An inductor for microwave frequencies has a substantially planar structure and is constructed of a transmission line designed as a linear microstrip element made of a central line comprising normal electrically conducting material, such as a suitable metal. The microstrip element has a width which is varied by making areas at sides of the central line superconducting. By changing the effective width of the microstrip, the inductance of the microstrip is changed accordingly. The areas at the sides of the microstrip element in the non-superconducting state may have some electrical conductivity. However, because they contact the central metal conductor only at a very narrow edge, instead of contacting it at a large surface, the side superconducting areas do not significantly affect the transmission characteristics of the transmission path when the superconducting areas are in the normal state.