provided that the per se conventional brake disk 3 is in direct contact with the flat annular piezoelectric element 9 however there is naturally nothing to prevent the brake disk 3 from being given another configuration and being adapted to attain maximum cooperation with the flat annular piezoelectric element 9 in the embodiment of the present invention illustrated in fig4 and 5 the apparatus is arranged at a flat brake of a per se conventional type which is intended to brake a thread 13 in that the thread under angular deflection is urged with a certain force against a brake plate 14 which is mounted on a pair of feet 15 and 16 secured to the machine naturally the brake plate 14 may be mounted directly on the machine it is further conceivable according to the invention that either one or both of the feet 15 16 are adjustable for regulating the braking force or alternatively that the signal generator or signal emitter 17 for urging the thread 13 against the brake plate 14 is adjustable on the side of the signal emitter 17 which faces the thread 13 there is disposed a brake spindle 18 which may be of a per se known type the brake spindle 18 is disposed on one side of a housing 19 which accommodates a circuit board 20 with electronics.