A measurement device comprising: a high permittivity dielectric resonator with a low microwave loss tangent and having at least a first symmetry axis (z 1); an electrically conductive resonance chamber containing and geometrically similar to the resonator and having a second symmetry axis (z 2) coincident with the first symmetry axis (z 1); the resonance chamber having a plurality of similar ports orthogonal to the first symmetry axis (z 1), each such port having a microwave antenna, either to inject microwaves into the resonance chamber, thereby to excite an electric field in the resonator, or to receive microwaves from the resonance chamber; and a comparator circuit (200, 300, 400, 500, 600, 700, 800) connected to a first one of the plurality of ports to inject microwaves into the resonance chamber and to another of the plurality of ports to receive microwaves from the resonance chamber; wherein the measurement device also includes: an electrically conductive tuning element in electrical contact with the resonance chamber, the tuning element being at least partially positionable in the electric field thereby excited in the resonator; and a source of magnetism to apply a magnetic field to a sample brought into proximity with a top surface of the resonator substantially parallel or anti-parallel to the first symmetry axis (z 1); and wherein one of the other of the plurality of ports to receive microwaves from the resonance chamber is orthogonal to the first one of the plurality of ports to inject microwaves into the resonance chamber. a high permittivity dielectric resonator with a low microwave loss tangent and having at least a first symmetry axis (z 1); an electrically conductive resonance chamber containing and geometrically similar to the resonator and having a second symmetry axis (z 2) coincident with the first symmetry axis (z 1); the resonance chamber having a plurality of similar ports orthogonal to the first symmetry axis (z 1), each such port having a microwave antenna, either to inject microwaves into the resonance chamber, thereby to excite an electric field in the resonator, or to receive microwaves from the resonance chamber; and a comparator circuit (200, 300, 400, 500, 600, 700, 800) connected to a first one of the plurality of ports to inject microwaves into the resonance chamber and to another of the plurality of ports to receive microwaves from the resonance chamber; wherein the measurement device also includes: an electrically conductive tuning element in electrical contact with the resonance chamber, the tuning element being at least partially positionable in the electric field thereby excited in the resonator; and a source of magnetism to apply a magnetic field to a sample brought into proximity with a top surface of the resonator substantially parallel or anti-parallel to the first symmetry axis (z 1); and wherein one of the other of the plurality of ports to receive microwaves from the resonance chamber is orthogonal to the first one of the plurality of ports to inject microwaves into the resonance chamber. an electrically conductive tuning element in electrical contact with the resonance chamber, the tuning element being at least partially positionable in the electric field thereby excited in the resonator; and a source of magnetism to apply a magnetic field to a sample brought into proximity with a top surface of the resonator substantially parallel or anti-parallel to the first symmetry axis (z 1); and wherein one of the other of the plurality of ports to receive microwaves from the resonance chamber is orthogonal to the first one of the plurality of ports to inject microwaves into the resonance chamber.