GENERAL ACCOUNT OF MAGNETIC ACTIONS 173 



These methods of making magnets give interesting illustrations 

 of magnetisation by induction, bub they are now little used 

 except sometimes for compass needles and for the needles used in 

 the determination of the dip of the earth's magnetic force, in which 

 case the divided touch is employed.* They are for general purposes 

 entirely superseded by 



The electro-magnetic method. In this method a coil of 

 wire is wrapped round a hollow cylinder, forming what is termed 

 a solenoid, the bar to be magnetised is inserted in the cylinder, and 

 a strong electric current is passed through the coil. When the 

 current is broken and the bar is withdrawn it is found to be 

 magnetised. 



Electro -magnets. The electro-magnetic method is not 

 only useful for thus making permanent magnets by the inser- 

 tion of bars and their withdrawal when magnetised. If a coil 

 of wire carrying a current surrounds an iron core that core is 

 a powerful magnet while the current is running, though on the 

 stoppage of the current the magnetisation in general falls off very 

 greatly. By far the strongest magnets are made by some such 

 arrangement as that represented in Fig. 118, where the two limbs 



U^k rm rm A^J 



FIG. 118. 



consist of soft iron surrounded by coils through which a current 

 may be passed, the limbs being joined by a soft iron cross-piece as 

 the base. With a strong current passing round the two limbs 

 from inside to outside in both limbs as seen from the front, 

 or in the opposite direction in both, the ends of the soft 

 iron become exceedingly strong poles, far stronger than can be 

 obtained in permanent magnets of steel. It is to be observed, 

 however, that the magnetism only maintains its great strength 

 while the current is passing. 



* A full account of the methods used before the introduction of the electro- 

 magnetic method will be found in the Library of Useful Knoulcdgt, arUc.te 

 Magnetism, chap. v. (1832). 



