Society, London.]

By Prof. J.J. THOMSON, M.A., F.R.S.


[Illustration: FIG. 1.--Coil of Glass Tube for Vacuum Discharge
Experiments. The primary coils are filled with mercury, the secondary
coils form continuous closed circuits.]

The phenomena of vacuum discharges were, he said, greatly simplified
when their path was wholly gaseous, the complication of the dark space
surrounding the negative electrode and the stratifications so commonly
observed in ordinary vacuum tubes being absent. To produce discharges
in tubes devoid of electrodes was, however, not easy to accomplish,
for the only available means of producing an electromotive force in
the discharge circuit was by electromagnetic induction. Ordinary
methods of producing variable induction were valueless, and recourse
was had to the oscillatory discharge of a Leyden jar, which combines
the two essentials of a current whose maximum value is enormous, and
whose rapidity of alternation is immensely great.

[Illustration: FIG. 2.--Exhausted Bulb Surrounded by Primary Spiral
Consisting of a Coiled Glass Tube Containing Mercury.]

[Illustration: FIG. 3.--Exhausted Bulb Surrounded by Primary Coils,
Inclosed in Bell Jar.]

The discharge circuits, which may take the shape of bulbs, or of tubes
bent in the form of coils, were placed in close proximity to glass
tubes filled with mercury, which formed the path of the oscillatory