CHAPTER III 

 HYSTERESIS AND EDDY CURRENTS IN IRON 



14. The Hysteresis Loop. Steel and iron possess a property 

 of retaining part of their magnetism after the external magnetomo- 

 tive force which magnetized them has been removed. Therefore, 

 the magnetization or the B-H curve of a sample depends some- 

 what upon the magnetic state of the specimen before the test. This 

 property of iron is called hysteresis. The curves shown in Figs. 2 

 and 3 refer to the so-called virgin state of the materials, which state 

 is obtained by thoroughly demagnetizing the sample before the 

 test. A piece of iron can be reduced to the virgin state by placing 

 it within a coil through which an alternating current is sent, and 

 gradually reducing the current to zero. Instead of changing the 

 current, the sample can be removed from the coil. 



Let a sample of steel or iron to be tested be made into a ring 

 and provided with an exciting winding, as in Fig. 1. Let it be 

 thoroughly demagnetized; in other words, let its residual mag- 

 netism be removed ; then let the ring be magnetized gradually or hi 

 steps to a certain value of the flux density. Let OA in Fig. 7 rep- 

 resent the virgin magnetization curve, that is to say the relation 

 between the calculated values of B and H from this test, and let 

 PA be the highest flux density obtained. If now the magnetizing 

 current be gradually reduced, the relation between B and H is no 

 more represented by the curve OA, but by another curve, such as 

 AC; this is because of the above-mentioned property of iron to 

 retain part of its magnetism. When the current is reduced to 

 zero, the specimen still possesses a residual flux density OC. Let 

 the current now be reversed and increased in the opposite direc- 

 tion, until H reaches the negative value OF, at which no magnetic 

 flux is left in the sample. The value of H=OF is called the coer- 

 cive force. When the magnetic intensity reaches the negative value 

 of OP' = OP, experiment shows that the magnetic density P'A' in 

 the sample is equal and opposite to P^. 



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