System for indicating the trace of a pencil or of an eraser on a writing medium comprising: - a magnetic ring (14; 110; 130; 152) that can be fixed to the pencil or the eraser, this ring comprising: - a cylindrical inner face centred on the longitudinal axis and an outer face opposite the inner face, - a magnetic material distributed in a layer (44; 112), called "magnetic layer", about the longitudinal axis so that the direction of the overall magnetic moment of the magnetic ring is parallel with the longitudinal axis, - a bearing face of a tablet, this bearing face constituting the writing medium or supporting the writing medium, characterized in that: - the magnetic ring (14; 110; 130; 152) can be fixed removably to the pencil or the eraser, - the smallest inner diameter of the ring is between 1.5 mm and 3 cm, - the ring comprises an elastic material (46; 116), distinct from the magnetic material, that is elastically deformable between: • a rest position corresponding to a first value of the inner diameter of the ring, and • a deformed position corresponding to a second value of the inner diameter of the ring, the difference between the first and second values of the inner diameter being greater than or equal to 1 mm, this elastic material exhibiting a Young's modulus at 25°C less than or equal to 1 GPa, - the system comprises: • an array of magnetometers that can measure the magnetic field generated by the magnetic ring, this array of magnetometers being fixed with no degree of freedom to the bearing face of the tablet, • a computer that can determine the position and the orientation of the magnetic ring to deduce therefrom the position of the point of the pencil or of the eraser with respect to the bearing face from the measurements of the array of magnetometers. - a magnetic ring (14; 110; 130; 152) that can be fixed to the pencil or the eraser, this ring comprising: - a cylindrical inner face centred on the longitudinal axis and an outer face opposite the inner face, - a magnetic material distributed in a layer (44; 112), called "magnetic layer", about the longitudinal axis so that the direction of the overall magnetic moment of the magnetic ring is parallel with the longitudinal axis, - a cylindrical inner face centred on the longitudinal axis and an outer face opposite the inner face, - a magnetic material distributed in a layer (44; 112), called "magnetic layer", about the longitudinal axis so that the direction of the overall magnetic moment of the magnetic ring is parallel with the longitudinal axis, - a bearing face of a tablet, this bearing face constituting the writing medium or supporting the writing medium, - the magnetic ring (14; 110; 130; 152) can be fixed removably to the pencil or the eraser, - the smallest inner diameter of the ring is between 1.5 mm and 3 cm, - the ring comprises an elastic material (46; 116), distinct from the magnetic material, that is elastically deformable between: • a rest position corresponding to a first value of the inner diameter of the ring, and • a deformed position corresponding to a second value of the inner diameter of the ring, the difference between the first and second values of the inner diameter being greater than or equal to 1 mm, this elastic material exhibiting a Young's modulus at 25°C less than or equal to 1 GPa, • a rest position corresponding to a first value of the inner diameter of the ring, and • a deformed position corresponding to a second value of the inner diameter of the ring, the difference between the first and second values of the inner diameter being greater than or equal to 1 mm, this elastic material exhibiting a Young's modulus at 25°C less than or equal to 1 GPa, - the system comprises: • an array of magnetometers that can measure the magnetic field generated by the magnetic ring, this array of magnetometers being fixed with no degree of freedom to the bearing face of the tablet, • a computer that can determine the position and the orientation of the magnetic ring to deduce therefrom the position of the point of the pencil or of the eraser with respect to the bearing face from the measurements of the array of magnetometers. • an array of magnetometers that can measure the magnetic field generated by the magnetic ring, this array of magnetometers being fixed with no degree of freedom to the bearing face of the tablet, • a computer that can determine the position and the orientation of the magnetic ring to deduce therefrom the position of the point of the pencil or of the eraser with respect to the bearing face from the measurements of the array of magnetometers.