 In addition, a rocking mechanism (B shown by fulcrum on the left and rotating eccentric on the right) agitates the mercury to transport the dissolved sodium metal from the outside cells to the center cell.The anode reaction in the center cell takes place at the interface between the mercury and the sodium hydroxide solution.Finally at the iron cathode (D) of the center cell the reaction is

The net effect is that the concentration of sodium chloride in the outside cells decreases and the concentration of sodium hydroxide in the center cell increases.