not shown which in this example is located on a printed circuit board 28 connection to the circuit is effected through terminals 29 and 30 a capacitor 31 is connected across terminal leads 26 and 27 for tuning coil 21 in the manner to be described a thin strip 32 of magnetically permeable material is disposed relative to coil 22 extending across its width under one side of the coil 22 between it and coil 21 through the coil 22 and over the other side at 33 the function of strip 32 is to provide a low reluctance flux path through coil 22 that is particularly effective for diverting flux normally parallel to the plane of coil 22 through coil 22 into linking relationship for a purpose that will also be discussed below a sheet 34 of magnetically permeable material preferably as large as the coil 21 is disposed parallel to coil 21 adjacent thereto on the side remote from coil 22 immediately adjacent the sheet 34 is the circuit board 28 containing the electronic circuit not shown to which the coil 21 is connected finally the circuit board 28 contacts a flat battery 35 of comparable size as shown the circuit on board 28 is brought out to terminals 36 and 37 that are connected by leads 38 and 39 to battery 35 the battery 35 having conductive metal components in which eddy currents can be induced will tend to modify the effective flux linkage between coil 21 and any ac magnetic field in which.