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I879-J 
PROCEEDINGS OF SOCIETIES. 
Royal Society, May 8.—“ On the Sensitive State of Eletfbrical 
Discharges through Rarefied Gases,” by William Spottiswoode, 
P.R.S., and S. Fletcher Moulton. It has frequently been re- 
marked that the luminous column produced by eledbric discharges 
in vacuum tubes sometimes displays great sensitiveness on the 
approach of the finger, or other conductor, to the tube. This is 
notably the case when with an indudbion-coil a very rapid 
break is used, or when with any constant source of eledbricity an 
air-spark is interposed in the circuit leading to the tube. The 
striking character of the phenomena, and the opportunity which 
they showed for affedbing the discharge from the outside during 
its passage, led the authors of this paper to consider that a spe- 
cial examination of this sensitive state would be desirable. If a 
conductor be made to approach a tube conveying a sensitive 
discharge, due to an air-spark in the positive branch of the cir- 
cuit, a series of effedbs is produced, of which the feeblest and the 
strongest are the most pronounced. In the first case, the lu- 
minous column is repelled by the conductor ; in the second, it 
is broken into two parts which stretch out in two tongues to- 
wards the point on the tube nearest the conductor, while a nega- 
tive halo appears between them. These effedts are proved to be 
due to the indudbive adbion of the conductor, or more particularly 
to re-distributions of eledbricity in it, co-periodic with the air- 
spark, and not to any permanent charge. Instead, however, of 
connedbing a point on the tube with a large condudtor or with 
earth, it may be connedbed with one or other terminal of the tube. 
And a further study of the subjedb shows that all the phenomena 
due to adbion from without may be produced by means of one or 
other of these connexions. The effedbs described need not be 
confined to a single patch or ring of conducting material placed 
upon the tube ; but they may be produced many times over in 
the same tube by a series of rings arranged at suitable distances. 
By this means the column may be broken into a series of sec- 
tions, all terminating with well-defined configurations towards 
the negative end, and having greater or less length, according to 
the position of the rings. The authors give evidence, derived 
mainly from the revolving mirror, and from the discharges of a 
partially charged Leyden jar, for the following conclusion : — 
That the passage of the discharge occupies a time sufficiently 
short in comparison with the interval between the discharges to 
prevent any interference between successive pulses. Certain 
experiments are then described which indicate that the discharge 
is effedbed, under ordinary circumstances, by the passage through 
VOL. IX. (N.S.) 2 G 
