Abstract: Apparatus for limiting the upward movement of the anodes of a computer-controlled aluminum reduction cell is disclosed. An air motor is connected with a jack for raising or lowering the superstructure supporting the cell anodes when air pressure is admitted to the motor in response to either a computer or cell-operator-initiated pilot air signal. Computer-generated pilot air signals of a time duration which could cause the air motor to raise the superstructure and create an open anode condition, are interrupted after a predetermined time period. A ball-cam valve is positioned in a fixed relation to the anode superstructure to interrupt either a computer or cell-operator-initiated "up" anode signal to the air motor when the anode superstructure reaches a predetermined upper limit.