INDUCTION FKOM TERRESTRIAL MAGNETISM. 135 



follows the same law, and therefore that the energy of 

 the induced magnetism is inversely as the square of the 

 distance. Consequently, the attraction between the two 

 magnetisms (the permanent magnetism of the magnet 

 and the induced magnetism of the iron), which is as the 

 product of these magnetisms directly and as the square 

 of the distance inversely, will be inversely as the fourth 

 power of the distance. With increase of distance there- 

 fore the attraction diminishes very rapidly. 



When the distance is so far increased that the effect 

 of the farther pole of the magnet, though diminished, is 

 less diminished than that of the nearer pole, and be- 

 comes comparatively sensible, it tends still more to 

 diminish the attraction. And, on the whole, the at- 

 traction diminishes with extreme rapidity, and is sensi- 

 ble only at very small distances. 



60. Induction of magnetism in soft iron, produced 

 by terrestrial magnetism. 



Take a bar of soft iron, which for convenience of 

 language we will suppose to have one end painted white 

 and the other end black : hold it vertical, with the 

 black end downwards. Upon applying any of the ordi- 

 nary tests, it will instantly be found that the bar in this 

 position is a genuine magnet, and that its black end is 

 charged with red magnetism and its white end with 

 blue magnetism. The easiest proof will be, holding it 

 parallel to itself, to carry it round a small compass : if 

 the black or lower end is at the level of the compass, it 



