Inductive heating very often means heating fiat objects, e.g. metal plates of different thickness. These are usually elements which are heated with a frequency within the range 5o Hz to 3 MHz, either thoroughly with respect to the plastic working or to the heat treatment of the metal element, or are heated on the surface only for the purpose of hardening. Flat metal objects as well as those of a cylindrical shape which are also frequently used are the typical machine elements used in induction heating. All the other possible shapes of objects can be nearly always reduced in theoretical and technical consideration to a flat or a cylindrical shape. The elements in question are mostly objects of mass industrial production for which the problem of heat treatment and induction heating must be solved in the best possible way, i.e. with the optimum heating parameters with regard to the thermal process itself, or because of economic and technical aspects of the heating process, e.g. high heating rate, minimum electric energy consumption, required temperature distribution, etc. It is common "practice in industry to heat flat objects unilaterally or bilaterally by induction i.e. by means of an appropriately shaped heating inductor which is applied to one side of the heated object, or by means of two inductors applied to both sides of it. The inductors are the source of an electromagnetic wave which penetrates the surface of the heated object, supplying the electric energy converted into heat. The electromagnetic field which has been generated in this way is characterized by the three vectors: tile intensity of the magnetic field H, of the electric fieldE, and by the Poynting vectorS. The corresponding induction system with the inductor w and with the three vectors marked on the surface of a flat object p of a thickness d are shown schematically in Fig. 1. In the case shown in Fig. 1 a the object is heated unilaterally, while in the other two cases the object is heated bilaterally by means of two inductors; in the case l b the induced currents are compatible with a regard to their direction and phase, e.g. both inductors w are supplied paralelly or in series from the same source; in case lC the induced currents are inversed, e.g. both inductors are supplied separately from two different sources.