We analyse the problem of defining and measuring the thermal contraction in the superconducting coil of the Large Hadron Collider main magnets. Since an unloaded coil length cannot be precisely defined, one has to use methods based on the pre-stress loss from room temperature to cryogenic temperature. A very strong dependence of the thermal contraction coefficient on the conventions used for defining coil deformations is discussed. Moreover, one finds different values of the thermal contraction

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... coefficient according to the pressure imposed at room temperature before the cooling down. A pressure-dependent thermal contraction coefficient can be therefore worked out. ABSTRACT We analyse the problem of defining and measuring the thermal contraction in the superconducting coil of the Large Hadron Collider (LHC) main magnets. Since an unloaded coil length cannot be precisely defined, one has to use methods based on the prestress loss from room temperature to cryogenic temperature. A very strong dependence of the thermal contraction coefficient on the conventions used for defining coil deformations is discussed. Moreover, one finds different values of the thermal contraction coefficient according to the pressure imposed at room temperature before the cooling down. A pressure-dependent thermal contraction coefficient can be therefore worked out.