184 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
will be found that by increasing the capacity of the system we can cause the relief- 
effect produced to vary from slight repulsion to the full to-earth effect. A very 
striking way of exemplifying this is by taking a thin copper wire, some 18 inches or 
2 feet in length, and fixing its two ends on two glass rods. Coil the wire round 
one till it is all wound round it, and place the other so that the end of the wire is in 
electrical contact with a piece of tinfoil placed on the tube at some spot where the 
discharge-effect is strongly manifested when the tinfoil is put to earth. The capacity 
of the system in such shape is so small that it will only produce a slight repulsion. If 
now the rod which is in contact with the tinfoil be held stationary, and the other be 
removed from it, the wire being allowed to unc'oil itself as the rods separate, it will be 
found that the repulsion gradually increases and passes almost imperceptibly into the 
discharge-effect. The removal of the other end of the wire from the tube has simply 
had the effect of increasing the capacity of the system to give relief to the tinfoil when 
under the action of the static induction of the electricity in the tube, and this, after 
causing the effect on the column to increase from faint to strong repulsion, further 
intensifies it by making it pass into the still stronger form of discharge-effect. 
There are other ways of passing continuously from the repulsion-effect to the 
discharge-effect; and these not only support our view of the identity of the two 
effects, but by comparison with the results just obtained throw some light upon the 
causes of these phenomena. If the finger be passed along a tube containing a 
discharge of considerable sensitiveness, it will usually be found that from the positive 
terminal to a certain distance from it the discharge-effect is produced, but beyond that 
the repulsion-effect. If we stop at any point where the discharge-effect is produced 
and vary the effect by substituting for the finger some conducting system capable of 
giving only imperfect relief, we shall find that the nearer we are to the positive 
terminal the more difficult it is to reduce the effect to mere repulsion ; in other words, 
the relief-effect tends to assume its more intense form the nearer we approach to the 
positive terminal. 
A third method of causing the repulsion-effect to pass continuously into the 
discharge-effect is by increasing the violence of the interruptions which render the 
current sensitive. If a very small air-spark be introduced into the “positive circuit” 
of an uninterrupted discharge, the luminous column will be rendered slightly sensitive, 
but such sensitiveness will be manifested solely in the form of the repulsion-effect. If 
the air-spark be increased so that the pulsations become more violent but less frequent, 
this sensitiveness will increase, and the relief effect near the positive terminal will 
assume the discharge form, while the rest of the luminous column will only show 
repulsion. If the air-spark be still further increased, it will be found that the limits of 
the discharge-effect gradually extend themselves further and further along the tube 
until the whole column is capable of manifesting it; and this state of things remains 
until the interruptedness of the discharge becomes so great by reason of the length of 
air-spark, and the consequent slowness of the intermittence, that the discharge 
