as shown by arrows f the servo control apparatus regulates the electromagnet 16 which pulls on the rod 12 while thus disengaging the element 3 borne by the block 4 from the element 2 itself borne by the body 1 when a defect immobilizes the strand in the calibrated space the vibration detector causes the opening of this space until the defect passes in the form of embodiment illustrated in fig9 to 11 a succession of separate calibrated spaces are provided so that the impregnated or coated linear materials are submitted to a succession of compressions and decompressions facilitating the elimination of excess matter in this form of embodiment the apparatus contains a frame 41 holding two opposite plates 42 these plates display grooves 43 in which are attached ceramic cylindrical small bars 44 particularly of tital an inset piece such as a thickness gauge is provided between the two plates in order to maintain their spacing at a desired value corresponding to the calibrated space 45 this arrangement allows for modification of the diameter number and spacing of the small bars between each pair of bars 44 a decompression interval 48 is located the example hereafter given as a.