4 ELEMENTS OF ELECTRICAL ENGINEERING. 



in equation (i) is an attraction. It is customary to consider a 

 north pole as positive and a south pole as negative. 



Equality of north and south poles of a magnet. The two 

 poles of a magnet are always equal in strength though opposite 

 in sign. When a magnet is broken in two, each piece is a com- 

 plete magnet with a north pole and a south pole. 



A bar of steel may be irregularly magnetized so as to have 

 one or more north poles and one or more south poles ; one or 

 more of the poles, sometimes called consequent poles, being near 

 the middle of the bar. In such a case the sum of the strengths 

 of all the north poles is equal to the sum of the strengths of all 

 of the south poles. 



It is often convenient to speak of an isolated magnet pole, mean- 

 ing one pole of a very long slim magnet, the other pole of which 

 is so far away as to be negligible in its effects. 



4, Magnetic field. A magnetic field is any region in which a 

 magnet pole if present is acted upon by a force tending to pull it 

 in one direction or another. For example, the region surround- 

 ing a magnet is a magnetic field ; the region surrounding a wire 

 carrying an electric current is a magnetic field ; the behavior of 

 a compass needle shows that the entire region surrounding the 

 earth is a magnetic field. 



A magnetic field has other important effects besides the force 

 action which it exerts upon a magnet pole ; however, this force 

 action is the effect which is usually employed as a basis for the 

 discussion of the ideas of direction and intensity of a magnetic 

 field at a point. Given a magnetic field, the field surrounding a 

 wire carrying electric current for example ; in order to direct at- 

 tention to the nature of this field at a given point we imagine 

 a magnet pole placed at that point, and we consider the force 

 with which the field acts upon this pole. A magnet pole 

 which is used in this way as an indicator of magnetic field we call 

 a test pole. 



Intensity of a magnetic field at a point, A magnetic field is 

 said to be intense when it exerts a relatively great force upon a 



