ON ELECTRICAL DISCHARGES THROUGH RAREFIED GASES. 
207 
individual pulse, and the repetition in successive pulses only serves to make them 
appear to the eye as continuous phenomena. 
The most valuable direct evidence of the velocity of the discharge through the tube 
and the independence of each discharge is obtained from the examination of the 
discharge by a revolving mirror. For this purpose a revolving four-sided prism, 
whose sides consisted of glass platinised on the upper surface, and which was rotated 
by hand at a rate of from 400 to 800 revolutions per minute, was made use of. It 
was found that this instrument was capable of showing the intermittence of the 
current when in a sensitive state however small the air-spark, showing that the 
superior limits of the rapidity of the pulsations when produced by the aid of an air- 
spark is by no means a very high one. The difficulty in increasing the rapidity of 
the pulsations arose from the tendency of the heated poles of the air-spark to produce 
an arc between them if they are kept too close to one another.* The revolving mirror 
was used in the ordinary way by placing a narrow slit upon the tube so that the 
image of the discharge was a bright line about one-eighth to one-sixteenth of an inch 
wide. On making the mirror revolve rapidly the appearance in the mirror consisted 
of parallel bands alternately bright and black ; the black bands becoming broader 
the greater the rapidity with which the mirror revolved and the less the frequency 
of the discharges. 
On trying the above experiments it was found that the discharge could always be 
resolved into such black and bright bands. There was no broadening out of the bands 
discernible, showing, so far as such means of observation were capable of demon¬ 
strating it, that the discharges were instantaneous. When the air-spark was reversed, 
i, e ., thrown from the positive into the negative, a change occurred in the appearance 
of the bands; but it was difficult to define the essential characteristics of such change. 
They seemed to consist in a slight variation of the circumstances of the discharge 
caused by what may be termed a want of electrical symmetry in some portion of the 
circuit rather than to any change in the nature of the discharge due to any funda¬ 
mental contrast in the properties of the two electricities. The definition seemed 
to be sharper when the air-spark was in the negative than when it was in the 
positive. 
The negative end of the tube was first observed, the slit used being sufficiently 
long to allow the negative pole, the negative dark space, and the negative end of 
the positive column to appear in the field at the same time. On the mirror being 
set to revolve, the black and bright bands appeared generally to extend in straight 
lines through the whole field right across the negative dark space, although they grew 
faint and sometimes even invisible in the region of the negative dark space. 
* The air-spark in these experiments passed between two small platinum spheres. Perhaps a valuation 
of the form of the poles might give air-sparks whose periodicity would be much more rapid. This 
tendency to form an arc was most strongly marked wben the air-interval was in the negative part of the 
circuit. 
