Electricity produced by Rotation—Direction of the Currents—Electricity
from the Rotation of a Magnet—M. Arago’s Experiment explained—Rotation
of a Plate of Iron between the Poles of a Magnet—Relation of
Substances to Magnets of three Kinds—Thermo-Electricity.
M. ARAGO discovered a source of magnetism in rotatory motion. If a
circular plate of copper be made to revolve immediately above or below a
magnetic needle or magnet, suspended in such a manner that it may rotate
in a plane parallel to that of the copper plate, the magnet tends to
follow the circumvolution of the plate; or, if the magnet revolves, the
plate tends to follow its motion; so powerful is the effect, that
magnets and plates of many pounds weight have been carried round. This
is quite independent of the motion of the air, since it is the same when
a pane of glass is interposed between the magnet and the copper. When
the magnet and the plate are at rest, not the smallest effect,
attractive, repulsive, or of any kind, can be perceived between them. In
describing this phenomenon, M. Arago states that it takes place not only
with metals, but with all substances, solids, liquids, and even gases,
although the intensity depends upon the kind of substance in motion.
Experiments made by Dr. Faraday explain this singular action. He found
that, if a piece of metal or a metallic wire forming a circuit of any
form be moved from right to left across the lines of force proceeding
from the pole of a bar magnet, these lines of force induce a current of
electricity flowing in one direction; and when the motion of the metal
or wire is reversed, the direction of the current is reversed also: the
rotation of the magnet about its axis has no effect on these results,
and no current is induced when the metal or wire is at rest. A plate of
copper, twelve inches in diameter and one fifth of an inch thick, was
placed between the poles of a powerful horseshoe magnet, consequently
crossing the magnetic lines of force at right angles, and connected at
certain points with a galvanometer by copper wires. When the plate was
at rest no effect was produced; but as soon as the plate was made to
revolve rapidly the galvanometer needle was deflected sometimes as much
as 90°, and by a uniform rotation the deflection was constantly
maintained at 45°. When the motion of the copper plate was reversed, the
needle was deflected in the contrary direction, and thus a permanent
current of electricity was evolved by an ordinary magnet. The intensity
of the electricity collected by the wires, and conveyed by them to the
galvanometer, varied with the position of the plate relatively to the
poles of the magnet.
The motion of the electricity in the copper plate may be conceived by
considering that, merely by moving a single wire, like the spoke of a
wheel, before a magnetic pole, a current of electricity tends to flow
through it from one end to the other. Hence, if a wheel be constructed
of a great many such spokes, and revolved near the pole of a magnet in
the manner of the copper disc, each radius or spoke will tend to have a
current produced in it as it passes the pole. Now, as the circular plate
is nothing more than an infinite number of radii or spokes in contact,
the currents will flow in the direction of the radii if a channel be
open for their return; and, in a continuous plate, that channel is
afforded by the lateral portions on each side of the particular radius
close to the magnetic pole. This hypothesis is confirmed by observation;
for the currents of positive electricity set from the centre to the
circumference, and the negative from the circumference to the centre,
and _vice versâ_, according to the position of the magnetic poles and
the direction of rotation; so that a collecting wire at the centre of
the copper plate conveys positive electricity to the galvanometer in one
case, and negative in another; that collected by a conducting wire in
contact with the circumference of the plate is always the opposite of
the electricity conveyed from the centre. It is evident that, when the
plate and magnet are both at rest, no effect takes place, since the
electric currents which cause the deflection of the galvanometer are
only induced by motion across the magnetic lines of force. When the
plate is placed edgewise so as to be parallel to these lines of force,
no revolution of it with the most powerful magnet produces the slightest
signs of a current at the galvanometer. The same phenomena may be
produced by electro-magnets. The effects are similar when the magnet
rotates and the plate remains at rest. When the magnet revolves
uniformly about its own axis, electricity of the same kind is collected
at its poles, and the opposite electricity at its equator.
The phenomena which take place in M. Arago’s experiments may be
explained on this principle. When both the copper plate and the magnet
are revolving, the action of the induced electric current tends
continually to diminish their relative motion, and to bring the moving
bodies into a state of relative rest; so that, if one be made to revolve
by an extraneous force, the other will tend to revolve about it in the
same direction, and with the same velocity.
When a plate of iron, or of any substance capable of being made either a
temporary or permanent magnet, revolves between the poles of a magnet,
it is found that dissimilar poles on opposite sides of the plate
neutralize each other’s effects, so that no electricity is evolved;
while similar poles on each side of the revolving plate increase the
quantity of electricity, and a single pole end-on is sufficient. But
when copper, and substances not sensible to ordinary magnetic
impressions, revolve, similar poles on opposite sides of the plate
neutralize each other; dissimilar poles on each side exalt the action;
and a single pole at the edge of the revolving plate, or end-on, does
nothing. This forms a test for distinguishing the ordinary magnetic
force from that produced by rotation. If unlike poles, that is, a north
and south pole, produce more effect than one pole, the force will be due
to electric currents; if similar poles produce more effect than one,
then the power is not electric. These investigations show that there are
really very few bodies magnetic in the manner of iron. Dr. Faraday
therefore arranges substances in three classes, with regard to their
relation to magnets:—those affected by the magnet when at rest, like
iron, steel, and nickel, which possess ordinary magnetic properties;
those affected when in motion, in which electric currents are evolved by
the inductive force of the magnet, such as copper; and, lastly, those
which are perfectly indifferent to the magnet, whether at rest or in
motion.
It has already been observed that three bodies are requisite to form a
galvanic circuit, one of which must be fluid. But, in 1822, Professor
Seebeck, of Berlin, discovered that electric currents may be produced by
the partial application of heat to a circuit formed of two solid
conductors. For example, when a semicircle of bismuth, joined to a
semicircle of antimony, so as to form a ring, is heated at one of the
junctions by a lamp, a current of electricity flows through the circuit
from the antimony to the bismuth; and such thermo-electric currents
produce all the electro-magnetic effects. A compass needle, placed
either within or without the circuit, and at a small distance from it,
is deflected from its natural position, in a direction corresponding to
the way in which the electricity is flowing. If such a ring be suspended
so as to move easily in any direction, it will obey the action of a
magnet brought near it, and may even be made to revolve. According to
the researches of M. Seebeck, the same substance, unequally heated,
exhibits electrical currents; and M. Nobili observed, that in all
metals, except zinc, iron, and antimony, the electricity flows from the
hot part towards that which is cold. That philosopher attributes
terrestrial magnetism to a difference in the action of heat on the
various substances of which the crust of the earth is composed; and, in
confirmation of his views, he has produced electrical currents by the
contact of two pieces of moist clay, of which one was hotter than the
other.
M. Becquerel constructed a thermo-electric battery of one kind of metal,
by which he has determined the relation between the heat employed and
the intensity of the resulting electricity. He found that, in most
metals, the intensity of the current increases with the heat to a
certain limit, but that this law extends much farther in metals that are
difficult to fuse, and which do not rust. The experiments of Professor
Cumming show that the mutual action of a magnet and a thermo-electric
current is subject to the same laws as those of magnets and galvanic
currents; consequently all the phenomena of repulsion, attraction, and
rotation may be exhibited by a thermo-electric current. M. Botto, of
Turin, has decomposed water and some solutions by thermo-electricity;
and the Cav. Antinori of Florence succeeded in obtaining a brilliant
spark with the aid of an electro-dynamic coil.
The principle of thermo-electricity has been employed by MM. Nobili and
Melloni for measuring extremely minute quantities of heat in their
experiments on the instantaneous transmission of radiant heat. The
thermo-multiplier, which they constructed for that purpose, consists of
a series of alternate bars, or rather fine wires of bismuth and
antimony, placed side by side, and the extremities alternately soldered
together. When heat is applied to one end of this apparatus, the other
remaining at its natural temperature, currents of electricity flow
through each pair of bars, which are conveyed by wires to a delicate
galvanometer, the needle of which points out the intensity of the
electricity conveyed, and consequently that of the heat employed. This
instrument is so delicate that the comparative warmth of different
insects has been ascertained by means of it.
The conservation of force is strictly maintained throughout the whole
science and different forms of electricity. In static electricity the
positive and negative forces exactly balance one another; they are
always simultaneous, and related often by curved lines of force; there
is no defect or surplus, and the existence of one kind without the other
is utterly impossible—it is absolutely a dual force. The very same may
be said of electric currents, whether produced by the Voltaic battery or
in any other way—the current in one part of the circuit is absolutely
the same in amount and dual character as the other; and in the insulated
Voltaic battery, where the sustaining power is internal, not the
slightest development of the forces of either of these can occur till
the circuit is completed or induction allowed at the extremities; for if
when there is no circuit the induction be prevented, not merely no
current, but no quantity of electricity at the poles ready to produce a
current, can be evolved in the slightest degree.[17]