A centrifuge rotor constructed of steel and Duralumin ST‐14 weighing 4.93 kilograms and having a radius of 65 mm to the center of the standard centrifuge cell, which contains the material to be centrifuged, is suspended magnetically in a high vacuum (pressure less than 10−5mm Hg). The rotor is driven to running speed by an air turbine below the vacuum chamber. The turbine is connected to the rotor by a small steel shaft along the axis of rotation and passing through vacuum‐tight oil glands into the vacuum chamber. When the rotor reaches operating speed, the shaft is disconnected and the rotor is allowed to coast freely during the sedimentation experiment. This is possible because of the very small deceleration of the rotor (less than a 0.1 revolution per sec per hour). As a result the temperature and rotor speed not only can be measured accurately, but can be maintained very nearly constant. The ultracentrifuge has been used both for rates of sedimentation measurements and for sedimentation equilibrium measurements, but is especially suited for the latter.