G28 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
These small time-quantities are as follows :— 
1. The period occupied by a discharge. 
2. The interval between two discharges. 
3. The time occupied by the discharge of the positive electricity from its ter¬ 
minal. * 
4. The time occupied by the discharge of the negative electricity from its 
terminal. 
5. The time occupied by molecular streams in leaving a negative terminal. 
6. The time occupied by positive electricity in passing along the tube. 
7. The time occupied by negative electricity in passing along the tube. 
8. The time occupied by the particles composing molecular streams in passing 
along the tube. 
9. The time occupied by electricity in passing along a wire of the length of the 
tube. 
To these there might be added the time that electric induction occupies in travers¬ 
ing a finite space. This we have taken to be zero, for we have not been able to dis¬ 
cover any symptoms of its being durational; or perhaps we should rather say that we 
have considered it either negligible in comparison with any of the above quantities, or 
included in them so as to be indistinguishable in order of magnitude from the shortest 
of them. 
Taking, then, the small time-quantities which are enumerated above, we know, in the 
first place, that the interval between two discharges is incomparably greater than any 
of the other small quantities to which we have referred. This is shown by the 
revolving mirror. For although this instrument easily separates the intermittent 
flashes, it never shows any splaying out in the luminous phenomena of the individual 
discharges ; for the haziness which it sometimes shows as attendant on the phos¬ 
phorescence does not testify to any durational character in the period of arrival of the 
molecular streams, but only to the power of the glass to retain phosphorescence for a 
short time after the exciting cause of it has ceased. We may therefore take the 
interval between two discharges as by far the largest of all the small quantities of 
which we have spoken. 
This conclusion is what we should naturally have anticipated. For although we 
are not at present in a position to say whether a discharge through a vacuum tube 
occupies a greater or smaller period of time than an electric spark in air, yet they are 
phenomena of a like nature, and they probably occupy periods which are to some 
degree similar in point of duration. Now it is well known that the time occupied by 
an electric spark in air is almost inconceivably small. However fast a wheel be 
rotating, it will appear to be at rest when illuminated by an electric spark. And 
* It will of course tie understood that under the term “terminal” are included all sources of electrical 
discharge, whether effective terminals or quasi-terminals. 
