One practical consequence of the 11 September outrages is that engineers are scurrying back to their drawing boards to work out ways of building safer skyscrapers. And Lord Corporation, of Cary, North Carolina, has devised one.
Lord's Rheonetic dampers are a metre long and can each deliver 20 tons (200,000 N) of force. The dampers use the magnetorheological (MR) principle. Each piston sits in a cylinder containing five litres of MR fluid surrounded by an electromagnet. Energise the magnet and the MR fluid changes from liquid to solid; turn off the current and it reverts to liquid again. Do it several thousand times per second and the damping is more effective than conventional means allow.