is prevented as long as the wire 20 does not move upwardly again and is released from the fabric if the weft w is not present or is broken it does not exert the above braking effect and the assembly 18 20 is urged by the springs 24 in the direction of the arrow f12 to follow within certain limits the initial movement of the sley 5 in a direction opposite to the arrow f10 this different behavior of the assembly 18 20 in the presence of and in the absence of the sound weft w causes the production of a signal by the action of the screen 22 in the case of a broken weft the screen 22 comes into alignment with the extensions 14a 14b before this would happen in the case of a sound weft capable then of resisting the thrust of the springs 24 in other words if the weft is broken or missing the assembly 18 20 22 advances before rather than after the wire 20 is lifted at this state the slidable unit 18 is moved to the position shown in fig1 wherein the springs 24 are compressed and the unit 18 is retained by the present weft just inserted and engaged by the warp exchanges in the absence of the weft or with slackening.