of the protective device according to the invention the device includes similarly to the first embodiment protective element 25 made of a ferromagnetic material comprising a ring shaped body 27 whose outer diameter is equal to the outer diameter of the outer collar of outer ring 1 and whose inner diameter is equal to the inner diameter of magnetic encoder 6 ring shaped body 27 has an oblique opening 21 diametrically opposed to tab 19 that runs radially with respect to the center of protective element 25 and has circular head 18 that fits inside the inner diameter of ring shaped body 27 like the first embodiment such a protective element 25 may be easily attached and magnetically held to the active face of magnetic encoder 6 during the manufacture of instrument equipped outer ring 1 for example this variant embodiment has the advantage of permitting protective element 25 to remain on magnetic encoder 6 during assembly of the measuring device shown in fig1 in fact as you can see in fig5 tab 19 on protective element 25 is designed to be able to fold back by pulling on head 18 so as to open the center of ring shaped body 27 accordingly assembly of the measurement device may be carried out with protective element 25 still attached to outer ring 1 by folding back tab 19 by pulling head 18 away and outside of ring shaped body 27 so as to permit the insertion of drive shaft 14 through outer ring 1 as shown in fig6 protective element 25 is therefore not removed.