An electrical coil, particularly a gradient coil for a magnetic resonance apparatus, contains at least one electrical conductor, a carrier structure, at least one component of a cooling device and a heat insulator, which is arranged for at least one section of the conductor between the conductor and the carrier structure. The valve further comprises at least one biasing member configured to bias the poppet shutter towards a closed position so as to block the flow passage through the flow inlet. The frequency and phase control allows neighboring stages to be closely spaced at a distance of from 1 to 2 times an inter-electrode distance within a stage, and, in any case, minimizing or avoiding production of a back corona current from a corona discharge electrode of one stage to an electrode of a neighboring stage.