[Footnote 1: From a recent communication made to the Physical
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
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 (Fig. 3), surrounded by a primary spiral, was contained in a bell
jar, and when the pressure of air in the jar was about that of the
atmosphere the secondary discharge occurred in the bulb, as is
ordinarily the case. On exhausting the jar, however, the luminous
discharge grew fainter, and a point was reached at which no secondary
discharge was visible. Further exhaustion of the jar caused the
secondary discharge to appear outside the bulb. The fact of obtaining
no luminous discharge either in the bulb or jar the author could only
explain on two suppositions, viz., that under the conditions then
existing the specific inductive capacity of the gas was very great, or
that a discharge could pass without being luminous. The author had
also observed that the conductivity of a vacuum tube without
electrodes increased as the pressure diminished until a certain point
was reached, and afterward diminished again, thus showing that the
high resistance of a nearly perfect vacuum is in no way due to the
presence of the electrodes. One peculiarity of the discharges was
their local nature, the rings of light being much more sharply defined
than was to be expected. They were also found to be most easily
produced when the chain of molecules in the discharge were all of the
same kind. For example, a discharge could be easily sent through a
tube many feet long, but the introduction of a small pellet of mercury
in the tube stopped the discharge, although the conductivity of the
mercury was much greater than that of the vacuum. In some cases he had
noticed that a very fine wire placed within a tube on the side remote
from the primary circuit would prevent a luminous discharge in that
tube.
[Illustration: FIG. 4.--Exhausted Secondary Coil of One Loop
Containing Bulbs. The discharge passed along the inner side of the
bulbs, the primary coils being placed within the secondary.]
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