magnet. The magnetization of a bar of
steel or iron, according to this theory consists in establishing within
the metal by induction a series of electrical currents, all revolving in
the same direction at right angles to the axis or length of the bar.

[Illustration: Fig. 3]

It was this theory which led Arago, as he states, to adopt the method of
magnetizing sewing needles and pieces of steel wire, shown in Fig. 3.
This method consists in transmitting a current of electricity through a
helix surrounding the needle or wire to be magnetised. For the purpose
of insulation the needle was enclosed in a glass tube, and the several
turns of the helix were at a distance from each other to insure the
passage of electricity through the whole length of the wire, or, in
other words, to prevent it from seeking a shorter passage by cutting
across from one spire to another. The helix employed by Arago obviously
approximates the arrangement required by the theory of Ampere, in order
to develop by induction the magnetism of the iron. By an attentive
perusal of the original account of the experiments of Arago, it will be
seen that, properly speaking, he made no electro-magnet, as has been
asserted by Morse and others; his experiments were confined to the
magnetism of iron filings, to sewing needles and pieces of steel wire of
the diameter of a millimetre, or of about the thickness of a small
knitting needle.

[Illustration: Fig. 4]

Mr. Sturgeon, in 1825, made an important step in advance of the
experiments of Arago, and produced what is properly known as the
electro-magnet. He bent a piece of iron _wire_ into the form of a
horseshoe, covered it with varnish to insulate it, and surrounded it
with a helix, of which the spires were at a distance. When a current of
galvanism was passed through the helix from a small battery of a single
cup the iron wire became magnetic, and continued so during the passage
of the current. When the current was interrupted the magnetism
disappeared, and thus was produced the first temporary soft iron
magnet.

The electro-magnet of Sturgeon is shown in Fig. 4. By comparing Figs. 3
and 4 it will be seen that the helix employed by Sturgeon was of the
same kind as that used by Arago; instead however, of a straight steel
wire inclosed in a tube of glass, the former employed a bent wire of
soft iron. The difference in the arrangement at first sight might appear
to be small, but the difference in the results p