inside surface of the annulus than over its outside surface moreover at least one strain gauge 107 is furthermore arranged on the inside face of the sector this making it possible to measure the stresses applied to the sectors at this inside face this strain gauge is for example made in the known form of a film supporting metal electrodes arranged in such a way that the extension or contraction of the surface on which the gauge is adhesively bonded causes a variation in the resistance of these electrodes according to a known law the set of 5 sectors is arranged inside a shaper annulus 108 which makes it possible to define the shape and the dimensions of the piezoelectric annulus this annulus is for example manufactured from epoxy glass with a carefully polished inside surface the dimensions of the sectors are contrived so that a clearance remains between the metal pieces of the ends of two adjacent sectors adjuster keys having the shape of wedges 109 fill this clearance these keys an example of which is represented in fig2 are therefore placed between the sectors and enable these sectors to be locked inside the shaper annulus 108 the angle between the two lateral faces of these keys is designed to correspond to the angle alpha of the end pieces of the sectors so that when the keys are in position these outside faces are applied to the outside faces of these end pieces with as small an angular clearance as possible so as to avoid excessive stresses at the.