ADVANCED 

 ELECTRICITY AND MAGNETISM. 



CHAPTER I. 



ELEMENTARY THEORY OF MAGNETISM. 



1. Ferromagnetism and electromagnetism. There are two 

 groups of magnetic phenomena, namely, (a) the phenomena of 

 ferromagnetism, that is to say, the phenomena which are asso- 

 ciated with magnetized iron and steel, and (6) the phenomena 

 of electromagnetism, that is to say, the magnetic phenomena 

 which are exhibited by the electric current. In developing the 

 theory of magnetism it is best to consider some phases of ferro- 

 magnetism first, because the phenomena of ferromagnetism are 

 more familiar than the phenomena of electromagnetism and 

 because important magnetic measurements are based upon 

 ferromagnetism. 



The phenomena of "electromagnetism are comparatively ob- 

 scure, and in many cases imperceptible, except where they are 

 enhanced by the presence of iron. Thus a dynamo or a trans- 

 former would operate if all iron parts were removed, but the 

 effects produced would be in most cases nearly imperceptible. 

 Practically, the phenomena of ferromagnetism and the phenomena 

 of electromagnetism are inextricably mixed up with each other. 



2. Poles of a magnet. The familiar property of a magnet, 

 namely, its attraction for iron, is possessed only by certain parts 

 of the magnet. These parts of a magnet are called the poles 

 of the magnet. For example, the poles of a straight bar-magnet 

 are usually at the ends of the bar. 



