nt in the charge of Mr. Christie, at his house at Woolwich,
where, by Mr. Christie's kindness, I was at liberty to work; and I have to
acknowledge my obligations to him for his assistance in all the experiments
and observations made with it. This magnet is composed of about 450 bar
magnets, each fifteen inches long, one inch wide, and half an inch thick,
arranged in a box so as to present at one of its extremities two external
poles (fig. 5.). These poles projected horizontally six inches from the
box, were each twelve inches high and three inches wide. They were nine
inches apart; and when a soft iron cylinder, three quarters of an inch in
diameter and twelve inches long, was put across from one to the other, it
required a force of nearly one hundred pounds to break the contact. The
pole to the left in the figure is the marked pole[A].

[A] To avoid any confusion as to the poles of the magnet, I shall
designate the pole pointing to the north as the marked pole; I may
occasionally speak of the north and south ends of the needle, but do
not mean thereby north and south poles. That is by many considered the
true north pole of a needle which points to the south; but in this
country it in often called the south pole.

45. The indicating galvanometer, in all experiments made with this magnet,
was about eight feet from it, not directly in front of the poles, but about
16 deg. or 17 deg. on one side. It was found that on making or breaking the
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 un