The present invention provides nuclear magnetic resonance apparatus comprising means for applying a static magnetic field (H.sub.0) to a sample under investigation, a radio frequency circuit arranged to be inductively coupled to the sample by means of an oscillating magnetic field disposed generally perpendicular to the static magnetic field for applying an excitation signal to the sample, and an electric potential sensor (E.sub.1, E.sub.2) for detecting the excitation of the sample and for generating a detection output. The electric potential sensor comprises an electrode placed generally perpendicular to the axis of the oscillating magnetic field for capacitively coupling the electric potential sensor to the sample.