in length of the unit comprising the articulating portion 1 and the rods 2 and 3 whereas any lateral displacement is avoided by means of the extending element 14 the other extremity of the threaded rod 11 cooperates with the housing 6 either by direct screwing or by means of a pin 15 as illustrated in fig2 another component of the articulating portion 1 is a lateral cover 16 whose general shape follows that of the housing 6 the visible face of that lateral cover 16 presents a scale 17 for the reading of relative angular movements between the sliding block 7 and the housing 6 and its lateral cover 16 some of the graduations of the rounded scale 17 are enlarged and extended to form viewing lines 18 aiming at the virtual axis a one could also envisage disposing viewing lines on the side of the housing opposite to the cover 16 not visible in the drawings the scale 17 and its corresponding mentioned numbers on the one hand and the viewing lines 18 on the other can either be painted or engraved in the cover and the housing as shown in the detailed cross section of fig7 the side of the housing 6 facing the lateral cover 16 presents an.