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shoe magnet, 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 45o. 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 from 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 versa, according to the position of the magnet 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 cease altogether. The same phenomena may be produced by electro-magnets. The effects are the same 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, for when both the copper plate and the magnet are revolving, the action of the electric current, induced in the plate by the magnet in consequence of their relative motion, tends continually to diminish that relative motion; that is, 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 a 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. Mr. 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 flow.
ing. 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. Nobili, the same substance unequally heated, exhibits electrical currents. The ex
periments of Professor Cumming show that the mutual action of a magnet and a thermo-electric current, is sub- ject 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. It is, however, so feeble, that neither heat, the spark, nor chemical action have been observed, nor can repulsion, attraction of light substances at sensible distances, or any other effects of tension, be perceived.
In all the experiments hitherto described, artificial magnets alone were used, but it is obvious that the magnetism of the terrestrial spheroid which has so powerful an influence on the mariner's compass, must also effect electrical currents. It consequently appears that a piece of copper wire bent into a rectangle, and free to revolve on a vertical axis, arranges itself with its plane at right angles to the magnetic meridian, as soon as a stream of electricity is sent through it. Under the same circumstances a similar rectangle, suspended on a horizontal axis at right angles to the magnetic meridian, assumes the same inclination with the dipping needle. So that terrestrial magnetism has the same influence on electrical currents as an artificial magnet.
But the magnetic action of the earth
also induces electric currents. When a hollow helix of copper wire, whose extremities are connected with the galvanometer, is placed in the magnetic dip, and suddenly inverted several times, accommodating the motion to the oscillations of the needle, the latter is soon made to vibrate through an arc of 80° or 90°. Hence it is evident, that whatever may be the cause of terrestrial magnetism, it produces currents of electricity by its direct inductive power upon a metal not capable of exhibiting any of the ordinary magnetic properties. The action on the galvanome ter is much greater when a cylinder of soft iron is inserted. into the helix, and the same results follow the simple introduction of the iron cylinder into, or removal out of the helix. These effects arise from the iron being made a temporary magnet by the inductive action of terrestrial magnetism, for a piece of iron, such as a poker, becomes a magnet for the time, when placed in the line of the magnetic dip.
M. Biot has formed a theory of terrestrial magnetism upon the observations of M. de Humboldt as data. Assuming that the action of the two opposite magnetic poles of the earth upon any point is inversely as the square of the distance, he obtains a general expression for the direction of the magnetic needle, depending upon the distance between the north and south magnetic poles; so that if one of these quantities varies, the corresponding variation of the other will be known. By making the distance between the poles vary, and comparing the resulting direction of the needle with the observations of M. de Humboldt, he found that the nearer the poles are supposed to approach to one another, the more did the computed and observed results agree; and when the poles