Ferromagnetic materials can be used to shield magnetic fields. Generally, we use ordinary iron plates. However, it is necessary to pay attention to the thickness of the iron plate. If the thickness is not reached, the iron plate is in the state of magnetic saturation, only a partial magnetic field can be shielded, and some of the magnetic lines of force will still be diverged. The key to guide magnetic field to form magnetic circuit is to select high magnetic material and suitable size and shape to reduce magnetic flux leakage.