Method and Apparatus Using Magnetic Flux for Container Security

A seal device includes a locking member with a magnetically permeable material portion, and structure that can receive the locking member. The structure supports a magnetic field generator and detector at locations spaced from each other and from a region that is occupied by the portion of the locking member when the locking member is received by the structure. The structure defines a main flux path as a loop having a first portion, a second portion and a remainder that are mutually exclusive, and that collectively define the entirety of the flux path. The first and second portions are respectively within the magnetic field generator and the region, and most of the remainder extends through magnetically permeable material of the structure. The detector is located where the magnetic field has different characteristics when the portion of the locking member is respectively present in and absent from the region.