Abstract Factors affecting the measurement of critical current density ( J c ) in high performance Nb 3 Sn strands are discussed. Sample holder design and sample mounting procedures are considered in the light of cryogenic heat flow considerations. Also discussed are: the steady state measurability of metastable strands at current densities ( J ) at and above their nominal J c , and mechanisms governing their quench at sufficiently high values of J greater than J c .