Static applications

Figure 1: An electromagnet.

Electromagnets consist of field coils to polarize an iron core, a yoke to guide the flux and pole pieces to concentrate flux in the airgap, Fig. 1. Flux guidance and concentration in electromagnets requires material with the highest polarization and very little remanence. Pure soft iron or Fe₆₅Co₃₅ is used. For best results, the pole pieces are tapered at an angle of 55◦.
Electromagnetic relays and solenoid valves are miniature electromagnets where an iron core is magnetized and exerts a force on another temporary magnet. When the gap flux density is B_g, the force per unit area is B²_g/2μ₀. Passive magnetic shielding of low-frequency AC fields or weak DC fields, such as the Earth’s, requires material to provide a low-reluctance flux path around the shielded volume. Reluctance is the magnetic analogue of resistance.

The shielding ratio R is the ratio of the field outside to the field inside. Values of R ≈ 100 are achieved in low fields. The thickness of shielding material is chosen so that its polarization is unsaturated by the flux collected. It is more effective to use several thin shields rather than one thick one. DC shields are often made of permalloy or mumetal, which have negligible anisotropy and magnetostriction, and are therefore immune to shock and strain. Flexible shielding woven from cobalt-rich metallic glass tape, or finmet sheet may be used directly.