=Magnetic Field.=--When a straight bar magnet is held under a piece of card board upon which iron filings are sprinkled, the filings will arrange themselves in curved lines radiating from the poles. If a horse shoe magnet be held at right angles to the plane of the card board, the filings will arrange themselves in curved lines, as shown in fig. 108. These lines are called _magnetic lines of force_ or simply _lines of force_; they show that the medium surrounding a magnet is in a state of stress, the space so affected being called the _magnetic field_. [Illustration: FIG. 100.--Badly magnetized bar. Properly magnetized magnets have only two poles; it is possible, however, by special or careless magnetization, to produce magnets with more than two poles, but no process will produce a magnet with a single pole. If an abnormal magnet with more than two poles be dipped into iron filings, the latter will adhere at places other than the two ends, as shown in the illustration. The polarities are alternately N and S; that is, the regions N, B, N, have north polarity, while A and C have south polarity. These are known as _consequent poles_.] [Illustration: FIGS. 101 to 107.--Effect of breaking a magnet into several parts. If a magnetized needle be broken, each part will be found to be a complete magnet having a N and S pole. The sub-division may be continued indefinitely, but always with the same result as indicated in the figure. This is evidence of the correctness of the molecular theory of magnetism, which states that _the molecules of a magnet are themselves minute magnets arranged in rows with their opposite poles in contact_.] =Ques. What is the extent and character of the magnetic field?= Ans. The influence of a magnet is supposed to extend in all directions indefinitely, however, the effect is very slight beyond a comparatively limited area. [Illustration: FIG. 108.--The region about a magnet in which its magnetic forces can be detected is called the _magnetic field_. This can be represented graphically by placing a piece of cardboard over the magnet and sprinkling iron filings on the paper, gently tapping at the same time. Each filing becomes a temporary magnet by _induction_, and sets itself, like the compass needle, in the direction of the _line of force_ of the magnetic field.] [Illustration: FIG. 109.--Tracing lines of force with a suspended magnet. If a small magnetic needle, suspended by a thread, be held near a magnet, it will point in some fixed direction depending on the proximity of the poles of the magnet. The direction taken by the magnet is called the direction of the force at the point, and if the suspended needle be moved forward in the direction of the pole, it will trace out a curved line which will be found to start from one of the poles, and end at the other. Any number of such lines can be traced; the space filled by these lines of force is called the _magnetic field_.]