01ST ELECTRICAL DISCHARGES THROUGH RAREFIED GASES. 
215 
actually prefers to return by the way it came rather than to pass through the tube to 
the other terminal. Such discharges we shall term unipolar discharges. 
This unipolar discharge is of course intermittent, and therefore sensitive. If we 
take a glass rod with a piece of tmfoil at the extremity electrically connected by a 
wire with the positive terminal of the tube, and hold it near to but a little beyond 
the end of the luminous column, we shall find the luminous column driven back ; 
and by carefully advancing it towards the positive terminal we can often succeed in 
driving the luminous column wholly back and preventing any visible discharge taking 
place into the tube (Plate 19, fig. 20). The explanation of this is obvious. At the 
moment that the charging-up, which causes the discharge, takes place in the positive 
terminal, there is also a charging up in the tinfoil, and this by its inductive effect 
tends to prevent the advance of any free positive electricity. Thus, however rapid 
the pulsation, the force tending to oppose the discharge keeps exact time with it, and 
causes the heading back of the luminous column. If the tinfoil and wire be connected 
with earth, or otherwise made a relieving system, we find the usual to-earth-effects 
produced on this unipolar discharge. 
A form of these experiments, which is in some respects even more striking, is obtained 
by taking a tube with an intermediate terminal and connecting the intermediate and one 
of the end terminals with the positive terminal of the machine, and the other terminal 
of the tube with the negative terminal of the machine. Interpose an air-spark in the 
positive circuit so that it forms part of the path to both of the terminals which were 
connected with the positive terminal of the machine. With an air-spark of proper 
dimensions it will be found that while the whole effective current passes from the posi¬ 
tive intermediate terminal to the negative terminal, there is seen, besides, first a tongue¬ 
shaped luminous column extending from the positive end terminal towards the inter¬ 
mediate terminal; and secondly a similar tongue-shaped luminous column stretching 
out from th'e intermediate terminal to meet it (Plate 19, fig. 21). Or again, if we 
arrange two tubes as first described, and connect both terminals of the second tube 
with the positive terminal of the machine, we shall have two positive unipolar columns 
as before. These two do not join; and it is clear that here we have naturally the same 
effect as that obtained by the use of the tinfoil in the former case (Plate 19, fig. 22). 
Each of these discharges acts repulsively on the other, and they drive each other back. 
If we use the tinfoil, as in the former experiment, we can drive each in turn back 
towards, and sometimes into its terminal, and, within considerable limits, when one 
column is driven back, the other advances, and vice versa. 
This experiment with the intermediate terminal, shows very forcibly how 7 the 
discharge from an air-spark terminal depends solely on the forces at work at the 
terminal itself and has but little reference to the condition of the other terminal of 
the tube. We see here that the positive electricity from the intermediate terminal 
actually issues copiously in the direction in which lies not only no negative terminal, 
but actually a positive terminal, which ultimately succeeds in repelling its advance. 
