MAGNETISM. 685 MAGNETISM. that in a uniform tiekl the forces acting on the two ends of a bar mag^iet are equal and opposite. This is sometimes expressed by saying that the nortii and south poles of a bar magnet have equal 'strengths.' If a bar magnet is broken in two, both parts are magnets with poles at the ends. This leads to the idea that magnetism is a property of the niolcculps of the magnetic body. In fact, when a bar of iron is magnetized, its volume changes, its elasticity changes, etc.; if it is magnetized and demagnetized rapidly by using an alternat- ing current in the magnetizing helix, its tem- perature rises. (If the temperature of a magnet is raised to red heat, it loses its magnetization.) Further, if a magnet is jarred or twi.sted. or if its temperature is raised, its magnetization is altered. In short, any physical action which affects the molecules of a magnet alters its magnetization ; and any change in the magnetiza- tion alters the molecular properties. This estab- lishes the fact that magnetism itself is a molecular property; and that if a molecule of a magnet could be obtained, it would have a north and a south pole like those of a bar magnet. It is impossible, therefore, to obtain a north pole apart from its equivalent south jiole. There are no such forms of matter as magnetic conductors which enable the poles to be sepa- rated. This fact is illustrated by the phenome- non of induction described below. If two bar magnets are brought near each other, it jnay 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 diflferent points: it is said to be 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 iiKiterial. 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 ea.sy 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 end being attracted toward the magnet's north pole. By this action all the little magnets are more or less arranged in order: and at the end near the north pole of the magnet there will be almost nothing but south poles of the molecules, etc. All the facts of magnetization of iron. e.g. saturation, hys- teresis (q.v.). etc.. can be explained by this idea of the molecules of iron, nickel, etc., being them- selves magnets. If a bar of iron i^ placed lengthwise between two magnets connecting two opposite poles, and This law may be expressed f=-^^, where /t 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 an 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 imit 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 m upon one whose charge is m' at a distance r cm. apait varies directly as the product mm' and inversely as r, and is different for different surrounding media. "•^-~ a quantity which differs for different material media. It is called the 'magnetic permeability' or the magnetic 'inductivity.' (Compare the quantity K in Electrostatics ^xaAc^[ Electricity.) The dimensions of a magnetic charge may be at once found. The square of a charge has the dimensions of fj.r'f, i.e. ;iL=MLT-=. Hence the charge itself has the dimensions L^M^T-V-i. This law of magnetic action has been verified approximately by Coulomb, it being called his law, so far as the statement that the force vanes as — ;- is concerned, and later bv Oauss, 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 a magnetic field so as to indicate by its direction at any point that of the field at that point; a 'iniiform field' is one in which the lines of force are all parallel, and the intensity is the same at all points. 8uch 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 sever.'il means ; the best for many reasons is that of Gauss. If a magnet of any kind or shape is suspended in a uniform macnetic field, free to turn around an axis perpendicular to the direc- tion of the field, it will turn and take some
