Magnetic induction

“ The principal points of difference are that (1) the magnetic permeability, unlike the electric conductivity, which is independent of the strength of the current, is not in general constant; (2) there is no perfect insulator for magnetic induction, which will pass more or less freely through all known substances. Nevertheless, many important problems relating to the distribution of magnetic induction may be solved by methods similar to those employed for the solution of analogous problems in electricity. ”

“ The coefficient of magnetic induction varies with the material of the
induced mass, and varies with the intensity of the magnetizing force.
This variation is due to the fact that as the induced magnetism in a
body increases, the magnetizing force required to maintain such
induction, increases in a more rapid ratio. The coefficient of magnetic
induction is the same as magnetic permeability, and in a certain sense
is the analogue of conductivity. It is also termed the multiplying power
of the body or core magnetized. ”

“ It is evident, from the last 3 experiments, that lines of force will pass through iron whenever possible, on their way from the N to the S pole of a magnet. When the armature of a horseshoe magnet is in place, most of the lines of magnetic induction crowd together and pass through it rather than push their way through the air. Air is not a good conductor of lines of force; and the magnet has to do work to overcome the resistance of the air, when the armature is removed, in order to complete the magnetic circuit. This work causes a magnet to become gradually weaker. ”

“ No substance has yet been discovered having a negative susceptibility sufficiently great to render the permeability (=1 + 4πκ) negative.
Magnetic Circuit.—The circulation of magnetic induction or flux through magnetic and non-magnetic substances, such as iron and air, is in many respects analogous to that of an electric current through good and bad conductors. Just as the lines of flow of an electric current all pass in closed curves through the battery or other generator, so do all the lines of induction pass in closed curves through the magnet or magnetizing coil. ”

“ For most practical purpose a knowledge of the exact position of the poles is of no importance; the magnetic moment, and therefore the mean magnetization, can always be determined with accuracy.
Magnetic Induction or Magnetic Flux.—When magnetic force acts on any medium, whether magnetic, diamagnetic or neutral, it produces within it a phenomenon of the nature of a flux or flow called magnetic induction (Maxwell, loc. cit., § 428) . Magnetic induction, like other fluxes such as electrical, thermal or fluid currents, is defined with reference to an area ”

“ The process of induction has ever been a great mystery in electrical science. Magnetic currents are known to act upon bodies in close proximity without the intervention of a spark, and to indue such bodies with magnetic force. This action, called induction, has been supposed to be limited to short distances. This we believe to be erroneous. In order that the inductive process take place, it is only necessary to suppose some impulse to be superinduced upon some pervading medium. This medium we recognize in the static vito-magnetic constituent of the atmosphere. ”

“ If is the intensity of magnetization, and A, B, C its components, then, by Art. 400,
We may call these the equations of magnetization, and they indicate that in the electromagnetic system the magnetic induction , considered as a vector, is the sum, in the Hamiltonian sense, of two vectors, the magnetic force , and the magnetization , multi plied by 4 π, or
In certain substances, the magnetization depends on the magnetic force, and this is expressed by the system of equations of induced magnetism given at Arts. ”

“ In the second place, considering the large dotted circle as the section of a diamagnetic cylinder, the dotted straight lines within it, together with the lines external to it, represent the effect of a diamagnetic substance in separating
the lines of induction and drawing together the equipotential surfaces, such a substance being a worse conductor of magnetic induction than the surrounding medium. Case of a Sphere in which the Coefficients of Magnetization are Different in Different Directions. ”

“ 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. ”

“ The facility with which electric currents are produced in metals when moving under the influence of magnets, suggests that henceforth precautions should always be taken, in experiments upon metals and magnets, to guard against such effects. Considering the universality of the magnetic influence of the earth, it is a consequence which appears very extraordinary to the mind, that scarcely any piece of metal can be moved in contact with others, either at rest, or in motion with different velocities or in varying directions, without an electric current existing within them. ”

“ Within the magnet, running through its substance, existed lines of force of the same nature as those without, exactly equal in amount to those without, and were, indeed, continuous with them. The conclusion must logically be that every line of force is a closed circuit.
Having thus established the exact quantitative laws of magneto-electric induction, he then advanced to make use of the induced current as a means of investigating the presence, direction, and amount of magnetic forces—in other words, to explore and measure magnetic fields. ”

“ If a wire of soft iron is substituted for the suspended magnetic needle, either pole of the bar-magnet will attract either end of the wire indifferently. The wire will in fact become temporarily magnetized by induction, that end of it which is nearest to the pole of the magnet acquiring opposite polarity, and behaving as if it were the pole of a permanent magnet. Even a permanent magnet is susceptible of induction, its polarity becoming thereby strengthened, weakened, or possibly reversed. ”

“ Let the current be returned to the wire after disappearance of magnetism, and the lines again seem to emanate from the wire and at the same time cut it and produce a counter potential in it, which is the index of the abstraction of energy from the circuit, and its storing up in the form of elastically strained lines of magnetism around the conductor. The effect is that of self-induction on making or upon increase of current, the measure of the amount being the energy stored in the magnetic circuits which have been extended or opened up by the current. ”

“ When the load on a hardened wire is gradually increased, the maximum value of I is found to correspond with a greater stress than when the load is gradually diminished, this being an effect of hysteresis. Analogous changes are observed in the residual magnetization which remains after the wire has been subjected to fields of different strength. The effects of longitudinal pressure are opposite to those of traction; when the cyclic condition has been reached, pressure reduces the magnetization of iron in weak fields and increases it in strong fields (Ewing, Magnetic Induction, 1900, 223) . ”

“ Greater self-induction in the armature coils;
4. Construction more expensive;
5. Leakage of magnetic lines through core, exterior to winding.
The generation of eddy currents in the polar faces may be overcome by making the air gap at least 50 per cent. of the distance between the teeth, so that the magnetic lines can spread from the corners of the teeth, and become nearly uniformly distributed over the polar faces. Magnetic leakage through the core may be reduced by making the amount of metal above the inductors very small. ”

“ Here, therefore, currents of electricity were produced by the direct inductive power of the earth's magnetism, without the use of any ferruginous matter, and upon a metal not capable of exhibiting any of the ordinary magnetic phenomena. The experiment in everything represents the effects produced by bringing the same helix to one or both poles of any powerful magnet (50.) ”

“ It thus appeared that an electrical current could be produced by the movement of the bar magnet—by dropping it into the coil or taking it out; and the current so produced he called an induced current. This operation is called magneto-electric induction. In 1845 Faraday greatly extended his magnetic discoveries. He not only established the magnetic condition of all matter by showing that every known body or thing could be influenced by magnetism, but he discovered a new property of magnetism, which he called diamagnetism. This was considered his greatest discovery. ”

“ They produce lines of force which may exhaust the
permeability of the iron, or may, if in an opposite direction, add to
its apparent permeability.
Magnetic Induction, Coefficient of.
The number, obtained by dividing the magnetization of a body, expressed
in lines of force produced in it, by the magnetizing force which has
produced such magnetization, expressed in lines of force producible by
the force in question in air. It always exceeds unity for iron, nickel
and cobalt. It is also obtained by multiplying the coefficient of
induced magnetization by 4 PI (4 * 3.14159) and adding 1. ”

“ Obviously, anything that will reduce the amount of air or other non-magnetic material that is in the magnetic circuit will tend to reduce the reluctance, and, therefore, to increase the total magnetization resulting from a given magnetizing force. ”

“ Therein he shows, as the result of his customary careful experimental explorations that the magnetism of every known substance (even tissues of the human frame) is manifested in one of two ways. Either the body is, like iron, attracted by the magnet, taking a position coincident with the magnetic forces which he calls paramagnetic (para beside or near, magnetes, magnes, magnet) or bodies—like bismuth, for instance—are repelled by the poles and should therefore be called diamagnetic (dia, across) for they set themselves across, equatorially, or at right angles to the magnetic lines. ”

“ The only assumption relative to induced magnetism which is involved in these equations is, that if a magnetic force X due to external magnetism produces in the iron of the ship an induced magnetization, and if this induced magnetization exerts on the compass needle a disturbing force whose components are X', Y', Z', then, if the external magnetic force is altered in a given ratio, the components of the disturbing force will be altered in the same ratio. ”

“ The plan we adopt is to excite the electro magnet by the whole or a part of the current which is to be measured. Since this current varies, the power exciting the core of the electro magnet must also vary; and since we require the core to have as nearly as possible a permanent magnetic force, we are brought face to face with the question, whether an electro magnet can be constructed that has a constant moment under varying exciting currents. ”

“ Let us assume a plain iron core, Fig. 7, magnetized as indicated, so that its poles, N, S, complete their magnetic circuits by what is called free field or lines in space around it. Let a coil of wire be wound thereon as indicated. Now assume that the magnetism is to be lost or cease, either suddenly or slowly. An electric potential will be set up in the coil, and if it has a circuit, work or energy will be produced or given out in that circuit, and in any other inductively related to it. Hence the magnetic field represents work or potential energy. ”

“ Electromagnets with iron armatures in contact across their poles will retain, after the current has been cut off, a very large part of their magnetism, even if the cores be of the softest of iron. But so soon as the armature is wrenched off, the magnetism disappears. An air gap in a magnetic circuit always tends to hasten demagnetizing. A magnetic circuit composed of a long air path and a short iron path demagnetizes itself much more rapidly than one composed of a short air path and a long iron path. ”

“ The magnetism acquired by soft iron while it is in a magnetic field. Soft iron will lose its induced magnetism when it is removed from a magnetic field. The strength and polarity of the induced magnetism will alter immediately as its magnetic latitude, or its orientation in a magnetic field, is changed. The induced magnetism has an immediate effect upon the magnetic compass as the magnetic latitude or heading of a craft changes. ”

“ If the direction of the current is reversed, the needle will be deflected in the opposite direction. From this experiment we see that the current has brought magnetic influences into play, or in other words has produced magnetism. If iron filings are brought near the wire while the current is flowing, they are at once attracted and cling to the wire, but as soon as the current is stopped they drop off. This shows us that the wire itself becomes a magnet during the passage of the current, and that it loses its magnetism when the current ceases to flow. ”