A plurality of air-tight corona generating cells of a corona generator are supported in a liquid-tight housing where they are immersed in a dielectric liquid such as transformer oil for preventing arcing between the edges of the two electrodes of each corona generating cell. The dielectric liquid is in heat exchange relationship with the external surface of each electrode and is circulated between a cooler and the housing for cooling the electrodes. A center support disc located between the electrodes of a cell unexpectedly increases the ozone yield.