form of a thicker or thinner piece of tow should pass into the intake rolls transport roll arrangement of the monitored machine this fault will show up as a change in the tow tension upstream of the intake rolls which will be readily measurable with tension measuring heads known per se if the transport roll arrangement consists of a multiple set of rolls a fault in the form of a thick place in the tow will lead to a tension increase and a thin place in the tow will lead to a tension decrease if the transport roll arrangement consists of a pair of squeeze rolls then a thick place in the tow on passing through the nip will lead to a tension increase and a thin place in the tow to a tension decrease provided the tow is also held under tension downstream of the pair of squeeze rolls if the tow is no longer under tension downstream of the squeeze rolls which is the case for example downstream of the intake rolls of a stuffer box crimping box the reverse will apply a thick place in the tow will lead to a tension decrease and a thin place will provoke a tension increase even so called 34 hard places 34 which are formed for example due to an accumulation of broken filaments which have coalesced to form a hot like polymer blobs will on passing through the squeeze type intake rolls of a monitored machine for example a stuffer box lead to a sharp tension.