A well-founded class of dependences, based on the theory of nonlinear electrical circuits, is proposed for the formal approximation of static characteristics for various electronic devices. The dependences are simple fractionally linear and fractionally quadratic expressions (a particular case of rational functions), convenient for engineering calculations. However, these expressions represent projective transformations in the sense of projective geometry. The projective transformations preserve an invariant in the form of a cross ratio (double proportion) of four samples of variable mode parameters (currents, resistances, voltage). The mutual reduction of measurement errors is an attractive practice property of the cross ratio. This leads to the direct formulas for recalculating the mode parameters that have group properties. An example of the approximation of the magnetization characteristic and relative magnetic permeability of the Nickel Steel Carpenter 49 is considered; invariants for the magnetic flux density and absolute permeability are established. The results are applicable in various fields of electronics and spintronics.