As wire is wound around and onto the surface of a former to form a coil, perturbations such as waves are introduced into turns of the coil, each perturbation being formed in a length of the wire less than the full length of a respective turn and projecting along the former surface, the perturbation having an initial shape and size. The coil is taken from the former and fixed in form. A surrounding jacket is radially collapsed onto the wire coil to uniformly press the coil against an internal compressible body in the course of which the overall diameter of the coil is reduced. Resulting stresses in the wire generated during the collapse are relieved by alteration of the perturbations from their initial shape and size to a subsequent shape and size.