The first thing that we will look at is the magnetic field which is always associated with every magnet. The magnetic field may be defined as that region or space around every magnet in which the magnetic force acts or is in operation. An illustration of a magnetic field may easily be obtained by taking a bar magnet and bringing near to it a magnetized needle, when it will be found that the needle will set itself in various positions relative to the magnet, on account of the lines of force which exist in the field. Thus let _A_ _B_ (Fig. 17) be a bar magnet with its North pole at point _A_ and South pole at point _B_. If a number of freely suspended needles be hung above it, as shown in the figure, they assume the positions indicated there. It will be seen that at the North and South poles the needles hang vertical, while midway between the two poles there is no dip of the needle, as it is parallel to the bar magnet; while between the place of no dip and the place of vertical dip, which is directly over each pole, the dip gradually changes, becoming more and more vertical as it gets nearer to the pole. If the bar magnet be a strong one, then its magnetic field will be manifested at a great distance; and any magnetized needle brought into the field will be, affected by the same, and will tend to set itself along the lines of force. As already stated in Art. 80, it was Faraday who originated the term "Lines of Force," and gave to the world some idea of the motions of the aetherial medium, which plays so important a part in electro-magnetic phenomena. A visible manifestation of these lines of force which gather round every magnet may be made by strewing iron filings over a piece of glass, underneath which are several bar magnets, when it will be found that the iron filings will set themselves in well-defined lines or curves, which Faraday termed "Lines of Force." [Illustration: Fig: 18.] As the bar magnets are placed in different positions, North pole to North pole, or North to South, and so on, the iron filings will change the figures assumed, indicating in, each case the effect of the lines of force of each magnet upon one another. The iron filings strewed over the magnet are magnetized by induction, with the result that the North pole of one filing attracts the South pole of the next one to it, and this is continued along the whole of one line of force, as revealed by the united iron filings. Faraday believed in the real physical existence of these lines of force, and that belief has been perfected by Clerk Maxwell in two papers which he wrote on "Physical Lines of Force," which will be considered in another article. We will simply deal with them at present as indications of the existence of the magnetic forces in the medium surrounding any magnet.