1909-10.] Observations on some Spark-Gap Phenomena. 
219 
IX. — Observations on some Spark-Gap Phenomena. By John 
M‘Whan, M.A., George A. Clark Scholar of the University of 
Glasgow. Communicated by Professor A. Gray, F.R.S. 
(MS. received November 26, 1909. Read January 10, 1910.) 
In a recent note to the Royal Society of Edinburgh,* Dr Dawson Turner 
remarks on a peculiarity observed by him in the behaviour of an induction- 
coil spark-gap when a piece of mica, glass, sulphur, ebonite, etc., is placed 
between the terminals, near or against the + one. The surprising variety 
of the phenomena exhibited on the interposition of a solid dielectric plate 
between the two poles of such a coil (or between the poles of an influence 
machine) may make, in view of Dr Turner s note, the following observa- 
tions of some interest. The observations in question were made during the 
course of an inquiry into the causes of the Lullin effect (according to which 
a thin dielectric plate interposed between two sparking terminals not 
opposite to one another is, as a general rule, perforated at the negative pole), 
and the apparatus employed consisted essentially of the details shown in 
fig. i., viz. of a motor-driven influence machine whose terminals were con- 
nected through a battery of Leyden jars in cascade to a specially designed 
spark-gap which allowed of any three-dimensional motion of the electrodes 
being accurately measured, and of any desired type or form of electrode 
(spherical, conical, disc type, etc.) being inserted at will. A second spark- 
gap, fitted with standard spherical electrodes and a micrometer screw motion 
to widen or narrow the gap and measure it accurately, was connected in 
parallel with the first. By adjusting this second just to spark synchronously 
with the first, and no more, the potential difference of the first could 
easily be deduced, no matter in what sort of medium it was immersed. f 
This method was found preferable to the use of an electrometer, very high 
potentials such as 100,000 volts being determined rapidly and with sufficient 
accuracy. In using such high potentials, even heavily insulated wires leak , 
i.e. give off, at weak points in the insulation, strong brush discharges into 
the surrounding air. These discharges are facilitated by abrupt bends in 
the wire, or by the proximity (within a few feet) of foreign bodies. The 
wires were therefore kept as straight or as gently curved as possible, were 
embedded in paraffine wax, and the main leads in addition enclosed in long 
* Proc. Roy. Soc. Edin ., 1908-9, vol. xxix. p. 414. 
t Landolt, Bornstein, and Meyerlioffer, Physikalische- Chemische Tabellen, p. 778. 
