This relates to the heating of thick slabs for rolling and like purposes wherein there is a complete penetration of the slab by induced electrical energy wherein the starting current frequency is very low and wherein a maximum heating is obtained by increasing the current frequency to control through penetration of the induced current in the slab as the temperature of the slab rises and thus the restivity of the metal of the slab increases. Several heating systems are envisioned. These include a heating system of a length corresponding to the length of the slab and wherein the slab is stationary during the heating to the desired temperature, after which the slab is moved out of the heating apparatus. The second arrangement is similar to the first, but wherein a lower coil arrangement moves relative to the upper coil arrangement so as to remove and expose a heated slab. In these two systems, the frequency is progressively increased. In a third system, the heating system is relatively short and the slab being heated is moved therethrough. In this arrangement, there are a plurality of coil sets each having a current supply of a different and gradually increasing frequency.