position of retaining grooves 54 respectively and the corresponding dimensioning of the sheet metal caps are such that the sheet metal caps secure the yarn guides in the axial direction the yarn guides 32 consist of materials commonly used for this purpose such as silicon titanium or aluminum oxides or of nitrided or chromium plated steel or the like preferably in the region in which they project from recess slot 30 the yarn guides 32 are conically beveled on their ends facing away from the base portion 16 as is indicated at 34 as a result the yarn guides 32 successively arranged in opposite walls 18 and 20 or 22 and 20 form in the cross sectional direction of the heating apparatus 10 respectively a v shaped groove 36 fig2 a which permits a yarn 38 to be guided in its stretched condition between yarn guides 32 without any special auxiliary measures or arrangements between successive yarn guides 32 in a movement substantially perpendicular with respect to base 16 there the yarn resting against the contact surfaces forms then a zigzagged yarn path as seen in fig3 as noted above the embodiment of the heating apparatus 10 as illustrated in fig1 consists of two rail segments 14a and 14b one following the other in direction of the advancing yarn while these segments differ in length they have otherwise the same cross sectional shape the purpose of such a bipartite arrangement may lie in the different.