ON ELECTRICAL DISCHARGES THROUGH RAREFIED GASES. 
187 
from the positive terminal is brought to an abrupt termination and ends in a sharply 
defined head, somewhat rounded at the extremity. Around this there is a well 
marked hollow cone of luminosity, springing from the inside of the tube immediately 
beneath the tinfoil, bright and sharply defined on the outside but hazy and blue on 
the inside, which is turned to and in fact surrounds the termination of the positive 
column above described. This hollow cone does not come to an apex on its external 
surface, but passes into a luminous column which stretches away towards the negative 
terminal of the tube, and supplies the place of the former luminous column, which 
it resembles in all respects (Plate 16, fig. 10). When the air-spark is considerably 
increased the truncated luminous column is very much altered; but as the various 
forms presented by the phenomena will require a close examination at a later stage we 
shall not detail them here. 
The one thing to which it is necessary to call attention at the present stage in regard 
to these effects is their complete dissimilarity to those we have described as relief-effects. 
As we have shown in a previous section, each part of a tube in which a sensitive dis¬ 
charge is passing has a standard relief-effect called the to-eartli effect, produced by con¬ 
necting it with a conducting system of practically infinite capacity. A continuous 
gradation of relief-effects from zero to this to-eai'th effect can be obtained by limiting 
the capacity of the relieving system so that it can no longer be subjected to the 
influence of the electrical disturbances that take place in the tube without its potential 
undergoing material alteration. Now, if we compare the appearances described in this 
section with any member of this chain, we see they are w T holly dissimilar, so that we at 
once learn that these effects are not due to the relieving capacity of the system with 
which the tinfoil is now connected, but to some special electrical interference to which 
the new arrangement has given rise. It is on the ground of this dissimilarity that we 
shall call these effects special or non-relief effects; and it must be borne in mind that 
these effects are only obtained when we connect the tinfoil with the air-spark terminal 
or make use of an arrangement electrically equivalent to this. 
If the negative be the air-spark terminal, there will be found to be an equally great 
contrast between the effects obtained by joining the tinfoil to earth and to the air- 
spark terminal. As before, the relief-effects can be obtained by joining the tinfoil to 
any conducting system of sufficient capacity, whatever be its potential, provided that 
it does not undergo variations of potential co-periodic with those within the tube. 
The special or non-relief effects can, on the other hand, be only obtained by connecting 
the tinfoil with the negative terminal. It will not, however, be necessary here to 
examine or contrast these effects, as they will subsequently be shown to fall under a 
general rule; and it suffices here to state that the contrast above referred to is found 
to exist. 
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