ELECTROMAGNETS. MAGNETISM OF IRON. 361 



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

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

 cB in the rod. That is : 



_* (;) 



Substituting cBs for <I> in equation (6) we have : 



d (8) 



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 evi- 

 dent when we consider in the first place that flux density in air is 

 identically the same thing as field intensity according to Art. 6, 

 Chap. I, 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 den- 

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

 in the part of the rod which is remote from the crevasse. 



11. 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, or in an electromagnet, is 

 called residual magnetism. A long slim bar, or a bar which 

 forms a closed or nearly closed magnetic circuit, retains a greater 

 portion of its magnetism than a short thick bar, because of the 

 fact that in case of a short thick bar the poles produce a strong 

 demagnetizing field along the bar. A long slim bar or a closed 

 ring of annealed wrought iron may retain as much as 90 per 

 cent, of its magnetism, but a very weak demagnetizing field or 

 a slight mechanical shock is sufficient to cause soft iron to lose 

 nearly all of its residual magnetism. Cast iron and hard-drawn 



* Frequently called magnetic induction or simply induction. 



