When a propeller shaft revolves it is electrically insulated from the rest of the hull by a film of lubricating oil. This can result in corrosion to the shaft bearings and sometimes the main bearings through the process of 'spark erosion'. Cathelco Jotun shaft earthing equipment brings the propeller and shaft back within the scope of the cathodic protection system by electrically connecting it to the hull. This is achieved by fitting a steel slip ring around the shaft with an inlaid silver track. A low resistance 'path' to earth is provided by two silver graphite brushes mounted in holders. As a result, the 'potential' difference between the shaft and hull is minimised to prevent corrosive activity.