ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
5 77 
of vacuum tube ramifies in all directions throughout the whole of the phenomena 
which they present, but also because there is no doubt that an attentive study of it 
would throw great light on the nature and capabilities of a vacuum tube considered as 
a system capable of being affected by electrical influences. 
It is well known that a vacuum tube after being used to convey a discharge is often 
strongly charged ; the gas within the tube, therefore, or the interior surface of the 
glass can retain a considerable quantity of free electricity. And further than this, a 
vacuum tube is capable of giving considerable relief to a sensitive discharge in another 
tube, just as a conductor would, showing that the inductive discharges which take 
place within the tube have much the same effect that the displacement of electricity 
in a conductor would have. This is rendered evident by the inductive discharges 
that become visible in the neighbouring tube, which must represent relief given to the 
sensitive discharge. The exact amount of this effect is difficult to measure, but it is 
plain that the result is to make the tube act for the moment much as a Leyden jar 
would act in which the inner tinfoil was in connexion with earth, for the superfluous 
electricity on the inside of the tube, though it cannot be driven out of the tube, is 
driven off from the inner surface of the jar, and remains for the moment as a charge in 
the rarefied gas within the tube. 
The capacity of a vacuum tube to act as a relieving system is immensely increased 
by passing a continuous discharge through it. This is seen in experiments with the 
standard-tube. If we mark the effect produced on the tube under examination by 
connecting a piece of tinfoil upon it with a piece of tinfoil upon the standard-tube 
(which is of course a relief-effect) and then suddenly stop the discharge in the 
standard-tube, we shall see an immediate diminution of the effect. In positive-relief, 
for instance, we have found the discharge-effect pass into mere repulsion, a change 
which as we have shown in our previous paper indicates a diminution in the capacity 
of the relieving system. And in some cases this difference is very marked, showing 
that the passage of the continuous discharge greatly facilitates the redistribution 
within the tube of the free electricity which is developed by the action of induction 
from the discharges that are passing through the sensitive tube. That this should be 
the case is not surprising when one has regard to the phenomena which have been 
described in Section XIV. 
The attention of the authors of the present paper was drawn specially to the subject 
of the capacity of vacuum tubes as receivers of electricity by a very peculiar pheno¬ 
menon. They noticed that on touching tubes containing a sensitive discharge, severe 
shocks were sometimes experienced, while at other times no such shocks were felt. 
It was found that this was not due to any greater length of air-spark in the former 
case, for the two effects would occur with the same length of air-spark. On 
examination it was found that the severe shocks were felt when that terminal of 
the machine was connected to earth which was separated from the tube by the air- 
spark interval, but not otherwise (Plate 26, fig. 6). In other words, we may say that 
