with a cover lever 27 attached to the cover the two joints 25 and 26 are each located at the corners of the outline of the insulating box 4 on the upper side of the cover facing away from the heating plate 2 a plurality of yarn guides 15 are mounted in the longitudinal direction these yarn guides have a sliding edge which terminates in the region of the heating plate in a trough in its operating position each cover is locked in position such that the cover with the yarn guides 15 mounted thereon slopes toward the center plane of the heating plate arranged above the two covers are yarn threading devices 17 in the embodiment of fig2 an air carrying tube 18 is shown which has a slot 22 extending over its entire length the tube itself extends over the entire length of the heating plate it can be moved on a guide rod 30 vertically to the center plane of the heating plate so that it does not interfere with the pivotal movement of the cover which will be described below the covers 6 1 and 6 2 are in their closed position in fig2 for purposes of threading the yarn is placed against the tube 18 in the region of the yarn inlet of the heating plate and advanced by the air current generated in the tube 18 to the opposite end of the tube 18 and the heating plate in so doing the yarn drops from the slot 22 onto the yarn guide 15 as a result of the developing yarn tension the yarn then slides.