ON ELECTRICAL DISCHARGES THROUGH RAREFIED GASES. 
175 
usually followed by an apparently complete cessation of discharge; it then recom¬ 
menced, and continued to exhibit during its entire period of existence variations of a 
very irregular kind. It is, in fact, probable that during the period subsequent to the 
first outburst and the momentary cessation of discharge that often follows it, the state 
of the tube is much the same as it would be during a continuous discharge, in 
consequence of the absence of any regularity of pulsation. The tube is always 
conducting a discharge, although perhaps the consequent disturbance is more violent 
in one part than in another. There is no alternation of a sharply-defined discharge and 
electrical emptiness. So important is the distinction between what may be called 
irregularly fluctuating and periodically intermittent discharges, that in the case of 
a fairly rapid coil discharge it is frequently found that, while the striae in the main 
luminosity give but very slight traces of sensitiveness, the faint ghostly striae which 
are seen to project far beyond the end of the positive column proper show very great 
sensitiveness (see Plate 16, fig. 6)."* And the experiments above referred to with 
the revolving mirror show that this faint column of luminosity is due solely to the first 
outburst of which we have spoken; so that such outbursts form a sensitive system, 
while the fluctuating discharges that follow are non-sensitive. As the action of the 
break gets more rapid it is found that the fluctuating portion of the discharge becomes 
less and less developed, and the discharge more and more nearly consists of the first 
portion only. It is a matter of observation that as we attain to still higher rapidity of 
break-action this becomes more and more the case, and that at very high speeds the 
discharge reduces itself to this first portion alone; and thus we understand how it is 
that with a very high-speed break we obtain the most intensely sensitive discharges 
from a coil. 
It will thus be observed that all the methods of producing the sensitive state agree 
not only in the intermittent character of the discharge, but also in the shortness 
of duration of the individual discharges themselves,! and this it is which has induced 
the authors of this paper to consider brevity of duration as much an essential feature o±‘ 
the individual discharges that produce the sensitive state, as rapidity or regularity of 
interval between these discharges. The latter characteristics are, in fact, of more 
importance for maintaining the persistence of the sensitive state during a finite interval 
of time than for actually producing it, since careful experiments fail to show any 
inferior limit of rapidity of the periodicity necessary to produce such a discharge. In 
* These faint sensitive striae projecting beyond the luminous column in a coil discharge, are the only 
case of a sensitive discharge of which the authors of the present paper are aware which is not produced by 
one or other of the methods previously described. 
t These remarks may seem to be inapplicable to the case of the Holtz machine with the wheel-break; 
for here the intervals during which the tube forms part of the circuit are usually equal to those during 
which it is not so. But it does not follow that the electricity is passing through the tube during the whole 
of the time that the tube is in circuit. For it will not commeiice to pass until there has been such a 
charging up of the terminals as is sufficient to effect a discharge. 
