182 



MAGNETISM 



in their positive direction, the direction of flow of the current in 

 the coil must be that in which the head is turned round. If the 

 current be reversed, the magnetisation is reversed. The force on a 

 pole inside the coil is proportional to the current. 



Now let us imagine that two equal coils, or solenoids, A and B, 

 are arranged as in Fig. 129 so that the same current flows in 

 opposite directions round them. Let a small needle be placed mid- 

 way between the near ends. It will be at a neutral point that is, 

 a point where the two coil magnets neutralise each other and if 



B 



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the coils are placed East and West the needle will set N and S 

 under the earth's force which is perpendicular to the common a\i-. 



Now let a bar of iron be placed within A. When the current 

 is put on, the bar at once becomes a magnet and there is nothing 

 on the other side, B, to counteract its magnetic action on the needle, 

 which will therefore be deflected and by an amount which will 

 indicate the magnetisation of the bar. We shall here assume that 

 the magnetisation is proportional to the tangent of the angle of 

 deflection, and that the magnetising force producing it is due to 

 the current in A and is proportional to that current, B being so far 

 away as to have no appreciable action. 



Imagine that we make a series of measurements of magnetising 

 force as given by the current and of the accompanying magnetisa- 

 tion as given by the deflection of the needle, gradually increasing 

 the current from zero to a large value, and let the magnetisation 

 be plotted against magnetising force on a diagram. The general 

 nature of the result is indicated in Fig. 130. 



The curve of magnetisation, though varying greatly with 

 different kinds of iron, shows in general three characteristic parts : 

 a part O A for very small magnetising forces, at a very small slope, a 

 part A B sloping very steeply, and a part B C tending to become a 

 nearly horizontal line for very great forces. These three parts 

 change from one to another more or less gradually. 



If, instead of merely increasing the magnetising force indefinitely, 

 we take it up to a certain point and then gradually decrease and 



