MAGNETISM AND ELECTRO-MAGNETISM.
If a small helix, or coil of covered wire, is arranged with an
unmagnetized steel needle within it, so that the discharge of a large
Leyden jar may take place through the coil, the needle will be found
strongly magnetic after the discharge of the electricity. (Fig. 194.)
Many years before this was known, it had been noticed that when a ship
was struck by lightning, the compasses were generally reversed; and in a
special case, where a house was struck, the electricity entered a box of
knives, fusing some, tearing the handles off others, but leaving them
strongly magnetic. Electricians tried to repeat the effect by sending
the discharge of powerful Leyden batteries through bars of steel without
any important result; and it was not until Oersted, in the year 1819,
made his important discovery that the copper wire conveying the
electricity possessed peculiar magnetic power, that the principle began
to be understood, and then the electricians succeeded in imitating the
effects of lightning on steel, as already described in the beginning of
this chapter. (Fig. 194.)
[Illustration: Fig. 194. A A. A glass tube supported on two uprights of
wood, with coil of copper wire passing round it, terminating in the
balls B B. C. Needle to place inside glass tube.]
[Illustration: Fig. 195.]
When the electricity has passed away from the Leyden jar through the
coil of [Page 207] copper wire, it no longer possesses any power to
affect a piece of steel or iron, but if the wires of the voltaic battery
are now connected with the coil of copper wire, which should be covered
with cotton or silk, and many yards in length, then a bar of steel or
soft iron is not only rendered magnetic, but remains permanently so, as
long as the current of electricity continues to pass along the coil of
wire, so that if some nails or iron filings are brought to the bar of
iron, one end of which projects from the coil, they cling to it with
great force, and a great number of nails may be hung on in this manner,
but they immediately fall off when the contact is broken with the
battery. (Fig. 195.)
Electricity thus becomes a source of magnetism, and the discoverer,
Oersted, found that only needles or bars of steel or iron were thus
affected, and not those of brass, shell-lac, sulphur, and other
substances; he termed the conducting wire "a conjunctive wire," and
described the effect of the electric current or "_electric conflict_,"
as he called it, as resembling a Helix (from [Greek: _helissô_], to turn
round; a screw or spiral), and that it is not confined to the conducting
wire, but radiates an influence at some distance. This latter statement
is exactly in accordance with our present notions, and hence the coil
conveying the current is said to _induce_ magnetism in the iron or
steel, just as the phenomena of induction are produced with frictional
electricity. The effect of Oersted's discovery, says Silliman, was truly
_electric_; the scientific world was ripe for it, and the truth he thus
struck out was instantly seized upon by Arago, Ampère, Davy, Faraday,
and a crowd of philosophers in all countries. The activity with which
this new field of research has been cultivated, has never relaxed even
to this hour, while it has borne fruit in a multitude of theoretical and
practical truths, and above all, in the electro-magnetic telegraph,
truly called, and especially in connexion with the Atlantic telegraph
wire, "_the great international nerve of sensation_."
[Illustration: Fig. 196. A loadstone mounted in brass or silver, with
the iron cheeks B B attached. C. The bit of soft iron called the
armature.]
Magnetism is not only the result of a current of electricity through any
good conductor, but there are certain oxides of iron, called magnetic
iron ores, which have the property of attracting iron filings, and are
mostly found in primitive rocks, being abundant at Roslagen, in Sweden,
and called the loadstone, from its always pointing, when freely
suspended, to the Polar, North, or Load Star. If a tolerably large
specimen of this mineral is examined, there will be found usually two
points where the iron filings are attracted in larger quantities than in
other parts of the same specimen. These attractive points are called
poles, and the loadstone being properly mounted with soft iron bars,
termed cheeks, bound round it (in old-fashioned loadstones) with silver
plate and duly ornamented with [Page 208] engraving, has its magnetic power
greatly increased, and is then said to be endowed with magnetic
polarity; and to prevent the loss of power, a soft piece of iron, called
the armature, is placed across and attracted to the poles of the
loadstone. (Fig. 196.)
_Second Experiment._
If a needle of tempered steel (fitted with a little brass cup in the
centre to work upon a point) is rubbed with the loadstone in one
direction only, it is rendered permanently magnetic, and will now be
found to take a certain fixed position, pointing always in a direction
due north and south. The end which points towards the north is called
the north pole, and the other extremity the south pole, and it is usual
to mark the north pole with an indent or scratch to distinguish it at
all times.
_Third Experiment._
If another bar of steel is magnetized, and the north pole duly marked,
and then brought towards the same pole of the suspended magnet, instant
repulsion takes place; the magnet, of course, grasped in the hand is not
free to move, but the small magnet immediately shows the same fact
noticed with electricity, viz., "_that similar magnetisms repel_." Two
north poles repel each other, but when the bar of steel is reversed, the
opposite effect occurs, and the suspended magnet is attracted, showing
that _dissimilar magnetisms attract_, and a north will attract a south
pole. (Fig. 197.)
[Illustration: Fig. 197. A magnetic needle, the north pole N being
attracted to the south pole of the bar magnet S, and repelled from the
north end.]
_Fourth Experiment._
[Illustration: Fig. 198. The horse-shoe magnet, and another one
unmagnetized, placed end to end; the one shaded and lettered N and S is
the magnet. A A. The piece of soft iron moved in the direction of the
arrow.]
By contact, the magnetic power is transferred from the magnet to a piece
of unmagnetized steel, and it is stated that the highest magnetizing
effect is that produced by the simple method of Jacobi. A horse-shoe
magnet has its poles brought in contact with the intended poles of
another bar of steel, likewise bent in the form of a horse-shoe, and by
[Page 209] drawing the feeder over the unmagnetized horse-shoe in the
direction of the arrow in the cut, and when it reaches the curve,
bringing it back again to the same place, say at least twelve times, and
after turning the whole over without separating the poles, and repeating
the same operation on the other side likewise twelve times, the steel is
then powerfully magnetized; and it is said that a horse-shoe of one
pound weight may be thus charged so as to sustain twenty-six and a half
pounds, and that by the old method of magnetizing it would only have
sustained about twenty-two pounds. (Fig. 198.)
_Fifth Experiment._
If the horse-shoe magnet is placed on a sheet of paper, and some iron
filings are dusted between the poles, a very beautiful series of curves
are formed, called the magnetic curves, which indicate the constant
passage of the magnetic power from pole to pole.
_Sixth Experiment._
The magnetic force exerted by a horse-shoe-shaped piece of soft iron,
surrounded with many strands of covered copper wire in short lengths, is
extremely powerful (Fig. 199), and enormous weights have been supported
by an electro-magnet when connected with a voltaic battery. Supposing a
man were dressed in complete armour, he might be held by an
electro-magnet, without the power of disengaging himself, thus realizing
the fairy story of the bold knight who was caught by a rock of
loadstone, and, in full armour, detained by the unfriendly magician.
[Illustration: Fig. 199. A. Powerful electro-magnet supporting a great
weight. B. The battery.]
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_Seventh Experiment._
When a piece of soft iron is held sufficiently near one of the poles of
a powerful magnet, it becomes by _induction_ endowed with magnetic
poles, and will support another bit of soft iron, such as a nail,
brought in contact with it. When the magnet is removed, the inductive
action ceases, and the soft iron loses its magnetic power. This
experiment affords another example of the connexion between the
phenomena of electricity and magnetism. It is in consequence of the
inductive action of the magnetism of the earth that all masses of iron,
especially when they are perpendicular, are found to be endowed with
magnetic polarity; hence the reaction of the iron in ships upon the
compasses, which have to be corrected and adjusted before a voyage, or
else serious errors in steering the vessel would occur, and there is no
doubt that many shipwrecks are due to this cause. No other metals beside
iron, steel, nickel, cobalt, and possibly manganese, can receive or
retain magnetism after contact with a magnet.
The remarkable effect of magnetism upon all matter, so ably investigated
by Faraday and others, will be explained in another part of this
book--viz., in the article on Dia-Magnetism.
[Illustration: Fig. 200. Magician and his loadstone-rock.--Vide _Fairy
Tale_.]
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