The ideal magnetic circuit concept in which leakage and fringing flux are absent has not heretofore been realized because of the absence of magnetic insulators under normal conditions. Below the transition temperature, however, a superconductor is a perfect magnetic insulator and a superconducting enclosure of PbBi around the source of mmf and the air gap of an experimental electromagnet operating in liquid helium has been found to reduce leakage flux from 5.1 to 0.39 maxwells per maxwell of air gap flux. The small residual leakage flux observed is accounted for by leakage in the space occupied by the winding. No detectable fringing flux was observed outside the superconducting magnetic insulator surrounding the air gap. The flux within the air gap follows the contour of the superconducting enclosure and the degree of field uniformity within is determined by the precision of alignment of the enclosure surfaces. The large reduction in size and weight possible under ideal magnetic circuit conditions makes operation in liquid helium feasible. In a magnetic circuit for maser application, a reduction in weight of nearly 2 orders of magnitude was possible partly because of the reduction in air gap length resulting from the inclusion of the magnetic circuit in the cryostat and partly because of ideal magnetic circuit operation.