254 Mr Whiddington, On a mechanical vacuum tube regulator. 
on being drawn out thus increasing the resistance of the tube. 
But even this apparently simple statement becomes complicated 
when we remember that the cathode beam is travelling in a 
strongly ionised gaseous layer so that the individual particles 
will be shielded from electrostatic action in some directions more 
than in others. It seems obvious in fact that any individual 
particle in the stream will only be repelled by the nearer portions 
of the charged sheath. No mere electrostatic explanation how- 
ever can be sufficient for the system under consideration is not 
in electrical equilibrium. 
Now the generally accepted theory of the discharge tube has — 
it that the cathode and positive rays are inseparably connected. 
The cathode rays in their passage through the gas produce positive 
ions which move towards the cathode thus becoming the positive 
rays. When these positive rays strike the cathode they liberate 
electrons which shoot off under the electric force at the cathode 
and become cathode rays and the process 1s repeated so long as 
the potential difference between cathode and anode is maintained. 
The difficulty now arising is that if the sheath repels the negative 
rays towards the axis of the discharge it should also attract the 
positive rays outwards away from the axis. Unfortunately there 
is not sufficient data as to the velocity of these positive rays 
at different distances from the cathode for the question to be 
discussed quantitatively, but it is important to notice that the 
positive rays will always be under the action of the sheath for — 
a very short distance for they acquire their velocity a short 
distance from the cathode and in any case a positive ray is not 
so long lived as a cathode ray owing to the ease with which it 
may become an uncharged atom by recombination. It would not 
be surprising therefore if the effect of the sheath on the cathode 
rays swamped the opposite effect on the positive rays. It is 
possible that this is the explanation of the sudden rise in the 
curve at P in the graph. 
To summarize in conclusion the more obvious complicating 
etfects which must be taken into account when attempting an 
explanation of this hardening action: 
(1) The electrostatic screening action of the cathode beam. 
(2) The similar screening action of the positive bundle 
which surrounds the cathode beam. 
(3) The attraction which must exist between the positive 
and negative streams. 
