A novel NMR principal detector element of the present invention comprises a NMR-active inductor in a radio frequency (RF) resonance circuit, and mitigates the distortions in a static magnetic field caused by the introduction of a toroid cavity NMR detector. The RF resonance circuit includes a novel principal detector element, capacitor elements, and a sample chamber (the cavity). The novel principal detector element defines the NMR active portion of the inductance of the electronic resonance circuit. In one embodiment of the invention, the novel principal detector element includes two hemispheres connected by a central rod. The central rod can be of any length and diameter, and the hemispheres can be of any diameter. A typical embodiment of the invention would include a rod that is 2 cm long having a diameter of 1 mm, and hemispheres having a diameter of 5 mm.