Abstract: A rotary mechanical amplifier for rapidly switching a controlled member between two positions, which comprises a rotary drum carrying a series of cylindrical rods regularly spaced in circular configuration about the axis of rotation of the drum. Each rod is selectively slidable axially between two positions in response to control signals, for example by means of electromagnetically controlled sprung bolts or catches. In one axial position, the rods tangentially engage a curved surface of a lever or other controlled member to move it to a position where it can be held without vibration by tangential engagement with successive rods.