Copper samples which have been measured in the capacitance dilatometer include Asarco (6N), OFHC, deformed, electrolytic tough‐pitch, phosphorus‐deoxidized, tellurium‐copper, and alloys containing 0.2% Fe and Mn. At room temperature and oxygen temperature they differ in linear coefficient a from the dilatometer cell (itself OFHC copper) by< ½% (<0.1 × 10−6 K−1). Below 30 K differences between annealed and 70% deformed material, and between Asarco and OFHC, are <1%, but the magnetic impurities inCuFe andCuMn alloys do have an increasing and marked effect on the expansion, a being increased byca4% at 20 K and 40% at 10 K.