222 A PHENOMENON ACCOMPANYING THE ELECTRIC DISCHARGE XII
tradicted, more directly than by these somewhat ambiguous
experiments, by the shapes which the jet takes up when the
discharge apparatus is completely altered.
12. By shortening the tube more and more, and changing
the distance and form of the electrodes, we may continuously
change the discharge apparatus so far used into any other form
we please; the jet then changes its shape, but does not dis-
appear, rather passes continuously into other forms. It is to
be observed that the discharge apparatus hitherto used has the
advantage of all others, only because it separates the appear-
ance considered from the mass of the luminous effects of the
discharge. The forms which occur are very various and often
very elegant; my observations do not suffice to represent them
in order. In general their shape appears to depend on the
direction of the current, and it is clearly seen that the portions
of gas set in motion have velocities along the path of the
current, of which the cause cannot be sought merely in the
rise of temperature.
Є
A sufficient confirmation is afforded by
the single example which I will mention
here. When we allow the jar discharge
to pass between spherical electrodes not
too far apart, the appearance analogous
to the jet is a bulge surrounding the
centre of the spark path (Fig. 27 a, a).
Its colour, like that of the jet, is yellow at
low pressure, reddish-brown at atmospheric
pressure. With this last tint the bulge
can, with some care, be seen on every
spark which passes between the electrodes
of a Holtz machine, when its condensers
(which must not be too small) are used.
The apparatus described in § 7 gives
interesting information as to the produc-
tion of the bulge. First the bright
straight spark appears, and during its
presence the yellow is still absent or
cannot be seen owing to the dazzling of the eye; it is followed
by the aureole (Fig. 27, b, B), which proceeds from the positive
electrode as a red band surrounded by the yellow light a; the
latter, somewhat more than halfway, banks itself up into a wall
d+
FIG. 27.