In the form of construction shown in the drawings (see particularly FIGS. 2 and 3), the detector 8 is constituted by a potentiometer, the circular track 9 of which is rigidly fixed to the sleeve 6 while the slider 10, fixed on a shaft 11, is driven in rotation about its axis, either directly or indirectly through the intermediary of two multiplying toothed wheels 12 and 13, of which one 12, coaxia