34 a 34 each zig zag in the line corresponds to a half turn of the roller 20 a guiding means 6 39 moves the yarn 1 39 tangentially or causes it to pass over roller 12 39 the guiding means 6 39 follows the helical surface 21 without penetrating between the rollers the yarn 1 39 is squeezed in the gripping line defined by the roller 20 coming in contact with the roller 12 39 as indicated at point 34 c 34 in fig4 with each zig zag portion of fig4 corresponding to a half turn of the roller 20 this movement is illustrated in fig4 by the curve 34 b 34 the measurement of the yarn begins when axial plane 22 comes into contact with rollers 12 39 as shown at point 34 c 34 and ends when guiding element 6 39 brings the yarn 1 39 out of the gripping line at point 34 d 34 by shifting movement 34 a 34 with respect to b or vice versa dotted line b 39 measuring time t is changed as a result the measured length also is changed in order to repeat the measurement it is advised to synchronize the speed of rotation of the notched roller 20 of the measuring device with that of the loom such that for every complete cycle of the loom the roller 20 rotates a complete cycle in other words if the roller 20 has only one aleveolus then the roller 20 should complete its cycle in synchronization with the cycle of the loom similarly if the roller 20 has two alevoli then the roller 20 should complete half of its cycle in synchronization with complete cycle of the loom therefore the number of rotations of the notched roller for every.