36. Similar effects were then produced by _ordinary magnets_: thus the
hollow helix just described (34.) had all its elementary helices connected
with the galvanometer by two copper wires, each five feet in length; the
soft iron cylinder was introduced into its axis; a couple of bar magnets,
each twenty-four inches long, were arranged with their opposite poles at
one end in contact, so as to resemble a horse-shoe magnet, and then contact
made between the other poles and the ends of the iron cylinder, so as to
convert it for the time into a magnet (fig. 2.): by breaking the magnetic
contacts, or reversing them, the magnetism of the iron cylinder could be
destroyed or reversed at pleasure.

37. Upon making magnetic contact, the needle was deflected; continuing the
contact, the needle became indifferent, and resumed its first position; on
breaking the contact, it was again deflected, but in the opposite direction
to the first effect, and then it again became indifferent. When the
magnetic contacts were reversed the deflections were reversed.

38. When the magnetic contact was made, the deflection was such as to
indicate an induced current of electricity in the opposite direction to
that fitted to form a magnet, having the same polarity as that really
produced by contact with the bar magnets. Thus when the marked and unmarked
poles were placed as in fig. 3, the current in the helix was in the
direction represented, P being supposed to be the end of the wire going to
the positive pole of the battery, or that end towards which the zinc plates
face, and N the negative wire. Such a current would have converted the
cylinder into a magnet of the opposite kind to that formed by contact with
the poles A and B; and such a current moves in the opposite direction to
the currents which in M. Ampère's beautiful theory are considered as
constituting a magnet in the position figured[A].