A consistent yarn thickness is a prerequisite in the textile industry for high quality products. Therefore, the naturally occurring yarn thickness fluctuations must be eliminated. The yarn is drawn to a constant thickness using a stretching tool. The measured data for controlling the stretching tool are provided by an eddy current sensor which indirectly measures the thickness. The thickness is transmitted to the eddy current sensor using sensing rollers. After the stretching, the result of the processing operation is also checked by an eddy current sensor.