fig1 in the above embodiments the rod 10 or 10 39 may be composed of a super conducting material in this case the rod has a permeability u03bc which is zero therefore the inductance can be effectively varied by using a small rod fig4 illustrates an embodiment for adjusting the inductance by changing the surrounding atmosphere of the inductance adjusting coil 4 the coil 4 is contained in a chamber 6 and the inductance of the coil 4 is finely adjusted by selectively supplying gases having dissimilar permeabilities such as oxygen nitrogen helium and the like from gas containers 9 into the chamber 6 by a blower 7 an exhaust unit 8 coupled to the chamber 6 is also used to replace the gas the coil 4 should preferably be composed of a high temperature super conducting material it is also allowable to change the atmosphere for the pick up coil 1 or the input coil 3 to change the inductance thereof without using an independent inductance adjusting coil like in the embodiment shown in fig3 it is further possible to contain the whole magnetic flux transfer circuit in a chamber to change the atmosphere for the whole circuit fig5 shows an embodiment for adjusting the inductance by changing the shape of the inductance adjusting coil if an adjusting rod 13 of a shape adjusting unit 11 added to the coil 4 is turned a pin 12 moves along the groove to change the ratio of a long axis and a short axis of.