in length may easily be drawn from the conductors. Each
turn of the machine, when worked moderately, occupies about 4/5ths of a
second.

291. The electric battery consisted of fifteen equal jars. They are coated
eight inches upwards from the bottom, and are twenty-three inches in
circumference, so that each contains one hundred and eighty-four square
inches of glass, coated on both sides; this is independent of the bottoms,
which are of thicker glass, and contain each about fifty square inches.

292. A good _discharging train_ was arranged by connecting metallically a
sufficiently thick wire with the metallic gas pipes of the house, with the
metallic gas pipes belonging to the public gas works of London; and also
with the metallic water pipes of London. It was so effectual in its office
as to carry off instantaneously electricity of the feeblest tension, even
that of a single voltaic trough, and was essential to many of the
experiments.

293. The galvanometer was one or the other of those formerly described (87.
205.), but the glass jar covering it and supporting the needle was coated
inside and outside with tinfoil, and the upper part (left uncoated, that
the motions of the needle might be examined,) was covered with a frame of
wire-work, having numerous sharp points projecting from it. When this frame
and the two coatings were connected with the discharging train (292.), an
insulated point or ball, connected with the machine when most active, might
be brought within an inch of any part of the galvanometer, yet without
affecting the needle within by ordinary electrical attraction or repulsion.

294. In connexion with these precautions, it may be necessary to state that
the needle of the galvanometer is very liable to have its magnetic power
deranged, diminished, or even inverted by the passage of a shock through
the instrument. If the needle be at all oblique, in the wrong direction, to
the coils of the galvanometer when the shock passes, effects of this kind
are sure to happen.

295. It was to the retarding power of bad conductors, with the intention of
diminishing its _intensity_ without altering its _quantity_, that I first
looked with the hope of being able to make common electricity assume more
of the characters and power of voltaic electricity, than it is usually
supposed to have.

296, The coating and armour of the galvanometer were first connected with
the discharging train (292.); the end B (87.) of the g