574 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
The way in which this is attained is as follows :—We have seen in the preceding 
section that rapid electrical impulses of a positive or negative type upon the outside 
of a tube carrying a continuous discharge, produce within the tube the familiar 
positive and negative effects respectively; that is to say, the phenomena of positive- 
special and relief effects. Now it is clearly a matter of indifference in what way these 
rapid electrical impulses are produced, provided that they have the sharpness and 
rapidity requisite for producing the luminous effects which are associated with sensitive 
discharges. Hence, instead of producing these impulses in the manner described in 
the previous section, it suffices to bring the wire from the tinfoil into contact with the 
surface of a tube through which a sensitive discharge is passing. If that sensitive 
discharge be one of positive intermittence, the positive charges that burst through the 
tube will drive off like discharges of positive electricity along the wire that is in 
contact with the tube, and we shall have positive effects (that is to say, phenomena of 
the same class as positive-special effects) in the standard-tube, i.e., the one through 
which the continuous discharge passes. If, on the other hand, the sensitive discharge 
be one of negative intermittence we shall have negative effects in the standard-tube, 
i.e., phenomena of the type of negative-special or positive-relief effects. Thus, the 
nature of the intermittence in the sensitive discharge will express itself in the appear¬ 
ances in the standard-tube. Nay, it is not only the type of the intermittence in the 
sensitive discharge that wall thus be indicated by the standard-tube. It is clear that 
the violence of that intermittence will affect the result, and this may be ascertained 
by examination of the standard-tube ; and there is no doubt that all other qualities 
which a sensitive discharge can possess will in some way or other express themselves 
in the phenomena which they cause in the standard-tube. 
It will be seen from what we have already stated that if the appearances in the 
standard-tube were thoroughly known it would be possible to read with accuracy the 
exact nature of the electrical disturbances that are going on in any tube. It is 
probable that at some stage of the investigation into the mechanism of vacuum 
discharges it may be necessary to do this, and it will then be desirable to ascertain 
what form of tube, what kind of gas, and what state of exhaust is best for a standard- 
tube. But at present this test has only been used by the authors of this paper to 
establish certain broad fundamental principles ; and as this could be done under 
circumstances very favourable to the action of the test, they have not yet pursued 
these questions further. They have, however, tried various sources for the continuous 
discharge that is used in the standard-tube, and on the whole they find that the most 
sensitive is obtained from a large condenser maintained at a very low state of charge 
so that the discharge through the standard-tube is not great in quantity. The use of 
a Holtz machine for the source of the discharge in the standard-tube has, however, so 
great an advantage in the matter of convenience that most of their experiments have 
been tried with discharges so produced. 
The arrangement, therefore, is as follows (Plate 25, fig. 4) :—The standard-tube, 
