A magneto-plasma-dynamic (MPD) generator, comprising: a conveying duct shaped for conveying a high velocity, conductive fluid; a magnetic field generator arranged to generate a magnetic field across the conveying duct, substantially perpendicular to the direction of travel of the fluid, such that the fluid passes through the magnetic field (2) when conveyed by the duct; electrodes provided in the conveying duct to conduct a current induced in the fluid as it is conveyed by the conveying duct through the magnetic field; and an electrode supplying mechanism configured to supply a conductive liquid for replenishing the electrodes.