704 ELECTEO-MAGNETS. 



while under the influence of a current or another piece 

 of iron which is magnetic. In the former case, when 

 the iron becomes magnetic under the influence of an 

 electric current, it is called an electro -magnet, and 

 the condition of the iron in that state, with various other 

 connected phenomena, are comprised under the name 

 electro - magnetism . 



If an electro-magnet, consisting of a small bar of iron 

 round which a covered copper wire is spirally coiled, 

 as shown in fig. 355, be freely suspended, so that the 

 free ends of the spiral may dip into the mercury con- 

 tained in the small wooden apparatus, figs. 351 B and 

 352 A, it will place itself with considerable energy 

 in a direction from north to south, or more exactly 

 from nearly north-north-west to south-south-east ; the 

 northern end will point somewhat to the west, the 

 southern a little to the east. The ends of an electro- 

 magnet, and generally of every piece of iron in the 

 magnetic state, are called its poles; that end of a freely 

 suspended electro-magnet which points towards north 



FIG. 355 (real size). 



is the north pole, the end which points towards south 

 the south pole. 



If, while the electro-magnet is in its north-south 

 position, the direction of the current in the spiral, and 

 therefore also of the Amperian currents in the bar of 



