The two main considerations are: * Hysteresis of the core: it's tendency to remain magnetized. This is analogous to form drag in hydrodynamics and it can be minimized by using a soft iron core, which has a narrow hysteresis loop. * Eddy currents in the core: circular currents that are induced in the same way that the transformer works. This is analogous to frictional drag in hydrodynamics and it can be reduced by dividing the core into laminae.