screw holes 24 for fixation on a mounting element 27 are arranged on the basis surface 22 a larger opening 23 is also arranged in the basis surface 22 through which the screws 26 26 39 can be tightened or slackened both the catching forks are mounted to be able to pivot in opposite directions and each held under tension by a pressure spring 30 arranged at a distance from the pivot axis the tensioned position is suggested by a dotted line each pressure spring is mounted in and acts against a bore in the holder 25 a proximity switch 29 is mounted on each outer side of the holder 25 so that it lies within the area of action of the outer end 31 of a catching fork each proximity switch generates a control signal as soon as a catching fork 19 is pivoted by contact with an incorrect yarn and the outer end 31 approaches said proximity switch each catching fork 19 is connected to a mounting element 27 mounted in a hollow chamber 33 in the holder 25 said hollow chamber being open towards its facing side the mounting element 27 also serves to compensate for inertia however in that it forms a compensation mass 32 which compensates the mass inertia taking effect in the direction of the arrow b on acceleration of the two sensors said acceleration taking effect in the direction of arrow a said sensors thus behave neutrally under the influence.