The dc bus to be measured passed through the center of the toroidal iron core saturable reactor and 120vac excitation was applied to the winding around the iron core. As the current in the dc bus increased, the iron saturated more and more, which in turn allowed a higher level of ac current to flow.
The direct current bus and the toroidal iron core saturable reactor were connected by a 120vac excitation applied to the winding around the iron core. The increasing DC bus current caused the iron to saturate, enabling higher levels of alternating current flow.