The effect of magnetic shaking on nonoriented silicon steel rings
The effect of magnetic shaking on the incremental permeability of nonoriented silicon steel rings was studied. 50H470 silicon steel is a very popular and cheap material, but does not have a high incremental permeability. However, we achieved a value of 27 000 for the target field of 10 Hz, 0.05 A/m, which is comparable to commercial PC permalloy. We also measured the target field, which ranged from 0.05-5 A/m, 1 Hz-1 kHz. Magnetic shaking was effective at 60 Hz is effective at shaking field amplitudes 40-60 A/m, corresponding to a flux density of 0.2-0.6 B/sub s/. We also made ring samples from 50H470 steel plates produced by a nibbling machine and evaluated the effect of stress on their magnetic properties.
The effect of shaking on magnetic shields
The increase of the shielding factor due to shaking was measured in a scale model for a magnetically shielded room. The increase was found to be 7 dB for a single-layer square cylinder biased by the Earth's magnetic field. The shielding factor of a large-volume three-layer Mumetal®room was estimated to increase by a factor of 30, thus confirming the feasibility of shaking in magnetic shields. The shaking parameters, amplitude, and frequency are not critical according to the experiments. Winding the shaking coils along the edges of the cubic shield leads to minimum disturbances inside the cube, and the winding can also be applied to demagnetize the shield by an alternating field of 25 A/m, 50 Hz. The relative incremental permeability of Mumetal was studied as a function of the shaking and biasing fields. The permeability was found to increase considerably by shaking and by decreasing the biasing field. With zero biasing and with shaking field of H s = 5 A/m root mean square (rms), 50 Hz, the permeability reached its maximum value of 89 000, which is sevenfold the value without shaking.
Magnetic Shielding Factors of a System of Concentric Spherical Shells
General expressions are developed for the shielding factors of concentric spherical shells of linear isotropic magnetic material. Approximate results are given for shells of highly permeable material which are thin compared to their radii.