GENERAL ACCOUNT OF MAGNETIC ACTIONS 177 



gravity, so the time of vibration of the compass needle is inversely 

 proportional to the square root of the force due to the magnet 

 acting on either pole. Hence by moving the needle about we 

 may investigate the direction of the force on a pole at any point 

 by the direction of the axis, and we may compare the value of the 

 force at different points by observing the time of vibration. 



Lines of force. If a curve is drawn in the magnetic field so 

 that at every point of the curve its direction coincides with that 

 of the magnetic force on a North-seeking pole placed at the point, 

 the curve is called a " line of force." It is to be regarded as having 

 for its positive direction that in which a North-seeking pole would 

 tend to move, and the opposite direction is negative. 



If, for instance, the line of force is in a horizontal plane and a 

 small compass needle is laid at a point in it, the needle will be a 

 tangent to the curve, and we must think of the line of force as 

 coming in to the SSP of the needle and going out of the NSP. 



We may trace a line of force in a horizontal plane by laying a 

 magnet on a table and then moving a small compass needle in its 

 field so that its centre is always travelling in the direction of its 

 length. 



What the compass needle shows in the horizontal plane would 

 be shown all round the magnet in other planes if we could suspend 

 a needle so that it was free to move in any direction round its 

 centre of gravity. 



Lines of force mapped by iron filings. There is a very 

 simple and beautiful method of exhibiting these lines of force in 

 the horizontal plane through the magnet by means of iron filings. 



The magnet is laid on or under a sheet of smooth cardboard, 

 and the card is sprinkled with iron filings. The filings are 

 elongated fragments of iron, and owing to the inductive action of 

 the magnet each filing becomes a little magnet and tends to set 

 with its longer axis in a line of force. The friction, however, 

 prevents immediate setting. If the card is gently tapped the 

 filings are thrown up a little from the card, are momentarily 

 relieved from friction, and so are pulled into the lines of force. 

 They thus map out the direction of the lines. 



Fig. 122 shows the general course of the lines of force round a 

 single bar magnet. 



It is important to notice how the lines of force starting from 

 the NSP curve round and enter the SSP. They start mostly from 

 near the end, and where they are most crowded the frequency of 

 vibration of a compass needle shows that the force there is strongest. 

 Near the middle of the magnet few lines issue or enter, and there 

 the force is less. As the lines issuing from the pole pass outwards 

 they diverge, and a needle shows by its diminishing frequency that 

 the force diminishes as the lines diverge. 



Fig. 123 shows the field due to two magnets arranged parallel, 

 with their unlike poles near each other. It should be noticed in 



