Abstract: An electrolytic cell having a body adapted to be located so that the inlet and outlet are each on or adjacent the lower surface thereof and including electrodes whereby electrolysis of water passing through the cell can be effected, the arrangement being such that, if the water remains in the cell, then gas formed will displace water from the electrode so as to reduce, and finally stop, electrolysis.The cell may also have a third electrode which is located in the cell adjacent the upper surface thereof, which is in connection with one of the other electrodes, the resistance between the two electrodes being low when the cell is full of liquid, but high when gas displaces the liquid and exposes the third electrode.