CHAP, ii.] ELECTRICITY AND MAGNETISM. 85 



reduced to regular shapes by cutting or grinding. 

 Formerly it was the fashion to mount them with soft iron 

 cheeks or " armatures " to serve as pole-pieces. 



For scientific experiments bar magnets of hardened 

 steel are commonly used ; but for many purposes the 

 horse-shoe shape is preferred. In the horse-shoe magnet 

 the poles are bent round so as to approach one another, 

 the advantage here being that so both poles can attract 

 one piece of iron. The " armature," or " keeper," as 

 the piece of soft iron placed across the poles is named, is 

 itself rendered a magnet by induction when placed across 

 the poles ; hence, when both poles magnetise it, the force 

 with which it is attracted to the magnet is the greater. 



1O2. Magnetic Saturation. A magnet to which 

 as powerful a degree of magnetisation as it can attain to 

 has been given is said to be " saturated. " Many of 

 the methods of magnetisation described will excite in a 

 magnet a higher degree of magnetism than it is able to 

 retain permanently. A recently magnetised magnet will 

 occasionally appear to be supersaturated, even after 

 the application of the magnetising force has ceased. 

 Thus a horse-shoe-shaped steel magnet will support a 

 greater weight immediately after being magnetised than 

 it will do after its armature has been once removed from 

 its poles. Even soft iron after being magnetised retains 

 a small amount of magnetism when its temporary mag- 

 netism has disappeared. This small remaining magnetic 

 charge is spoken of as residual magnetism. 



Strength of a Magnet. The "strength" of a 

 magnet is not the same thing as its " lifting-power." The 

 " strength " of a magnet is the " strength " of its poles. 

 The " strength " of a magnet pole must be measured by 

 the magnetic force which it exerts. Thus, suppose there 

 are two magnets, A and B, whose strengths we compare 

 by making them each act upon the N. pole of a third 

 magnet C. If the N. pole of A repels C with twice as 

 much force as that with which the N. pole of B placed 



