218 A PHENOMENON ACCOMPANYING THE ELECTRIC DISCHARGE XII
exhausted down to 10 to 20 mm. pressure, and was kept well
dried by placing under the receiver a small dish with sulphuric
acid or phosphorus pentoxide; a large Leyden jar was joined
up, and the glare of the discharge itself screened off by using
as outside electrode a metal tube placed round the glass tube,
and projecting slightly beyond it. Under these conditions
the jet was in form like a tree, which reached up to 12 cm.
in height; the part corresponding to the stem projected up
straight from the tube a distance of 1 to 5 cm., while the top
consisted of flames, which shot violently apart in all directions.
The brightness may be judged from the fact that the appear-
ance was still visible in a lighted room, but all details could
be observed only in a darkened room.
3. When the wall opposite to the jet is too close, so that
the jet cannot be fully developed, it spreads out over the wall.
When it meets it perpendicularly, it forms a circular mound
round the point of impact; but when it is inclined at an
angle, it creeps along the wall in the direction in which a
body would be reflected after impinging on the wall (in the
direction of the flame). The phenomena which here occur
may be most simply described by saying that the jets behave
as liquid jets would do if they emerged from the mouth of
the tube.
4. A magnet has no action on the jet. Neither have
conductors, when brought near, not even when they are
charged, e.g. when they are connected with one of the two
electrodes.
5. The jet generates much heat in the bodies which it
encounters. A thermometer brought into the jet shows a rise
of ten or more degrees according to circumstances. When the
jet encounters the glass receiver it heats it perceptibly; small
objects are melted off from wires on which they have been
stuck by wax. When the jet is produced in the open air (see
§ 10) the heat generated may be felt directly. On the other
hand it was found impossible to cause a platinum wire,
however thin, to glow when hung in the current.
6. The jet exerts considerable mechanical force. A wire
suspended in it is set in violent oscillation, so also is a mica
plate, used to deflect the jet. A mica plate, placed on the
mouth of the tube, is violently thrown to a distance by the