element is held firmly in a fixed position by means of this circuit fluctuations in yarn tension that are detected by the evaluation unit 17 as nonperiodic fluctuations inyarn tension are balanced out because a decrease in the yarn tension immediately causes the compressive force 24 to be raised optionally it is also possible during the unwinding of a cop on the winding machine for the rising yarn tension to be balanced out first by lowering of the compressive force 24 while toward the end of the cop the winding speed is lowered as well fig3 shows a further variant of the present yarn tension sensor and unlike the embodiments described thus far here the plunger coil 5 34 and yarn guide 6 34 are mounted on a lever 27 the rotary bearing of the lever 27 may be a known air bearing for instance which has a negligibly low rotary resistance since the deviations of the plunger coil from the zero position are extremely slight as already noted the bearing shown in fig 3 again has no disadvantageous effect in terms of changing the air gap as a function of the lever length that forms the pivotal radius the length of the lever 27 may also be selected such that the radius of curvature is large enough that normal pivotal movements of the lever.