, 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
discharge. The parts thus corresponded to the windings of an induction
coil, the vacuum tubes being the secondary and the tubes filled with
the mercury the primary. In such an apparatus the Leyden jar need not
be large, and neither primary nor secondary need have many turns, for
this would increase the self-induction of the former and lengthen the
discharge path in the latter. Increasing self-induction of the primary
reduces the E.M.F. induced in the secondary, while lengthening the
secondary does not increase the E.M.F. per unit length. Two or three
turns (Fig. 1) in each were found to be quite sufficient, and on
discharging the Leyden jar between two highly polished knobs in the
primary circuit, a plain uniform band of light was seen to pass round
the secondary. An exhausted bulb (Fig. 2) containing traces of oxygen
was placed within a primary spiral of three turns, and, on passing the
jar discharge, a circle of light was seen within the bulb in close
proximity to the primary circuit, accompanied by a purplish glow,
which lasted for a second or more. On heating the bulb the duration of
the glow was greatly diminished, and it could be instantly
extinguished by the presence of an electromagnet. Another exhausted
bulb (F