ELECTROMAGNETS. MAGNETISM OF IRON. 87 



$ = 4irck + Ms (2) 



The part ^m or 471-^5 of the flux through an iron rod is 



due to the magnetized condition of the rod, and the part Ms 

 is due to the magnetizing force directly. This part, <?&, is the 

 amount of flux which would pass through the region occupied 

 by the rod if the rod were removed. 



54. Flux density in iron.* The magnetic flux per unit sec- 

 tional area of an iron rod, namely 3>/s, is called the flux density 

 in the rod, and it is represented by the letter cB. That is: 



Substituting $s for $ in equation (2) of Art. 53, we have: 

 & = 47^ + cV (2) 



The flux density in an iron rod is equal to the intensity of the 

 magnetic field in a thin crevasse cut across the rod. This is 

 evident when we consider in the first place that flux density in 

 air is identically the same thing as field intensity according to 

 Art. 25, and in the second place, that a very thin crevasse does 

 not sensibly disarrange the flux through a rod, so that the flux 

 density in the crevasse is the same as in the rod. If the crevasse 

 is not very narrow, a portion of the flux passes out of the rod, 

 around the crevasse, and back into the rod again. In this case 

 the flux density, or field intensity in the crevasse, is less than the 

 flux density in the part of the rod which is remote from the 

 crevasse. 



55. Residual magnetism. Permanent magnets. An iron rod 

 retains a portion of its magnetism when it is removed from the 

 magnetic field in which it has been magnetized, or, in case of an 

 electromagnet, when the magnetizing current is reduced to zero. 

 The magnetism thus left in a bar of iron (in the core of an electro- 

 magnet) is called residual magnetism. The poles of a short 

 thick bar produce a strong demagnetizing field along the bar, and 



* Frequently called magnetic induction or simply induction. 



