has a short leg whose right edge is in register with the edge 62a of the upper plate 62 thus a free space serving as an insert gap e is provided at the right side of edge 62a and between the plates 61 and 62 allowing insertion of a thread or yarn in radial direction into yarn channel k the upper plate 62 is provided with an extension 63 having a bore 64 for mounting the sensing device on a machine as may be seen from fig6 a the yarn channel k is confined by the plate shaped electrodes 61 62 in alternate sequence this embodiment of the sensing device may advantageously be used in the place of the one shown in fig2 a and 2b for sensing a ballooning yarn and producing a signal indicative of a ballooning motion only during time intervals in which the yarn is contacting collector electrode 61 at the edge k1 thereof beneath insert gap e that embodiment may be modified variously and accommodated to any use in question by way of example the upper plate 62 may be made of an insulating material rather than metal or it may be covered by a hard insulating material collector electrode 61 may also be provided with a hard insulating layer preferably ceramic oxide of great surface hardness is used as an insulating material for this purpose in an alternative embodiment there may be attached to the lower plate 61 serving as a sensing element a piezoelectrical element 65 which may be vibrated.