An apparatus and process are provided for controlling the heating or melting of an electrically conductive material. Power is selectively directed between coil sections surrounding different zones of the material by changing the output frequency of the power supply to the coil sections. Coil sections include at least one active coil section, which is connected to the output of the power supply, and at least one passive coil section, which is not connected to the power supply, but is connected in parallel with a tuning capacitor so that the at least one passive coil section operates at a resonant frequency and the output frequency of the power supply is changed so that the induced power in the at least one passive coil section changes as the frequency is changed.