(A) Magnetic lines of force run side by side and do not cross one another. (See magnetic fields.) (B) Magnetic lines of force are believed to form "_closed curves_" or to be continuous. The part outside of the magnet is a continuation of the part within the magnet. (See Fig. 180.) [Illustration: FIG. 180.--Diagram of the field of a bar magnet.] (C) The attraction of a magnet is strongest where the magnetic lines are thickest, hence they are believed to be the means by which a magnet attracts. (D) Since like poles repel and unlike poles attract, it is known that the action along a line of force is not the same in both directions. It has therefore been agreed by physicists to indicate by an arrow head (Fig. 180), the direction that a north-seeking pole tends to move along a line of force. The lines of force are considered as leaving the north-seeking pole of a magnet and entering the south-seeking pole. (See Figs. 181 and 182.) [Illustration: FIG. 181.--Magnetic field between like poles showing repulsion.] (E) A freely suspended small magnet in a magnetic field places itself parallel to the lines of force. (Test this by holding a magnetic compass in different portions of a magnetic field). Note the position of the needle and the lines of force. This fact indicates that the compass needle points north on account of its tendency to turn so as to be parallel to the earth's magnetic held. [Illustration: FIG. 182.--Magnetic field between unlike poles showing attraction.] (F) Each magnet is accompanied by its own magnetic field. When a piece of iron is brought within the field of a magnet the lines of force passing through the iron tend to arrange the iron molecules in line or to magnetize the iron. =207. Magnetic Induction.=--The action of magnetic lines of force in magnetizing iron when they pass through it, is called _Magnetic Induction_. This may now be defined as _the production of magnetism in a body by placing it within a magnetic field_. Freely suspended magnets place themselves parallel to the lines of force in a magnetic field, therefore when an iron rod is placed in a weak field, or one with few lines of force, the iron is but slightly magnetized; that is, but few molecules are brought into line. Increasing the strength of the magnetizing field, gives stronger magnetization to the iron up to a certain point. After this, stronger fields give no increase in magnetizing effect. When iron exhibits its greatest magnetization it is said to be _saturated_. [Illustration: FIG. 183.--Effect of a piece of iron in a magnetic field.]