MAGNETISM. 68G MAGNETISM. 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 anioimt 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 710 magnetization exce])t 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 be 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 ordinaiy piece of ribbpr 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 4r)«, 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 be an induced current in the metallic circuit; the total quantity of electricity carried by this current varies directly as the change in the inimher 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 4-. The energj' 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 mu.st be such as to decrease the potential energy and increase the kinetic, at least temporarily. JIagnetic Properties of Iron. We have seen that when an electric current is sent through a coil or spiral of wire the coil is found to have the properties of a magnet; that is to say, it w'ill at- tract bits of iron, dellect a magnetic needle, etc. The region around the spiral when the current ia flowing in it is called a magnetic field. The 24 32 H FlO. 1. CURVES OF .MAGXETIZATIOS. intensity or strength of the field is defined as the force which would act on a unit north pole, and so varies from point to point in the neighborhood of the spiral. In describing this field the con- vention has been adopted of imagining the ficlil traversed by lines of force which indicate by their direction the path the unit north pole would follow if free to move. When the field has unit strength it is said to contain one line <if force per square centimeter perpendicular to the direction of the line. It may be shown that the intensity of the field, or the number of lines of force per square centimeter within the spiral carrying the current, is represented by the formula H= 0.47rj?i', H being the intensity of the field, n the number of turns per unit length of the spiral, and i the current strength in amperes. If now the coil is wound on a core of iron, nickel, or cobalt, its properties as a magnet are found to be greatly intensified ; that is to say, the number of lines of force per square centi- meter is increased ; i.e. the core has become mag- netized. The number of lines of force per square centimeter within the core is called the induc- tion, and is usually designated by the letter B. The ratio of the induction B to the intensity of the field H is called the'pfrmenhiliti/, fi; i.e. B = /iH. For air B is evidently equal to H, k
