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