Discovery of Electro-magnetism—Deflection of the Magnetic Needle by a
Current of Electricity—Direction of the Force—Rotatory Motion by
Electricity—Rotation of a Wire and a Magnet—Rotation of a Magnet about
its Axis—Of Mercury and Water—Electro-Magnetic Cylinder or
Helix—Suspension of a Needle in a Helix—Electro-Magnetic
Induction—Temporary Magnets—The Galvanometer.
THE disturbing effects of the aurora and lightning on the mariner’s
compass had been long known. In the year 1819 M. Oersted, Professor of
Natural Philosophy at Copenhagen, discovered that a current of Voltaic
electricity exerts a powerful influence on a magnetized needle. This
observation has given rise to the theory of electro-magnetism—one of the
most interesting sciences of modern times, whether it be considered as
leading us a step farther in generalization, by identifying two agencies
hitherto referred to different causes, or as developing a new force,
unparalleled in the system of the world, which, overcoming the
retardation from friction, and the obstacle of a resisting medium,
maintains a perpetual motion as long as the action of a Voltaic battery
is continued.
When the two poles of a Voltaic battery are connected by a metallic
wire, so as to complete a circuit, the electricity flows without
ceasing. If a straight portion of that wire be placed parallel to, and
horizontally above, a magnetized needle at rest in the magnetic
meridian, but freely poised like the mariner’s compass, the action of
the electric current flowing through the wire will instantly cause the
needle to change its position. Its extremity will deviate from the north
towards the east or west, according to the direction in which the
current is flowing; and, on reversing the direction of the current, the
motion of the needle will be reversed also. The numerous experiments
that have been made on magnetism and electricity, as well as those on
the various relative motions of a magnetic needle under the influence of
galvanic electricity, arising from all possible positions of the
conducting wire, and every direction of the Voltaic current, together
with all the other phenomena of electro-magnetism, are explained by Dr.
Roget in some excellent articles on these subjects in the Library of
Useful Knowledge.
All experiments tend to prove that the force emanating from the electric
current, which produces such effects on the magnetic needle, acts at
right angles to the current. The action of an electrical current upon
either pole of a magnet has no tendency to cause the pole to approach or
recede, but to rotate about it. If the stream of electricity be supposed
to pass through the centre of a circle whose plane is perpendicular to
the current, the direction of the force exerted by the electricity will
always be in the tangent to the circle, or at right angles to its radius
(N. 223). Consequently, the tangential force of the electricity has a
tendency to make the pole of a magnet move in a circle round the wire of
the battery.
Rotatory motion was suggested by Dr. Wollaston. Dr. Faraday was the
first who actually succeeded in making the pole of a magnet rotate about
a vertical conducting wire. In order to limit the action of the
electricity to one pole, about two-thirds of a small magnet were
immersed in mercury, the lower end being fastened by a thread to the
bottom of the vessel containing the mercury. When the magnet was thus
floating almost vertically with its north pole above the surface, a
current of positive electricity was made to descend perpendicularly
through a wire touching the mercury, and immediately the magnet began to
rotate from left to right about the wire. The force being uniform, the
rotation was accelerated till the tangential force was balanced by the
resistance of the mercury, when it became constant. Under the same
circumstances the south pole of the magnet rotates from right to left.
It is evident, from this experiment, that the wire may also be made to
perform a rotation round the magnet, since the action of the current of
electricity on the pole of the magnet must necessarily be accompanied by
a corresponding reaction of the pole of the magnet on the electricity in
the wire. This experiment has been accomplished by a vast number of
contrivances, and even a small battery, consisting of two plates, has
performed the rotation. Dr. Faraday produced both motions at the same
time in a vessel containing mercury; the wire and the magnet revolved in
one direction about a common centre of motion, each following the other.
The next step was to make a magnet, and also a cylinder, revolve about
their own axes, which they do with great rapidity. Mercury has been made
to rotate by means of Voltaic electricity, and Professor Ritchie
exhibited in the Royal Institution the singular spectacle of the
rotation of water by the same means, while the vessel containing it
remained stationary. The water was in a hollow double cylinder of glass,
and, on being made the conductor of electricity, was observed to revolve
in a regular vortex, changing its direction as the poles of the battery
were alternately reversed. Professor Ritchie found that all the
different conductors hitherto tried by him, such as water, charcoal,
&c., give the same electro-magnetic results when transmitting the same
quantity of electricity, and that they deflect the magnetic needle in an
equal degree when their respective axes of conduction are at the same
distance from it. But one of the most extraordinary effects of this
force is exhibited by coiling a copper wire, so as to form a helix or
corkscrew, and connecting the extremities of the wire with the poles of
a galvanic battery. If a magnetized steel bar or needle be placed within
the screw, so as to rest upon the lower part, the instant a current of
electricity is sent through the wire of the helix, the steel bar starts
up by the influence of this invisible power, and remains suspended in
the air in opposition to the force of gravitation (N. 224). The effect
of the electro-magnetic power exerted by each turn of the wire is to
urge the north pole of the magnet in one direction, and the south pole
in the other. The force thus exerted is multiplied in degree and
increased in extent by each repetition of the turns of the wire, and in
consequence of these opposing forces the bar remains suspended. This
helix has all the properties of a magnet while the electrical current is
flowing through it, and may be substituted for one in almost every
experiment. It acts as if it had a north pole at one extremity and a
south pole at the other, and is attracted and repelled by the poles of a
magnet exactly as if it were one itself. All these results depend upon
the course of the electricity; that is, on the direction of the turns of
the screw, according as it is from right to left, or from left to right,
being contrary in the two cases.
The action of Voltaic electricity on a magnet is not only precisely the
same with the action of two magnets on one another, but its influence in
producing temporary magnetism in iron and steel is also the same with
magnetic induction. The term induction, when applied to electric
currents, expresses the power which these currents possess of inducing a
particular state upon matter in their immediate neighbourhood, otherwise
neutral or indifferent. For example, the connecting wire of a galvanic
battery holds iron filings suspended like a magnet as long as the
current continues to flow through it: the iron becomes magnetic by the
induction of the current. The most powerful temporary magnets are
obtained by bending a thick cylinder of soft iron into the form of a
horseshoe, and surrounding it with a coil of thick copper wire covered
with silk to prevent communication between its coils. When this wire
forms part of a galvanic circuit the iron becomes so highly magnetic by
the induction of the current flowing through the wire that a temporary
magnet of this kind made by Professor Henry of the Albany Academy in the
United States sustained a weight of nearly a ton. Another by Mr. Gage
has been applied with considerable success as a moving power: its spark
is a bright flash, and the snap as loud as a pistol. But the most
powerful known is that employed by Mr. Joule in his experiments, which
sustains a weight of 2080 lbs. The iron loses its magnetism the instant
the electricity ceases to flow, and acquires it again as instantaneously
when the circuit is renewed.
The action of an electric current causes a deviation of the compass from
the plane of the magnetic meridian. In proportion as the needle recedes
from the meridian, the intensity of the force of terrestrial magnetism
increases, while at the same time the electro-magnetic force diminishes;
the number of degrees at which the needle stops, showing where the
equilibrium between these two forces takes place, will indicate the
intensity of the galvanic current. The galvanometer, constructed upon
this principle, is employed to measure the intensity of galvanic
currents collected and conveyed to it by wires. This instrument is
rendered much more sensible by neutralizing the effects of the earth’s
magnetism on the needle, which is accomplished by placing a second
magnetised needle so as to counteract the action of the earth on the
first—a precaution requisite in all delicate magnetical experiments.
It has been ascertained by means of this instrument that the action of
an electrical current upon a magnet is inversely as the square of the
distance, and the energy with which an electro magnet acts is directly
as the power of the galvanic battery and the number of coils round the
core, and inversely as the resistance of the wire.