Segway HTThe Segway Human Transporter is a self-balancing motorized scooter with two wheels on a single axle. It was invented by Dean Kamen. The scooter is designed to be used in pedestrian areas, hence its footprint is not much bigger than that of a human being. The inspiration behind it came from the balancing technology of his innovative wheelchair, which has four wheels, climbs stairs, and can prop itself onto two wheels to give the user the same eyelevel as standing persons. The Segway HT has electric motors powered by batteries which can be charged from house current. It balances with the help of internal computers and gyroscopic sensors. (The gyroscopes do not affect the balance; they are merely used as sensors.) The motors turn the wheels forwards or backwards as needed for balance or propulsion. The rider accelerates or decelerates by leaning forward or backwards in the direction she wishes to travel. Steering is controlled by a twist grip on the left handlebar. An alternative power source currently being explored by Kamen's company is the use of a Stirling engine to power the Segway. Stirling engines use hot and cold in a phased rhythm to move a piston by gas expansion. Stirling engines could run themselves, eliminating the need for charging via external power. The Segway HT's maximum speed is 12 mph (19 km/h). Maximum power is 2 horsepower (1500 watt). The Segway HT has been known by the names Ginger and IT in the past. There were various reports of a revolutionary invention, but no details were available until its unveiling on December 3, 2001. The name Segway is a play on "segue" (a transition).