ELECTROMAGNETS. MAGNETISM OF IRON. 359 



the cavity around / is very long and narrow as shown in Fig. 7, 

 then the poles n and s at the ends of this cavity are weak and 

 far away from /, so that these poles n and s do not produce at / 

 any perceptible field, and therefore the field intensity in the nar- 



Coil 



Coil 

 Fig. 7. 



row longitudinal cavity shown in Fig. 7 is the resultant of e^' 

 and M" of Fig. 5. That is, the field intensity in a narrow longi- 

 tudinal * cavity is the actual value of the magnetizing field in 

 iron. 



8. Intensity of magnetization. Let m be the strength of the 

 magnetic pole at the end of a magnetized iron rod of which the 

 sectional area is s square centimeters. The pole strength per 

 unit sectional area, namely m/s, is called the intensity of magnet- 

 ization, $, of the rod. That is : 



S= $ (4) 



9. Magnetic flux through a magnetized rod. Consider a long 

 slim iron rod, s square centimeters in sectional area, placed in 

 and parallel to a magnetic field, which, but for the presence of 

 the rod, would be a uniform magnetic field of intensity etf. Then 

 the actual intensity of the magnetizing force along the middle por- 

 tions of the rod is 3f y as explained in Art. 7. The magnetic field 

 in the neighborhood of each end of the rod is a composite field 

 which is the resultant of two distinct parts, a and b ; and the mag- 

 netic flux which comes up to one end of the rod, flows through 

 the rod, and passes out from the other end of the rod, consists of 

 two distinct parts corresponding to the two parts, a and b, of the 

 field near the ends of the rod. 



* Parallel to the direction of magnetization of the iron. 



