CHAP, ii.] ELECTRICITY AND MAGNETISM. 87 



LESSON X. Distribution of Magnetism. 



104. Normal Distribution. In an ordinary bar 

 magnet the poles are not quite at the ends of the bar, 

 but a little way from it ; and it can be shown that this is 

 a result of the way in which the magnetism is distributed 

 in the bar. A very iong, thin, uniformly magnetised bar 

 has its poles at the ends ; but in ordinary thick magnets 

 the " pole " occupies a considerable region, the " free 

 magnetism " falling off gradually from the ends of the 

 bar. In each region, however, a point can be determined 

 at which the resultant magnetic forces act, and which 

 may for most purposes be considered as the pole. In 

 certain cases of irregular magnetisation it is possible to 

 have one or more poles between those at the ends. 

 Such poles are called consequent poles (see Fig. 51). 



105. Magnetic Field. The space all round a 

 magnet pervaded by the magnetic forces is termed the 

 "field" of that magnet. It is most intense near the pole 

 of the magnet, and is weaker and weaker at greater dis- 

 tances away from it. At every point in a magnetic field 

 the force has a particular strength, and the magnetic 

 induction acts in a particular direction or line. In the 

 horse-shoe magnet the field is most intense between the 

 two poles, and the lines of magnetic induction are curves 

 which pass from one pole to the other across the field. 

 A practical way of investigating the distribution of the 

 lines of induction in a field is given in Art. 108, under the 

 title " Magnetic Figures." When the armature is placed 

 upon the poles of a horse-shoe magnet, the force of the 

 magnet on all the external regions is weakened, for the 

 induction now goes on through the iron of the keeper, 

 not through the surrounding space. In fact a closed 

 system of magnets such as that made by placing four 

 bar magnets along the sides of a square, the N. pole of 

 one touching the S. pole of the next has no external 

 field of force. A ring of steel may thus be magnetised 



