hough the
appearances may not be seen, to determine the element of _time_. So when,
by bringing the hand towards a projecting rod or ball, the pitch of the
tone produced by a brushy discharge increases, the effect informs us that
we have increased the induction (1374.), and by that means increased the
rapidity of the alternations of charge and discharge.

1432. By using wires with finer terminations, smaller brushes were
obtained, until they could hardly be distinguished as brushes; but as long
as _sound_ was heard, the discharge could be ascertained by the eye to be
intermitting; and when the sound ceased, the light became _continuous_ as a
glow (1359. 1405. 1526-1543.).

1433. To those not accustomed to use the eye in the manner I have
described, or, in cases where the recurrence is too quick for any
unassisted eye, the beautiful revolving mirror of Professor Wheatstone[A]
will be useful for such developments of condition as those mentioned above.
Another excellent process is to produce the brush or other luminous
phenomenon on the end of a rod held in the hand opposite to a charged
positive or negative conductor, and then move the rod rapidly from side to
side whilst the eye remains still. The successive discharges occur of
course in different places, and the state of things before, at, and after a
single coruscation or brush can be exceedingly well separated.

[A] Philosophical Transactions, 1834, pp. 581, 585.

1434. The _brush_ is in reality a discharge between a bad or a
non-conductor and either a conductor or another non-conductor. Under common
circumstances, the brush is a discharge between a conductor and air, and I
conceive it to take place in something like the following manner. When the
end of an electrified rod projects into the middle of a room, induction
takes place between it and the walls of the room, across the dielectric,
air; and the lines of inductive force accumulate upon the end in greater
quantity than elsewhere, or the particles of air at the end of the rod are
more highly polarized than those at any other part of the rod, for the
reasons already given (1374.). The particles of air situated in sections
across these lines of force are least polarized in the sections towards the
walls and most polarized in those nearer to the end of the wires (1369.):
thus, it may well happen, that a particle at the end of the wire is at a
tension that will immediately terminate in discharge, whilst in those