph gives some idea of the range of hardness to be 
expected from a tube fitted with such a regulator. The tube in 
this case was a litre bulb fitted with a cathode 1°5 cm. in diameter 
