XII A PHENOMENON ACCOMPANYING THE ELECTRIC DISCHARGE 219
A
first discharge.
Radiometer-like vanes of various kinds may
be set revolving continuously by the jet. But the impulse
does not act in one direction only-away from the mouth.
mica plate set up before the mouth, so as to be only movable
towards the mouth, is also set in vibration, which fact shows
that each impulse directed away from the mouth is followed
by a return impulse, though one of less strength.
7. The jet does not appear instantaneously, but takes a
conveniently measurable time to develop. I have examined
its time-changes, first with a rotating mirror, and secondly
with an apparatus specially constructed for the purpose; this
has, however, been already described by others, and is arranged
as follows: A disc with a narrow radial slit is fixed to the
axis of a Becquerel's phosphoroscope; at every revolution of
the disc in one particular position of it the apparatus breaks
the primary circuit. When the disc is rapidly rotated it
appears to be transparent, but if we look through it at
different places, we see the phenomena as they occur at certain
definite different times after break. This apparatus usually
gives better results than the rotating mirror, but in this case
the latter is sufficient. Both methods of observation lead to
the following results. The phenomenon is not instantaneous, but
lasts about sec. The different parts of the jet do not all
appear at once; the lower portions emit light before the upper
ones commence; the upper parts are visible after the lower
ones have gone out. Thus the phenomenon is a jet only to
the unaided eye; in reality it consists of a luminous cloud,
which is emitted from the tube with a finite velocity. When
no Leyden jar is used, this velocity is for the whole path of
the order of 2 m. per second, but it appears to be much greater
at the commencement of the phenomenon: so also it seemed
much greater for Leyden jar sparks; for such sparks it may be
that often only the after-glow of the gas and not the develop-
ment of the jet was observed.
8. Analogous phenomena to those described occur in other
gases, but the jets show characteristic differences as regards
colour, form, effect of density, etc. In oxygen the jet is very
beautiful, much like that in air, but the tint is a purer yellow.
The appearance in nitrous oxide resembles that in oxygen
almost exactly. In nitrogen it was possible to produce only