The Standard Electrical Dictionary

1. Magnetic Screen

   A box or case of soft iron, as thick as practicable, for protecting bodies within it from the action of a magnetic field. The lines of force to a great extent keep within the metal of the box on account of its permeability, and but a comparatively few of them cross the space within it. Such screens are used to prevent watches from being magnetized, and are a part of Sir William Thomson's Marine galvanometer. A magnetic screen may be a sphere, an infinite or very large plane, or of the shape of any equipotential surface. Synonym--Magnetic Shield.