connexion of the poles by soft iron, the instrument was slightly affected;
but all error of observation arising from this cause was easily and
carefully avoided.

46. The electrical effects exhibited by this magnet were very striking.
When a soft iron cylinder thirteen inches long was put through the compound
hollow helix, with its ends arranged as two general terminations (39.),
these connected with the galvanometer, and the iron cylinder brought in
contact with the two poles of the magnet (fig. 5.), so powerful a rush of
electricity took place that the needle whirled round many times in
succession[A].

[A] A soft iron bar in the form of a lifter to a horse-shoe magnet,
when supplied with a coil of this kind round the middle of it,
becomes, by juxta-position with a magnet, a ready source of a brief
but determinate current of electricity.

47. Notwithstanding this great power, if the contact was continued, the
needle resumed its natural position, being entirely uninfluenced by the
position of the helix (30.). But on breaking the magnetic contact, the
needle was whirled round in the opposite direction with a force equal to
the former.

48. A piece of copper plate wrapped _once_ round the iron cylinder like a
socket, but with interposed paper to prevent contact, had its edges
connected with the wires of the galvanometer. When the iron was brought in
contact with the poles the galvanometer was strongly affected.

49. Dismissing the helices and sockets, the galvanometer wire was passed
over, and consequently only half round the iron cylinder (fig. 6.); but