If two bar magnets are brought near each other, it .may be shown that like poles repel each other, and unlike poles attract each other: and further the action of a north pole of any one magnet on the north pole of another is equal and opposite to that of the south pole if the former is placed at the same distance from the north pole of the second magnet.
If a bar magnet is placed in air, and if a piece of any matter different from air is brought near it, this piece is observed to manifest mag netic forces at different points; it is said to he magnetized by 'induction,' and the forces of attraction or repulsion ordinarily observed with magnets acting on iron. etc., are due to the presence of these 'induced charges' of magnetism. If the piece of matter is of iron or any magnetic material, it is magnetized in such a manner that, if it is nearest the north pole of the magnet, its face which is next this pole is a south pole. (If the piece of matter is bismuth, the opposite is true.) It is perfectly easy to explain the induction of iron or other magnetic substances if it is assumed that each molecule of the mag netic substance is a magnet. Then, before this body is put near the magnet, the molecular mag nets are standing at random, and there is no ex ternal action ; but, when it is brought near the magnet, each molecular magnet is acted upon by a couple which tends to make it turn and point toward the magnet, its south mid being attracted toward the magnet's north pole. By this action all the little magnets are or less arranged in order: and at the end near the north pole of the magnet there will be almost nothing hut south poles of the molecules. etc. All the facts of magnetization of iron. e.g. saturation. hys teresis (q.v.). etc.. can he explained by this idea of the molecules of iron, nickel, etc.. being them selves magnets.
If a bar of iron is placed lengthwise between two magnets connecting two opposite poles, and if two bar magnets with their opposite poles together are held nearly vertically at the middle point of the iron bar and then drawn slowly apart along the iron bar, it becomes magnetized, especially if the process is repeated several times. This is known as the method of 'divided touch.' Its explanation is evident from the theory of molecular magnets.
It is observed that if a small magnet is pivoted, free to turn, inside a helix of wire, it will place itself parallel to the axis of the helix when au electric current is passed through the latter. It is evident, then, why a bar of iron placed through the helix becomes magnetized by the action of an electric current.
The law of action of magnets on each other may be given if the words 'equal poles' and a unit magnetic charge or 'unit pole' are defined. Two magnetic poles are defined as being equal if they have the same action on any third pole; and a 'unit pole' is chosen to be such that when acting on another unit pole at a distance of 1 cm. in a vacuum the force is 1 dyne. To find the numerical value to give any pole it is necessary to find what combination of unit poles has the same action on a third pole. Experi ments then show that the action of a pole whose magnetic charge is at upon one whose charge is ne at a distance r em. apart varies directly as the product min' and inversely as and is different for different surrounding media. This Ulm l law may be expressed f = 77- , where It is a quantity which differs for different material media. It is called the 'magnetic permeability' or the magnetic 'inductility.' (Compare the quantity K in Mcetrostaties under ELECTRICITY.) The dimensions of a magnetic charge may be at once found. The square of a charge has the dimensions of /A i.e. g Hence the charge itself has the dimensions This law of magnetic action has been verified approximately by Coulomb, it being called his law. so far as the statement that the force anal varies as is concerned. and later by Gauss, r- who used an indirect method, and by the count less experiments and calculations made daily by electrical engineers. The importance of recog nizing the effect of the surrounding medium was first emphasized by Faraday.
The region around a magnet is called its 'field of force:' the 'intensity' of the field at any point is the force which would act on a unit north pole if placed there: the 'direction' of the field is the direction of the intensity; a 'line of mag netic force' is a line drawn in it magnetic field so as to indicate by its direction at any point that of the field at that. point : a 'uniform field' is one in which the lines of force ore all parallel, and the intensity is the same at all points. Such a field is a limited region on the earth due to magnetic action of the earth, or the space be tween the poles of a horseshoe magnet or be tween the field-magnets of a dynamo. The in
tensity of any uniform field may be measured by several means; the best for many reasons is that of Gauss. If a magnet of any kind or shape is suspended in a uniform magnetie field. free to turn around an axis perpendicular to the direc tion of the field, it will turn and take some definite position; in order to keep this magnet in a position at 90° from this against the action of the field of force will require the application by some external agency of a certain couple or moment; the amount of this couple, if the in tensity of the uniform field is unity, is called the 'magnetic moment' of the magnet about the given axis. If the magnet is uniformly mag netized—i.e. if in case it is broken into two, three, etc., equal parts, they will all be alike in every way—the magnetic moment divided by the volume of the magnet is called its 'intensity of magnetization.' If the magnet is a bar or rod or wire, uniformly magnetized, so that there is no magnetization except at the ends, it is called a `solenoid,' and the distribution of magnetic charges is called 'solenoidal' (thus the magnetic action of a long helix of wire carrying an electric current is approximately solenoidal). If the magnet consists of a thin sheet with all the molecular magnets side by side, having their north poles on one face of the sheet and their south poles on the other, it is called a 'lamellar' distribution. (Thus the magnetic action of a single loop of wire carrying an electric current has this lamellar property.) Lines of magnetic force may be drawn by placing a small magnetic needle at different points in the field of force and noting its direc tion. Still another method is to sprinkle iron filings through the field, e.g. over a glass plate or smooth piece of paper; each filing becomes magnetized by induction, and if the filings are jarred slightly each will turn and place itself along the line of force at the point where it is. (Actually there will be a force of attraction toward the magnet; but it is resisted by the friction between the filing and its supporting plate or paper.) It may be observed imme diately that lines of force join opposite poles of magnets in the air; and, since the molecules of a magnetic substance are also magnets, the lines of force can be imagined as proceeding from molecule to molecule inside the magnet. In this sense, lines of magnetic force are continuous closed curves—not like lines of electric force which end on charged surfaces. Tubes can he imagined constructed, by choosing somewhere in a magnetic field a small closed curve, and draw ing through each point of it the line of force. Such tubes are continuous and closed like an ordinaly piece of rubber tubing with its two ends brought together. If the cross-section of these tubes is so chosen that the number passing out at the north pole of a magnet on which there is a magnetic• charge m is 47rnt. they are called `tubes of induction.' It is shown in electricity that if the number of such tubes passing through a closed circuit of wire is changed, there will he an induced current in the metallic circuit; the total quantity of electricity carried by this current varies directly as the change in the number of tubes of induction. This gives the simplest and most accurate method of deter mining the distribution of magnetic charge over the surface of a magnet. If the magnet is in the form of a rod, a coil of wire having its two ends joined to a galvanometer may be slipped over it. The tubes of induction enter the magnet wherever there is any south magnetic charge, i.e. where the south poles of the molecular magnets reach the surface, and leave it where there is any north magnetic charge, i.e. where the north poles of the molecular magnets reach the surface. Tubes are crowded together about the middle of the rod and then escape at the sides and south ends, returning into the sides and end at the other pole of the rod. As the small explor ing coil is pushed along the rod from the centre to the north end, the number of tubes inside decreases, owing to the passage out from the rod of the tubes of induction; the induced quantity of electricity measures the decrease in the tubes; that is, the number of tubes leaving the sides of the magnet, and therefore the magnetic charge over them, because one tube corresponds to a magnetic charge 417-.
The energy of a magnetic field is identical with that associated with an electric current. (See ELECTRICITY.) Whenever attraction or repulsion is observed to take place, the motions must be such as to decrease the potential energy and increase the kinetic, at least temporarily.