The cathode is connected to the + side of the DC load, the other side being connected to the center tap of the transformer secondary winding, which always remains at zero potential with respect to ground or earth. For each AC phase, a wire from each end of that phase winding is connected to a separate anode "arm" on the mercury-arc rectifier. When the voltage at each anode becomes positive, it will begin to conduct through the mercury vapor from the cathode.