Abstract: A protection device (70) for providing a drive system (60) for imparting rotary drive between two mechanical members (28, 44) about an axis by magnetic coupling with protection from an external magnetic field, said drive system including at least two axially magnetized elements (62a, 62b, 64a, 64b) each secured to at least one of said mechanical members and designed to interact with one another, said magnetized elements being axially offset, said protection device comprising two parts (72, 74; 92, 94; 96, 98) of magnetic material surrounding said drive system, each of said parts having a minimum radial extent greater than the radial extent of said magnetized elements, the protection device being characterized in that the two parts (72, 74; 92, 94; 96, 98) of magnetic material leave between them an axial space (76) so as to reduce the gradient of the external magnetic field at the drive system, and have an axial extent such that they encompass said drive system axially.