RESIDUAL MAGNETISM:

        Soft iron will become magnetized when placed in a field of a magnet but will lose most of its magnetism when removed from this field. In soft iron is placed in a magnetic field and then removed, only a few of the molecules will remain in magnetic alignment. These few molecules will produce a very weak magnetic field. This is known as residual magnetism. 

          Residual magnetism is the factor that makes it possible for the DC generator to start its generating cycle. It is a form of self excitation, without which, the DC generator would not function once it had been stopped.

RESIDUAL MAGNETISM