l plane.
It is so arranged that the poles N' s' are opposite to the central or
neutral point of the helix, so that when a current is sent through the
latter, the magnets, as before explained, are unaffected. Here then
we have an excited helix which itself has no action upon the magnets,
and we are thus enabled to examine the action of a body placed within
the helix and excited by it, undisturbed by the influence of the
latter. The helix being 12 inches high, a cylinder of soft iron 6
inches long, suspended from a string and passing over a pulley, can be
raised or lowered within the helix. When it is so far sunk that its
lower end rests upon the table, the upper end finds itself between the
poles N'S' of the astatic system. The iron cylinder is thus converted
into a strong magnet, attracting one of the poles, and repelling the
other, and consequently deflecting the entire astatic system. When
the cylinder is raised so that the upper end is at the level of the
top of the helix, its lower end comes between the poles N'S'; and a
deflection opposed in direction to the former one is the immediate
consequence. To render these deflections more easily visible, a
mirror m is attached to the system of magnets; a beam of light thrown
upon the mirror being reflected and projected as a bright disk against
the wall. The distance of this image from the mirror being
considerable, and its angular motion double that of the latter, a very
slight motion of the magnet is sufficient to produce a displacement of
the image through several yards.

This then is the principle of the beautiful apparatus [Footnote:
Devised by Prof. W. Weber, and constructed by M. Leyser, of Leipzig.]
by which the investigation was conducted. It is manifest that if a
second helix be placed between the poles SN with a cylinder within
it, the action upon the astatic magnet may be exalted. This was the
arrangement made use of in the actual enquiry. Thus to intensify the
feeble action, which it is here our object to seek, we have in the
first place neutralized the action of the earth upon the magnets, by
placing them astatically. Secondly, by making use of two cylinders,
and permitting them to act simultaneously on the four poles of the
magnets, we have rendered the deflecting force four times what it
would be, if only a single pole were used. Finally, the whole
apparatus was enclosed in a suitable case which protected the magnets
from air-currents, and the