182 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
IV .—The relief-effect (when the intermittence is effected near the positive terminal )* 
assumes the form either of repulsion or of discharge from the interior surface of 
the glass. These two effects are identical in nature, and the form actually 
assumed depends, in the same tube., solely on* the intensity of the action which 
calls it forth. 
We shall in the present section suppose that the air-spark or wheel*break, or other 
contrivance used to cause intermittence, is placed between'the positive terminal of the 
tube and the source of electricity. This being so, the most cursory examination of 
vacuum-tubes enables us to distinguish two types of effects when a conductor is made 
to approach to the tube. In the one case the luminous column is repelled by the 
conductor ; in the other the luminosity appears to leap towards the conductor, usually 
in two tongues, which approach, but do not necessarily touch the glass, one on each 
side of the place where the conductor most closely approaches the glass, while between 
the two tongues there appears a luminous haze similar to that which ordinarily sur¬ 
rounds the negative terminal. We shall call these respectively the repulsion-form and 
the discharge-form of the relief-effect (Plate 16, fig. 7). 
Further examination show's that these two forms of the relief-effect! include all 
cases. It is not uncommon to find both forms present at once. The luminous column 
is not always wdiolly interrupted ; the two tongues which branch from it towards the 
conductor being often connected at them bases by the remains of the luminous 
column. This connecting piece usually shows signs of having been repelled by the 
conductor, and though not remarkable in other respects, it is in most cases less bright 
than the remainder of the luminous column. The discharge-form again is often 
destitute of one or other of its members ; the two tongues may be very feeble, and the 
haze may be so faint as to be practically invisible. The magnitude of the repulsion- 
effect also varies greatly, depending, as will be seen, upon all the causes which are at 
work in producing the sensitive discharge. In many cases the sensitiveness is only 
indicated by a slight depression of the luminous column when the finger is placed on 
or very close to the glass; in others, the finger cannot be brought anywdiere near the 
glass without the luminous column being strongly repelled. But these variations do 
* In thus examining the effects produced when the air-spark is in the “positive circuit” (he., between 
the positive electrode and the source of electricity), before dealing with the matter in its more general 
form, the authors of this paper are following the actual course of them investigations. It has been thought 
advisable so to do because the class of effects examined in this section fully merit a separate treatment, 
and are more conveniently dealt with in this way than in connexion with the more general theory. 
f By using the term relief-effect, the authors of this paper do not mean anythin^furtlier or other than 
the effect, on the luminous discharge, of a neighbouring conductor, whose condition is such that its potential 
remains more or less completely unchanged by the electric disturbances in the tube. No theory of the 
modus operandi of these effects upon the luminous discharge is intended to be conveyed in the term. It 
was selected because the electric strain in the tinfoil was relieved by the process of joining it to a larger 
conductor, and has been retained because no more convenient term has been found. 
