Abstract: A mechanical clutch with two elements (11, 21) which, for the purpose of positively engaging one another, are provided with opposed end toothings which are adapted to one another, the first element (11) being rotatably mounted and axially supported, the second element (21) also being rotatably mounted and axially supported, and further being axially blocked in a first position for the purpose of being non-rotatably engaged with the first element (11), and being axially moveable into a position in which it is freely rotatably and disengaged from the first element (11), with a device for selectively axially blocking the second element (21), the clutch further comprising: a cavity system whose shape is variable and which is filled with a magneto-rheological fluid; a displacing member which can be moved between a first position and a second position, which delimits the cavity system and which is connected to the second element (21); and an electro-magnet (37) which can magnetize at least part of the magneto-rheol