Abstract: An electrical coil is formed from a core of magnetizable material and surrounding winding. Through the core is an axially disposed circular passage into which is positioned and sealed with a suitable mastic a hollow cylindrical sleeve of a non-magnetizable material having one closed end aligning with the outer surface of the core. The sleeve is internally threaded for receiving a like threaded shaft of non-magnetizable material containing a tube core of a magnetizable material. Rotation of the shaft adjusts the location of the tube core so as to adjust the inductance of the coil when energized.