A simple electrolysis cell of small capacity has been constructed. A needle valve system maintains a constant pressure in the cell enabling the electrolysis of heavy water to continue for hours without attention. The system also eliminates the exposure of a large amount of mercury to the atmosphere. 160 ml h^-1 of D₂ gas at normal pressure are collected in the reservoir under vacuum at a current of 0.4 A through the cell. Deuterium prepared in this way can be used as an RF ion source.