Abstract: A device for measuring strain in an elongated metallic bar, the device comprising a housing arranged to be secured to the metallic bar, at least one optical sensor, at least one light source arranged to emit light across an interior space of the housing, and a thread arranged freely movable within a sheath and having a proximal end extending to a distal end of the housing and a distal free end arranged to be attached to the metallic bar at a distance from the housing such that longitudinal deformation of the metallic bar causes displacement of the proximal end of the thread in relation to the housing, and wherein the at least one optical sensor is configured to measure the displacement of the proximal end of the thread by measuring light emitted from the at least one light source.