ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
567 
of tinfoil is then placed round the tube nearly midway between the ends, and is 
connected by a wire to one of the terminals of the smaller Holtz machine. The 
arrangement is then complete. Its electrical effect is easy to comprehend. When 
the terminals of the first machine are separated (those of the second machine being 
closed) the discharge from that machine passes through the tube in the form of an 
ordinary non-sensitive discharge wholly unaffected by the presence of the tinfoil or 
its connexion with the terminals of the second machine, inasmuch as these terminals— 
although serving as the channel through which passes the current generated by the 
second machine—are electrically inert for all purposes of interference. But when the 
terminals of the second machine are slightly parted a wholly different state of things 
is set up. Sparks pass from the one terminal to the other, and each such spark 
represents an impulsive alteration of electric tension in opposite directions at the two 
terminals. At the positive terminal it causes a sudden downfall of positive potential ; 
at the negative it causes a sudden downfall of negative potential, or as we might 
better describe it, a sudden rise of potential. And these changes communicate them¬ 
selves to all bodies in metallic connexion with these terminals in the form of sudden 
impulses of negative and positive electricity respectively. 
We will assume first of all that the tinfoil on the tube is connected with the nega¬ 
tive terminal of the smaller Holtz machine, i.e., the machine that is used for producing 
the interfering system. Then in the interval between the passage of two consecutive 
sparks between the terminals of the machine, the wire to the tinfoil and the tinfoil 
itself will be charging up with negative electricity. But when the spark comes this 
will disappear; in other words, there will be a change ecpiivalent to a sudden rush of 
positive electricity to the tinfoil. This will be repeated every time a spark passes, 
which may amount to many hundred times a second if the distance between the 
terminals of the machine be small. Thus we shall have a like number of sudden 
impulsive positive chargings-up in each second, the intervals between two consecutive 
chargings-up being occupied by a gradual and continuous (though of course very rapid) 
in-pour of negative electricity, the two actions neutralizing each other in each com¬ 
plete period. In other words, we shall have reproduced in the tinfoil ring precisely 
the action that goes on when we place a similar ring round a tube through which is 
passing an intermittent discharge with the air-spark in the positive, and connect it 
with the positive terminal. 
Since the electrical action within the tinfoil is identical in the two cases, it follows 
that the immediate consequences of that action will be alike, however much its ulti¬ 
mate effects may be modified by the difference in the electrical conditions in the 
interior of the tubes. And this is so. Through the influence of induction each one 
of these positive impulses causes a similar discharge of positive electricity from the 
inner surface of the glass beneath it, leaving on that surface a like quantity of nega¬ 
tive electricity. This becomes gradually freed during the interval which elapses before 
the arrival of the next positive impulse—an interval which, though actually occupying 
4 D 2 
