each conductive only in the left portion thereof and are nonconductive in the right portion thereof in this manner the position sensor emits a signal when the associated yarn guiding element is in the feeding area position while in the nonfeeding area position no signal is produced of course this arrangement could also be reversed the transition from the nonconductive portion to the conductive portion of each contact plate is arranged so that a signal is produced exactly when the associated yarn becomes disengaged from the belt 5a 5b 5c 5d or moves under the belt namely the switching points of the position sensors lie exactly in the threshold regions in which a positive delivery of the yarn starts or stops the respective signal is then maintained unchanged during further movement of the yarn guiding element within the non feeding area or the feeding area the contact plates 7a 34 7b 34 7c 34 7d 34 are connected by a connecting line to a control relay cr which on the other hand is connected to a positive voltage source in the housing 1 the contact pins 7a 39 7b 39 7c 39 7d 39 are connected by a common line to a negative voltage source also in the housing 1 the control relay cr has its relay contacts cr c connected in a circuit which includes the stop motion relay of the knitting machine the stop motion relay for example turns off the knitting machine when it is energized this circuit in the housing forms an electric logic switching circuit which operates in the aforementioned manner and switches off the knitting machine when all position sensors produce specific signal combinations which will be explained later fig2.