preferred embodiments of a resonant circuit in particular for a sensing device according to the invention fig2 a shows a resonant circuit produced by providing an electrical conductor i e electrical conducting means e g a metallic conductor such as aluminium or an electrically conducting coating in form of two serially connected coils 21 22 terminating in plates 24 25 or a substrate 23 e g a textile a paper or a suitable plastic foil the substrate is folded along the line a u2014 a as shown in fig2 b providing the back surfaces of the substrate against each other and the front surfaces of the substrate carrying the coil away from each other thereby the formation of short circuits between the coils is avoided as further shown in the folded substrate in fig2 c and 2d depending on the degree of folding the distance between the plates 24 25 is changed thereby providing a variable capacitance it should be noted that the skilled person can select the number of windings and their directions with respect to each other for either enhancing coupling or decoupling of the magnetic fields of the coils of the folded substrate fig2 d shows a cross section view of the resonant circuit along the line b u2014 b in fig2 c together with an equivalence diagram of the resonant circuit the two serially connected coils 21.