"Induction heating. Basically the idea is you pass an alternating current through the copper coils. This induces an alternating magnetic field which in turn induces eddy currents on the surface of metals that are placed inside (the workpiece). The eddy currents cause joule heating in the workpiece while the coils don't heat up because usually they're water cooled." Another quote: "The alternating electricity in the copper coil makes a magnetic field. When the metal thing is put inside that field it makes its own electricity and that causes the atoms of the metal to move back and forth so fast that the "friction" between them makes heat."