It is a circumference length correction device with a metal ring, which are a pair of rollers of a rotation drive side and a follower side is wound, wherein, during the separation distance of the pair of rollers is regulated and a predetermined voltage is applied to the metal ring, the metal ring - circumferential length corrected by amplifying the predetermined voltage; a circumference length measuring device, which measures the separation distance, in order to provide a measured value, and the metal ring - circumferential length of the measured values is calculated; and a surface defect test device, wherein, during the separation distance of the pair of rollers is regulated and a predetermined voltage is applied to the metal ring,according to any existing surface defect of the metal ring checks the of a scraper, wherein each roller of the pair of rolls with its peripheral surface in contact with a blade edge of the scraper is.
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