The apparatus (1) has a movable wall (69B) mechanically connected to a conducting brush (40), and moved relative to a fixed wall (12B) when the brush moves between conduction and isolation positions. A viscous fluid substance (57B) is sandwiched between the walls in the conduction and isolation positions and in each of intermediate positions between the conduction and isolation positions. A movable contact (5) carried by the brush is applied against a fixed contact (4B) in the conduction position, and is separated from the fixed contact in the isolation position.
