is a system that works as follows: it is based on the highly non-linear change in the electrical resistance of the fireproof material. As the crucible wall thickness is reduced, the melt penetrates further forward in the direction of the installed elec- trode plates. This causes the temperature of the fireproof material in front to rise, while its electrical resistance falls. For the measurement current, this means that it flows from one electrode to the melt, and from there back to the other electrode. A certain proportion of the measurement current flows directly from electrode to electrode. This proportion tends toward zero with reducing wall thickness, while the measurement current through the melt increases sharply. This alteration in the condition of the fireproof lining is assessed by the complex measurement sys- tem, and the wear to each electrode plate is calculated and displayed.