190 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
Tlie first takes place at the moment that the electricity in the tube passes the spot 
where the tinfoil is; the second keeps time with the charging-up of the terminal 
which leads to the discharge, and is therefore most intense at the moment of 
discharge. And, further, we have the experimental fact that under ordinary circum¬ 
stances these effects are of a diametrically opposite kind. 
This is, perhaps, the most suitable place for explaining the connexion between 
intermittence and sensitiveness in vacuum discharges. The effects observed in 
sensitive discharges have been traced in the preceding part of the paper to the 
effect of static induction from the free electricity that passes into the tube at each 
pulse. Now, in order that this static induction outside the tube may be strong, it 
is clearly necessary that the quantity of free electricity within the tube while the 
discharge is passing should be great. In other words, the current must pass in large 
pulses and not in driblets. This is exactly what intermittence effects. The electricity 
is penned back until it lias accumulated in large quantities, and then in one pulse it 
bursts into and sweeps through the tube. In the so-called continuous current, also, the 
electricity doubtless passes into the tube in pulses, but as these pulses are perhaps 
many thousand times more numerous in any given time than is the case with the 
sensitive discharge, the quantity in each pulse is proportionately smaller, and is thus 
insufficient to produce a static induction which is of sufficient magnitude to be capable 
of causing in its turn discharges within the tube. Hence the so-called continuous 
current is really a sensitive current of infinitely small sensitiveness. The difference 
between the two is much the same as between the effect of letting the superfluous 
waters of a river overflow a weir each moment, and of penning them back for a whole 
day and then suddenly letting them free. 
VII.— Examination and interpretation of the special or non-relief-effect when the 
positive is the air-spark terminal. 
For the purposes of this section we shall take first the form of special or non-relief- 
effect already described. It is a very marked form, but by no means an unusual 
one, and indeed it is probable that it can be obtained in a more or less complete form 
in most vacuum tubes ; and it possesses, as we shall see, the important property 
of being the typical form from which all others can be derived by modifications of 
known and definite kinds corresponding to peculiarities in the circumstances of the 
tube or the discharge. 
If we place round the tube a narrow ring of tinfoil, and connect it with the positive 
terminal (where the air-spark is supposed to be) by a wire passing at a sufficient 
distance from the tube to prevent its directly affecting the luminous column, the 
following appearances will be noticed 
(f.) The column which starts from the positive terminal will be found suddenly 
to terminate at the tinfoil ring in a bright column of small diameter 
