An annular fluidic device includes a closed annular tube, wherein the tube includes at least one portion of transparent wall and contains a rigid torus bathed in a fluid filling the internal volume of the tube, in that the torus is arranged to be movable in the tube, and wherein at least one permanently polarised magnetic element integral with the torus allows it to undergo a moment of force causing it to be aligned in the direction of the external magnetic vector. A watch case can include this device.