This part of IEC 61788 addresses a test method for the determination of the residual resistance ratio (RRR), r_RRR, of cavity-grade niobium. This method is intended for high-purity niobium grades with 150 < r_RRR < 600. The test method is valid for specimens with rectangular or round cross-section, cross-sectional area greater than 1 mm² but less than 20 mm², and a length not less than 10 nor more than 25 times the width or diameter.