12 



ELEMENTS OF ELECTRICAL ENGINEERING. 



iron (south pole). When the magnetic flux is confined wholly 

 to air as in case of the magnetic field surrounding a wire carrying 

 an electric current there are of course no magnet poles. When 

 the magnetic flux is confined wholly to iron as in case of an endless 

 iron rod in the form of a ring there are likewise no magnetic poles. 



8. Magnetization. When a piece of iron or other magnetic 

 substance is placed in a magnetic field it becomes a magnet. 

 Thus a bar of iron or steel placed in the intense magnetic field 

 inside of a coil of wire through which an electric current is flow- 

 ing, becomes strongly magnetized. A neutral or unmagnetized 

 bar of iron when held in the direction of the earth's magnetic 

 field shows north polarity at the north-pointing end and south 





Fig. 4. 



polarity at the other end. If the bar is turned end for end, the 

 magnetism of the bar is reversed. A sharp blow with a ham- 

 mer renders the bar more susceptible to the weak magnetizing 

 action of the earth's field. The polarity of the ends of the 

 bar may be easily determined by means of a compass needle. 

 This action of a magnetic field upon iron and steel is called 

 magnetization. 



