Abstract: An apparatus for testing the pipe-to-soil potential of a buried pipeline having a plurality of rectifiers electrically connected to it in spaced-apart intervals for applying an electrical potential to the pipe includes a plurality of interrupter units electrically connected to the rectifiers for turning them on and off at intervals. A clock reference unit is electrically connectable to each of the interrupters for synchronizing them for turning all of the rectifiers on and off in unison. A portable sensing device produces signals indicative of the pipe-to-soil potential of the pipe and directs these signals to a demultiplexer unit which is also synchronized with the clock reference unit for separating the signals corresponding to the ON and OFF potentials which are then separately recorded.