It is concentrated at the top. The five most active assignees hold 53.0% of all 83 records in scope, and the ten most active hold 77.1%. Below that, the ranking runs to 33 companies total, with most holding only a handful of records each, so the field combines a dense top group with a long tail of occasional filers. Anyone entering this space should map their intended claims against that top-ten group first, since it accounts for more than three-quarters of the documented activity.

Filings peaked in 2021 at 6 for the year, then fell to 2 by 2024, a 67% decline across those three complete years. That is a genuine pullback in new filings over that period, though it does not by itself indicate the underlying engineering problem is resolved. The 2025 and 2026 counts in the dataset are lower still but should be read cautiously, since publication typically lags actual filing by around 18 months and those years are not yet complete.

G01R, the electric and magnetic measurement classification, appears on 36.1% of the 83 records in scope, making it the single densest area. H05K printed-circuit assemblies (18.1%) and B60L electric vehicle propulsion (14.5%) follow, with power systems, motors and generators, medical diagnosis, geophysics and semiconductor devices each sitting near 10-12%. Because a single record can carry several IPC classes, these figures add to more than 100% of the record total, which is expected and reflects overlapping claim scope rather than double-counting error.

US5757183A, assigned to Eastman Kodak, claims a magnetic field shielding structure built from two or more concentric annular rings of high-permeability, magnetically soft ferromagnetic material, separated by nonmagnetic spacer layers, with ring diameter, thickness and spacer distance all specified as design variables. It defines a geometric family of concentric-ring shields rather than one fixed design, so it can read on later designs that follow the same ring-and-spacer arrangement even with different dimensions. It does not, on its own, block shielding approaches that use a fundamentally different geometry, such as active compensation coils or non-ring high-permeability sheet layouts.

Based on the IPC composition, the least-claimed branches relative to the dominant measurement and EV-propulsion clusters include active compensation coil control loops, saturation-limit modeling for thin high-permeability sheets, and shielding-factor calculation methods for non-planar geometries. These sit below the 10-12% range that the smaller application classes occupy, suggesting less claim density even though the underlying engineering need is documented across the dataset. A prior-art search focused specifically on those sub-areas, rather than on the broad G01R measurement class, is likely to surface fewer blocking references.