to control the yarn quality of a yarn produced by a textile machine , particularly a false - twist jet spinning apparatus , there is provided at the end of the spinning operation , however , upstream of the pair of withdrawal rolls for the outfeed of the produced yarn , a yarn tension measuring device . upon falling outside of a predeterminate yarn tension tolerance or tension range the yarn tension measuring device controls operation of the false - twist jet spinning apparatus such that , as required , there is accomplished an appropriate increase or decrease of the yarn tension of the yarn produced by the false - twist jet spinning apparatus . alteration of the yarn tension can be achieved , for instance , by varying the blowing intensity or energy of the air utilized for the spinning operation as well as by varying the angle at which there is blown in the air used for the spinning operation or by varying the rotational speed of the delivery rolls which deliver the formed yarn or by selected combinations of these measures .