ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
573 
It must not be thought that the arrangement described above is the only arrange¬ 
ment by which these effects can be produced. Instead of the Holtz machines, any 
two sources of high tension electricity giving out continuous currents may be used, 
providing that it is possible, as must generally be the case, to render the interfering 
system intermittent, either by the introduction of an air-spark or otherwise. For 
some purposes it is very convenient to use a current from a large condenser which is 
filled by a coil for illuminating the tube. Other arrangements might be made to give 
the rapid impulsive electrical actions which are requisite in the interfering system. 
But all these modes of arriving at the results would be electrically equivalent, and do 
not require further mention here. 
XV .—On the standard-tube method of examining intermittent vacuum discharges. 
The results given in the last section have been utilized by the authors of the present 
paper to furnish them with a new method of testing the intermittence of vacuum 
discharges, which has been, and promises still to be, of very great utility in the 
investigation of the mechanism of such discharges. It differs essentially from almost 
all the modes of testing intermittence described in the former paper in that the 
luminous effects, by which the nature of the intermittence is recognised, are produced, 
not in the tube under examination, but in a separate tube which may be chosen on 
account of its manifesting such effects very readily, and which, being used throughout 
the whole of the experiments, may be termed the standard-tube. This avoids the great 
difficulty that otherwise must have been faced in extending the results of our former 
paper to vacua of a different character to those used in the experiments there 
described. So long as the nature of the intermittence could only be judged from the 
appearances in the tube itself, each separate tube presenting phenomena in any way 
differing from those which had been previously observed and classified, had to be 
subjected to a separate examination until the observer became familiar with its 
peculiarities. But with the new method this labour is avoided. Only one tube has 
to be known thoroughly, and all others, however much they may differ among them¬ 
selves, are made to express the nature of the intermittence of the discharge that is 
passing through them in terms of the appearances in the standard-tube. 
the negative terminal of the interfering machine, it was found that the formation was rendered more 
perfect by leading a wire from the positive terminal of the interfering machine to a point on the tube 
between the rings near the stria due to the first ring (i.e., that nearest the negative end of the tube). 
This, in fact, supplied synchronous impulses of negative electricity in the manner figured in fig. 16 a of the 
plate above mentioned. The circumstances under which these auxiliary impulses were found useful were 
doubtless those of an excess of positive thrown in from the surface of the tube below the tinfoil, or as it 
may also be described a deficiency of negative to satisfy the positive coming up from the second ring. 
On the one hand these facts supply an interesting corroboration of what has been said above ; and on 
the other they illustrate the additional power which the present method (viz.: that of an independent 
source of electricity) furnishes for experiments on interference in general, and especially with discharges 
not in themselves sensitive. 
4 E 
MDCCCLXXX. 
