Abstract: The connecting member has a sleeve body and a resistance heating wire embedded therein with terminals to a power supply. By heating the resistance heating wire, a welding area between the sleeve member is plasticized and welded. For checking the weld quantity, an indicator is provided, which comprises a cavity having a piston with an indicator pin movable therein in the welding area. During welding the material which becomes plastic as a result of the welding pressure is forced into the cavity, so that the indicator pin covered prior to welding is forced out through a passage and is consequently visible and tangible. As the travel of the piston is limited by a cover at the end of the cavity and the distance from the bottom of the cavity to the welding area can be selected corresponding to the desired welding pressure, the end position of the exiting indicator pin corresponds to a temperature and pressure range necessary for completely satisfactory welding.