62 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



Such an iron rod with its winding of wire is called an electro- 

 magnet. The iron rod is called the core, the coil of wire is called 

 the ivinding, and the electric current which flows through the 

 coil is called the exciting current of the electromagnet. 



The permanent magnet. When the core of an electromagnet 

 is made of soft iron it loses * its magnetism very quickly and 

 almost completely when the exciting current ceases to flow. 

 When the core of an electromagnet is made of hardened steel, 

 however, it retains its magnetized condition very persistently after 

 the exciting current has ceased to flow, and of course 

 such a bar of magnetized steel may be removed from 

 the magnetizing winding. A steel bar magnetized in 

 this way is called a permanent magnet. A perma- 

 nent magnet, so-called, loses its magnetism more or 

 less rapidly when it is subjected to mechanical shocks 

 or temperature changes. 



Poles of a magnet. Compass. Naming the poles. 

 Certain parts only of a magnet possess the power of 

 attracting iron. These parts of a magnet are called its 

 poles. The poles of a bar magnet, for example, are 

 usually at its ends. Thus, Fig. 28 shows a bar mag- 

 net with iron filings clinging to its ends. A horizontal magnet 

 which is free to turn about a vertical axis places itself, at most 

 places on the earth approximately north and south. This beha- 

 vior of a magnet is exemplified in the ordinary magnetic com- 

 pass which consists of a pivoted magnet playing over a divided 

 circle. The terms magnetic north, magnetic east, etc., are occa- 

 sionally used in referring to the cardinal points of the compass 

 as indicated by the compass needle. 



The north pointing pole of a magnet is called its north pole, 

 and the south pointing pole of a magnet is called its south pole. 

 Mutual force action of two magnets. When a magnet is sud- 

 denly brought near to a compass, the compass needle is set more 



* Except when the core is long and slim, or when the core is part of a complete 

 iron circuit. 



