The paper deals with a connection for mercury-arc rectifier equipments, in which secondary windings on the interphase transformer are connected to a small auxiliary rectifying device. By this means, 24-phase waveforms are obtained from a 12-phase set, without any appreciable impairment of the other performance characteristics. The connection has been named the phase-doubled 12-phase connection. Test results and elementary theory are given. It is possible to install initially simple 12-phase equipments, and convert these to phase-doubled 12-phase operation at a later date if disturbing effects from harmonics on the a.c. side occur.