the generator, typically about long and in diameter, contains a stationary stator and a spinning rotor, each containing miles of heavy copper conductor. there is generally no permanent magnet, thus preventing black starts. in operation it generates up to 21,000 amperes at 24,000 volts ac (504 mwe) as it spins at either 3,000 or 3,600 rpm, synchronized to the power grid. the rotor spins in a sealed chamber cooled with hydrogen gas, selected because it has the highest known heat transfer coefficient of any gas and for its low viscosity, which reduces windage losses. this system requires special handling during startup, with air in the chamber first displaced by carbon dioxide before filling with hydrogen. this ensures that a highly explosive hydrogen–oxygen environment is not created.