A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to perform mechanical work on a rotating output shaft. Its modern manifestation was invented by Sir Charles Parsons in 1884. Because the turbine generates a rotary motion, it is particularly suited to be used to drive an electric generator: approximately 90% of all electricity generation in the United States (1996 ) is by use of steam turbines. The steam turbine is a form of thermal engine that derives much of its improvement in thermodynamic efficiency from the use of multiple stages in steam expansion, resulting in a closer approach to the ideal reversible expansion process.