A transmission has a gear system (34) which may be in the form of an orbit system (32, 35) or a planetary orbital spur system. The gear systems include a control gear (32) which is controlled to determine the drive ratio of the transmission. The control gear is controlled by a mommentum control which includes a disc (23) and a slide (25) which carries a magnet (30) for selectively allowing or preventing rotation of the disc (23) to in turn change the speed of rotation of the control gear (32). Alternatively the momentum control can be formed from moveable masses which move radially outwardly relative to the control shaft to change the speed of rotation of the control gear.