It builds a magnetic field in one direction, then collapses that and builds a magnetic field in the other direction. Doing this repeatedly tends to cause the residual magnetism to take on the average value of the field(s) created, which is no net magnetism.
If the AC current is interrupted while the magnetic field is still present, the field may remain--depending on the material. This is why the degaussing process requires slow reduction in the strength of the AC current (or slow removal of the degaussing coil).
If the AC current is interrupted while the magnetic field is still present, the field may remain--depending on the material. This is why the degaussing process requires slow reduction in the strength of the AC current (or slow removal of the degaussing coil).