GENERAL ACCOUNT OF MAGNETIC ACTIONS 175 



manent magnetisation is usually near the surface of a bar, for if the 

 surface layers be dissolved away by nitric acid the inner core will be 

 found to be nearly unmagnetised. Or if a steel tube with a steel 

 core be magnetised the core on removal is only slightly magnetised. 



Saturation and supersaturation. It is found that there 

 is a limit to the magnetisation of a given bar, depending on its 

 composition, temper, and shape. When the limit is reached the 

 bar is said to be saturated. Just after being magnetised the bar 

 is frequently found to have attained a degree of magnetisation 

 greater than it can permanently retain, and it is then said to be 

 supersaturated. The supersaturation disappears in time if the bar 

 is left to itself, and rapidly if it is shaken. 



Preservation of magnetisation. Armatures. A single 

 magnet falls below even the normal saturation in course of time, 

 especially if subjected to rough usage. To preserve the magnetisa- 

 tion it is usual to keep bar magnets in pairs with their like poles 

 turned in opposite ways, cross-pieces of soft iron termed armatures 

 connecting the unlike poles. Horseshoe magnets are provided 

 with a single armature connecting the poles. The armatures have 

 poles developed by induction as marked in Fig. 120, and each pole 



FIG. 120. 



Pole 

 oF soFt 



pieces 

 iron. 



being faced by an unlike pole, they are supposed to tend to 

 strengthen rather than to weaken each other. 



Mr. Hookham * has found that if a bundle of bar magnets has 

 large soft iron " pole pieces," Fig. 121, 

 fixed on the ends, the surfaces of the pole 

 pieces being separated only by a very 

 small air space, the horseshoe magnet 

 which is thus virtually formed retains its 

 magnetisation indefinitely without an 

 armature. 



Long thin bars retain their mag- 

 netisation much better than short thick 

 ones. 



Portative force. Experiments have 

 been made to find a general formula for 

 the weight which a horseshoe magnet 

 can sustain, or rather the force which is required to pull away its 

 armature. Hacker gives 



* I'hil. M:/.,[5], vol. xxvii, p. 186. 



> a r ri a q n e c s 



FIG. 121. 



