great electric density.

A most curious form of discharge is observed with such a coil when the
frequency and potential are pushed to the extreme limit. To perform
the experiment, every part of the coil should be heavily insulated,
and only two small spheres--or, better still, two sharp-edged metal
discs (dd, Fig. 11) of no more than a few centimetres in
diameter--should be exposed to the air. The coil here used is immersed
in oil, and the ends of the secondary reaching out of the oil are
covered with an air-tight cover of hard rubber of great thickness.
All cracks, if there are any, should be carefully stopped up, so that
the brush discharge cannot form anywhere except on the small spheres
or plates which are exposed to the air. In this case, since there are
no large plates or other bodies of capacity attached to the terminals,
the coil is capable of an extremely rapid vibration. The potential may
be raised by increasing, as far as the experimenter judges proper, the
rate of change of the primary current. With a coil not widely
differing from the present, it is best to connect the two primaries in
multiple arc; but if the secondary should have a much greater number
of turns the primaries should preferably be used in series, as
otherwise the vibration might be too fast for the secondary. It occurs
under these conditions that misty white streams break forth from the
edges of the discs and spread out phantom-like into space. With this
coil, when fairly well produced, they are about 25 to 30 centimetres
long. When the hand is held against them no sensation is produced, and
a spark, causing a shock, jumps from the terminal only upon the hand
being brought much nearer. If the oscillation of the primary current
is rendered intermittent by some means or other, there is a
corresponding throbbing of the streams, and now the hand or other
conducting object may be brought in still greater proximity to the