ON ELECTRICAL DISCHARGES THROUGH RAREFIED GASES. 
217 
negative terminal, forming part first of tlie discharge that occupies the further por¬ 
tion of the tube, and then of that which occupies the part of the tube nearer to the 
positive terminal. 
We have already spoken of the results of connecting a small condenser with one 
of the terminals of the tube, when a coil discharge is used, so as to depress the 
electrical tension produced at that terminal, and shift the position of the neutral 
zone. It is very instructive to compare these effects with the analogous effects in 
the case of unipolar discharges. If we join the effective terminal of the tube con¬ 
taining the unipolar discharge to a small condenser composed of, say, two pieces 
of tinfoil about three square inches in area, with a plate of mica between them, we 
shall see the luminous tongue slightly shorten. If, now, we connect the other side 
of this condenser to earth we see a further shortening of the column, which will often 
almost disappear. If we connect the terminal with a larger condenser or a Leyden 
jar the discharge wholly disappears. Thus we see that the condenser or Leyden jar 
has, just as in the case of the coil discharges, the effect of muffling or toning down 
the intensity of the impulsive changes of electrical tension at the terminals and 
thereby lessening the violence of the discharges into the tube. 
We conclude, then, from these experiments that the independence of the discharge 
from each terminal is so complete that we can at will cause the discharges from the 
two terminals to be equal in intensity hut opposite in sign (as in the case of the 
coil) or of any required degree of inequality (as in the case of the coil with a small 
condenser). Or we can cause the discharge to he from one terminal only, the other 
terminal acting merely receptively (as in the case of the air-spark discharge) ; or 
we can cause the discharge to pass from one terminal only and return to it, the 
other terminal not taking any part in the discharge; or, finally, we can make the 
two terminals pour forth independent discharges of the same sign each of which passes 
back through the terminal from whence it came. 
XIII .—On the state of the tube during the occurrence of the discharge. 
We have already shown that each pulsation may be considered as a separate 
disturbance the effects of which have passed away before the next pulse comes. It 
now remains for us to ascertain what is the state of the tube during the continuance 
of the almost instantaneous pulse which may be considered as the unit of which the 
visible discharge is built up. 
For the purpose of experimentally ascertaining this, it is obvious that if we employ 
any tests which are not themselves affected with the same periodicity as the discharge 
that is being examined, we shall get no results that speak with any degree of 
certainty of the state of the tube at the instant in question. Thus, if we listen 
with a telephone in circuit between the earth and a piece of tinfoil on the tube, we 
hear a sound identical in pitch with that of the air-spark, but this only tells us 
MDCCC LX XIX. 2 F 
