System for providing rotary drive between two mechanical members by means of magnetic coupling, and a fluid meter including such a system

A drive system for rotating two mechanical members about an axis by means of magnetic coupling, the system comprising a magnetic element secured to each of said mechanical members, the two magnetic elements being concentric, the first of said magnetic elements possessing N magnetic poles regularly distributed about the axis of rotation in a ring having a minimum radius R.sub.1 and a maximum radius R.sub.2, where N is even, the second magnetic element possessing N magnetic poles regularly distributed about said axis of rotation in a first ring and N magnetic poles regularly distributed about the axis of rotation in a second ring identical to the first ring, where both rings have a minimum radius R.sub.3 and a maximum radius R.sub.4, said rings of said second magnetic element being mechanically secured to each other and each having a respective midplane perpendicular to the axis of rotation that is axially offset by a distance d from the midplane of the ring of the first magnetic element in such a manner as to lie on opposite axial sides of said first magnetic element, wherein said first and second magnetic elements are magnetically coupled by attractive forces, the magnetization of each magnetic element having a direction that is contained in a plane perpendicular to the axis of rotation, and wherein the ratio 2d/.vertline.(R.sub.3 +R.sub.4)-(R.sub.1 +R.sub.2).vertline. lies in the range 0.2 to 0.8.