An elongated member including an outer sleeve-like rigid insulator shell surrounding a high voltage conductor extending in the longitudinal direction of the shell and a gap inside the shell next to an internal wall of the shell at least partially surrounding the conductor and filled with a medium including a material of with electrically insulating properties. The medium is formed by an electrically insulating material including hollow spaces at least partially filled with gas. The material is adapted to expand upon a temperature rise thereof by reversibly compressing the hollow spaces and reducing the volume thereof.