mercury was put into a glass tube for the purpose. The dense carbon
deposited in coal gas retorts, also produced the current, but ordinary
charcoal did not. Neither could I obtain any sensible effects with brine,
sulphuric acid, saline solutions, &c., whether rotated in basins, or
inclosed in tubes and passed between the poles.

133. I have never been able to produce any sensation upon the tongue by the
wires connected with the conductors applied to the edges of the revolving
plate (88.) or slips of metal (101.). Nor have I been able to heat a fine
platina wire, or produce a spark, or convulse the limbs of a frog. I have
failed also to produce any chemical effects by electricity thus evolved
(22. 56).

134. As the electric current in the revolving copper plate occupies but a
small space, proceeding by the poles and being discharged right and left at
very small distances comparatively (123.); and as it exists in a thick mass
of metal possessing almost the highest conducting power of any, and
consequently offering extraordinary facility for its production and
discharge; and as, notwithstanding this, considerable currents may be drawn
off which can pass through narrow wires, forty, fifty, sixty, or even one
hundred feet long; it is evident that the current existing in the plate
itself must be a very powerful one, when the rotation is rapid and the
magnet strong. This is also abundantly proved by the obedience and
readiness with which a magnet ten or twelve pounds in weight follows the
motion of the plate and will strongly twist up the cord by which it is
suspended.

135. Two rough trials were made with the intention of constructing
_magneto-electric machines_. In one, a ring one inch and a half broad and
twelve inches external diameter, cut from a thick copper plate, was mounted