the monitoring interval not only has the weft thread been inserted or introduced but also has been inserted at the correct moment of time on its output side 50a the pulse generator 50 is connected to a clock pulse generator 60 the clock pulse generator 60 is feed back connected to define the start of the monitoring interval the clock pulse generator 60 as shown in fig3 comprises a first and gate 61 having two inputs 38 39 and an output 87 and further comprises a third and gate 65 having two inputs 51 52 and an output 53 the first inputs 38 and 51 of the first and third and gates 61 and 65 respectively are connected to the pulse generator 50 the output 87 of the first and gate 61 is connected firstly to an adjustable monostable or one shot multivibrator 62 which acts as a timing element and secondly to the second input 101 of a counter 83 in a counting and indicating unit or circuit 80 which also forms part of the monitoring and control circuit 17 in the clock pulse generator 60 there is further provided a first monostable or one shot multivibrator 63 having two inputs 44 45 and an output 46 the first input 44 or r input is connected to the output 53.