ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
629 
although the interval between two of the discharges considered in this paper may 
seldom exceed a thousandth part of a second, it is not surprising that it should be 
out of all proportion greater than the time actually occupied by the discharge. It is 
to this fact that we owe the isolation of the individual discharges of the sensitive 
discharge, which has formed the basis of the present investigation : an isolation which 
renders the examination of this type of discharge equivalent, as we have already men¬ 
tioned, to the examination of the electric spark itself. * 
We need not examine the magnitude of the period occupied by the whole discharge. 
It would be obtained by adding together a proper selection of the other small time- 
quantities with which we are dealing, and which are in reality the component parts of 
which the whole discharge is made up. And, further, it would possess no scientific 
value so far as the investigation of the mechanism of the discharge is concerned, for it 
is the relative duration of the various processes which go to constitute the complete 
discharge that it is important for us to learn. 
We shall next consider the magnitudes of the periods occupied by the positive and 
negative electricities in passing along the tube, by which we mean the interval that 
elapses from the instant of their emission from one terminal to the instant of their arrival 
at a point situated in the neighbourhood of the opposite terminal. And we shall first 
show that the time occupied by the passage of either electricity along the tube is of a 
greater order of magnitude than the time reqidred for it to pass along an equal length 
of wire. 
The experiments given in Section XIII. of our former paper suffice to demonstrate 
this proposition. For when a piece of tinfoil near the air-spark terminal was connected 
by a wire with a piece near the opposite terminal, the former derived at least as much 
relief from the latter as if it had not been on the tube, while the special effect was 
manifested at the latter. This showed conclusively that at the time the electric dis¬ 
turbance arrived at the former piece of tinfoil the latter was unaffected by it; and 
further that there was time for the impulsive electrical action which was exerted upon 
the former piece of tinfoil to be communicated along the wire to the latter, and to 
affect it and the tube near it before the electricity itself passed up within the tube to 
the place where the latter lay. A similar phenomenon was pi-esented when a long- 
strip of tinfoil was placed along the tube. In that case there was a gradual shading 
from the relief to the special effect. It will be remembered that in this way we got 
perfect examples of the typical positive effect near the negative terminal by means of 
impulses from a piece of tinfoil laid on the tube near the positive end (the air-spark 
being in the positive), showing that the positive impulses had had time to run along 
the wire, to form the hollow cone of positive luminosity by means of the positive dis- 
* In our previous paper an experiment was described in which the luminous phenomena of a single 
discharge from a coil were observed, such single discharge having the characteristics which an individual 
discharge of a sensitive' vacuum discharge must have. It was found to exhibit all the phenomena of 
sensitiveness. 
MDCCCLXXX, 
4 M 
