(EP963616-A1)      Determination of material conversion during surface electrochemical reactions is carried out in a local region between a counter-electrode surface and an electrode surface having a contact element segment, which corresponds to the surface region, and a contact element which contacts the remaining surface and which is electrically insulated from the contact element segment. The novelty is that (a) both the contact element (34) and the contact element segment (34s) are connected to a reaction current source or sink (90); and (b) the magnetic field, generated about a current conductor (44) between the contact element segment (34s) and the current source or sink (90), is determined as a measurement of the material conversion in the local region (40). Also claimed is an electrochemical unit in which the above process is carried out.    USE -   For determining material conversion in e.g. electrolytic reactions or reactions in fuel cells or batteries.    ADVANTAGE -   The process allows material conversion determination in a simple manner using a single current source or sink for both the contact element and the contact element segment.    (US6036838-A)      Determination of material conversion during surface electrochemical reactions is carried out in a local region between a counter-electrode surface and an electrode surface having a contact element segment, which corresponds to the surface region, and a contact element which contacts the remaining surface and which is electrically insulated from the contact element segment. The novelty is that (a) both the contact element (34) and the contact element segment (34s) are connected to a reaction current source or sink (90); and (b) the magnetic field, generated about a current conductor (44) between the contact element segment (34s) and the current source or sink (90), is determined as a measurement of the material conversion in the local region (40). Also claimed is an electrochemical unit in which the above process is carried out.    USE -   For determining material conversion in e.g. electrolytic reactions or reactions in fuel cells or batteries.    ADVANTAGE -   The process allows material conversion determination in a simple manner using a single current source or sink for both the contact element and the contact element segment.