A device for hardening the surface of a workpiece, wherein an energy supply part and a working part each include a magnetic core, wherein the magnetic cores are at a small distance from each other such that electrical energy is transmitted contactlessly from the energy supply part of the inductor to the working part of the inductor, and wherein the working part includes a heating conductor branch for inductive heating of a section of the surface of the workpiece; a control instrument coupled to the inductor, wherein the control instrument displaces the working part relative to the surface to be hardened and relative to the energy supply part, and wherein the magnetic core of the working part is displaced relative to the magnetic core of the energy supply part; and an electrical energy supply connected to the energy supply part of the inductor.