the input of a differentiator upon yarn breakage the sudden voltage change u03b4u results in the generation at the differentiator output of a voltage spike for example a negative voltage spike which is passed by a half wave rectifier to a schmitt trigger if the negative voltage spike is of a magnitude so great as to correspond to the sudden pressure decrease associated with yarn breakage the schmitt trigger triggers a dynamically triggered monostable multivibrator which in turn generates a well shaped pulse constituting the yarn breakage indicating signal the schmitt trigger is provided to distinguish between high magnitude voltage spikes associated with yarn breakage and low magnitude voltages associated with lesser pressure variations attributable to other causes the half wave rectifier is provided to block the voltage spike which is generated by the differentiator when the piezoelectric voltage u undergoes an opposite sudden change upon resumption of yarn travel the yarn breakage signal whether generated by photoelectric or by piezoelectric means can be processed in the same manner instead of the proximity detectors already discussed use could also be made of an air pressure responsive proximity detector which in the event the ring traveller 9 comes to a standstill as a result of yarn breakage causes a signal to be generated in a manner analogous to what has been described above in response to the absence of air flow.