Magnetic Shielding

Author: N.H. Crowhurst

There are three kinds of shielding: magnetic, electromagnetic, and electric. Magnetic shielding prevents the magnetic field that causes the induction from reaching the low-level circuit. The entire circuit or transformer is surrounded by the shield. The inducing field passes into the shield, around the circuit to be shielded and out at the other side. This reduces the induction inside the shield by a factor dependent upon the permeability of the magnetic material of which the shield is made, the thickness of this material that provides a magnetic conducting path, and the frequency of the magnetic fluctuation.

Magnetic shielding. At higher frequencies, field changes too fast for shield to work. Shield produces delayed reactionary field, which does not eliminate induction any more.

Magnetic shielding is most effective against steady magnetic fields and low-frequency induction, such as hum. At higher frequencies, it becomes less and less effective because the magnetism takes time to be induced in the material of the shield. The faster the inducing magnetic field fluctuates, the less effective the shield is in conveying the magnetic field around the shielded circuit.