identical and in this case they attract one another when they are face to face in a second particular case their north south orientations can also be inverted and in this case they repel one another when they are face to face it is the latter arrangement that will be generally preferred as it produces forces more efficient in displacing the mobile elements in addition the diameters of holes 20 26 of the stator and of the rotor are also rigorously identical and the holes are geometrically distributed in the same way according to an advantageous form of embodiment of key 14 the key is composed of a body of amagnetic material such as aluminium for example magnets 16 are placed inside this body in holes 15 the diameter and geometrical distribution of which are identical with those of cylinder 1 they are for example perpendicular to the front face 10 of the key magnets 16 being for example small in size and of an elongated shape according to fig3 a and 3b sheaths 22 of ferro magnetic material are formed by an amagnetic sleeve 42 and a yoke 43 of ferro magnetic material they are thus capable of concentrating the magnetic flux from magnets 16 and 21 in a given direction to increase the coding accuracy the two magnets are gripped in their sheaths in which they are fixed for example with araldite they are placed exactly opposite one another with the poles of the same sign facing each other because of ferro magnetic yokes 43 the lines of the field are highly concentrated and rigorously symmetrical thus in the case of fig3 a they produce a force of maximum mutual.