if imperfectly produced, as they are likely to be this evening, they
are sufficiently striking to interest an intelligent audience.

Before showing some of these curious effects I must, for the sake of
completeness, give a short description of the coil and other apparatus
used in the experiments with the disruptive discharge this evening.

[Illustration: FIG. 3.--DISRUPTIVE DISCHARGE COIL.]

It is contained in a box B (Fig. 3) of thick boards of hard wood,
covered on the outside with zinc sheet Z, which is carefully soldered
all around. It might be advisable, in a strictly scientific
investigation, when accuracy is of great importance, to do away with
the metal cover, as it might introduce many errors, principally on
account of its complex action upon the coil, as a condenser of very
small capacity and as an electrostatic and electromagnetic screen.
When the coil is used for such experiments as are here contemplated,
the employment of the metal cover offers some practical advantages,
but these are not of sufficient importance to be dwelt upon.

The coil should be placed symmetrically to the metal cover, and the
space between should, of course, not be too small, certainly not less
than, say, five centimetres, but much more if possible; especially the
two sides of the zinc box, which are at right angles to the axis of
the coil, should be sufficiently remote from the latter, as otherwise
they might impair its action and be a source of loss.

The coil consists of two spools of hard rubber RR, held apart at a
distance of 10 centimetres by bolts c and nuts n, likewise of hard
rubber. Each spool comprises a tube T of approximately 8 centimetres