XII A PHENOMENON ACCOMPANYING THE ELECTRIC DISCHARGE 217
the anode by the dark space, one or more millimetres
wide, and from its end to the anode the path of the current
is marked by a red band 1 to 2 mm. in diameter. For both
directions of the current this band occupies the greater
part of the length of the glass tube, and at its opening
bends round sharply towards the electrode outside. But in
addition I observed a jet, brownish-yellow in colour, and
sharply defined, which projected in a straight line from the
mouth of the tube; it was some 4 cm. long, and its form was
like that shown in the drawing, Fig. 25. The greater portion
of the jet appears to be at rest, and only at the tip does it
split into a few flickering tongues. The jet
does not change its shape appreciably when
the current is reversed. But when a Leyden
jar is joined up, an important change occurs:
the jet becomes brighter, and is straight for a
distance of only 1 to 2 cm.; then it splits up
into a brush of many branches, which are
violently agitated and separate in all directions,
in the way shown in Fig. 26.
FIG. 26.
If the
2. If we increase or diminish the pressure
of the air, neglecting for the present the
effect of the jar, then in both cases the
jet becomes less striking, but in different ways.
pressure be increased, the path of the spark no longer
completely fills the cross-section of the mouth of the tube,
neither does the escaping jet do so, but it only emerges at
that side of the mouth where the spark appears; it becomes
narrower, shorter, and assumes a darker, reddish-brown tint.
If the pressure be diminished, the jet is again shortened, but
at the same time it widens out, and assumes a lighter yellow
tint and becomes less bright. When the first striæ form in
the tube, it is only just perceptible, and then occupies a small
hemispherical space just outside the mouth of the tube.
When a Leyden jar is used, a similar succession of appearances
is observed, but the greatest development occurs at smaller
pressures, and it is advisable to choose a wider-mouthed tube.
I obtained the most striking forms in air with the follow-
ing arrangement. The glass tube was 5 mm. wide and 3 cm.
long, and without any contraction at the mouth: the air was