A method for determining a length parameter of an electrode during operation of an electric arc furnace. An internal duct extends through the length of the electrode and is open at its lower end. A waveguide having a solid core is received in the internal duct and comprises a material having a low dielectric constant and high temperature resistance. The solid core of the waveguide includes at least one target. An electromagnetic radiation signal is emitted from a source and transmitted through the waveguide and the signal is diffracted and/or reflected from at least one target to produce at least one return signal which is transmitted back through the waveguide. A time or frequency difference between the emitted signal and the return signal is measured and the length parameter is calculated based on this difference.