the air to the segment 14 of the axial cavity of the nozzle body the needle 11 has a thread 21 in its upstream portion which is the maximum diameter portion which thread is adapted to engage the female thread of the segment 13 of the body and on which a ring 22 and a ceramic thread guide 50 are mounted for purposes that will be set forth hereinbelow proceeding in the direction of the yarn travel the needle has an upper cylindrical surface 23 which has the same diameter as the segment 14 of the body cavity and engages it tightly an air tight gasket 24 being provided in this zone proceeding further the shape of the needle changes and it acquires as particularly shown in fig3 a square outer cross section in the zone indicated at 25 the diagonal of the square being equal to the diameter of the segment 15 of the body cavity and the square cross section portion axially extending sufficiently to engage at least a part of the aforesaid segment 15 a suitably shaped surface 26 connects the cylindrical surface 23 to the square outer surface of the segment 25 as is seen a substantially annular chamber 27 having a prismatic inner surface and a cylindrical outer surface is formed between the segment 25 of the needle and the segment 14 of the body cavity in which chamber the compressed air which enters the passage 18 of the nozzle body flows from chamber 27 the air can proceed longitudinally along the body.