A magnetic sensing element detects the presence of magnetic particles in a binding assay. The magnetic sensing element has at least one planar layer of electrically conductive ferromagnetic material that has an initial state in which the material has a circular magnetic moment within the plane of the layer. The magnetic sensing element has molecules of a first specific binding member attached to it. The device also includes a fluid test medium to which the magnetic sensing element is exposed during the course of a binding assay. The fluid test medium includes magnetizable particles that become immobilized during the assay in relation to the amount of analyte in the test medium. The relative size of the magnetic particle and the magnetic sensing element and the location of the molecules of the first specific binding member on the magnetic sensing element arc selected so that when the magnetic particle becomes immobilized with respect to the magnetic sensing element, the radial fringing field of the magnetic particle causes the magnetic moment of at least one layer of electrically conductive ferromagnetic material to shift from circular to radial, thereby causing a detectable change in the electrical resistance of the magnetic sensing element.