sensor device 24 and is sent via the signal line 25 as an electric signal i for further processing to the control circuit 18 the embodiment of a scanning sensor 22 shown in fig4 corresponds essentially to the embodiment of a scanning sensor already known from fig3 this means that the scanning sensor 22 has a clip 26 b which is concave relative to the yarn balloon b and mounted pivotably at the ends the clip 26 b bears against the yarn balloon b from the inside and therefore has a curvature 31 b outwards in the direction of the yarn balloon b by means which as already explained above the contact of the running outer yarn 5 with the clip 26 b is minimised and the yarn is thus protected which has a positive effect on the yarn quality the pivot axis 29 b of the clip 26 b is at right angles to the axis of rotation 30 of the yarn balloon b and is arranged so that the clip 26 a is lifted by the yarn balloon b and thereby positioned in an operating position bs which as in the example embodiment of fig2 and 3 is recognised by the associated sensor device 24 and sent via the signal line 25 as an electric signal i for further processing to the control circuit 18 in a further advantageous embodiment the scanning sensor 22 can also be designed however as a spring loaded scanning feeler 32 bearing on the yarn balloon b from the outside the scanning feeler 32 can be designed to be linear as shown in fig5 or as shown in fig6 can have a concave curvature 33 relative to the yarn balloon b in the linear design.