(UF6) A rotating magnetic field at the bottom of the rotor, as used in an electric motor, is able to spin it quickly enough that the UF6 is thrown towards the outer wall, with the 238UF6 enriched in the outermost layer and the 235UF6 enriched at the inside of this layer. The centrifugal force creates a pressure gradient: On the axis of the centrifuge there is practically vacuum, so that no mechanical feedthrough or seal is needed for the gas inlet and outlets; near the wall the UF6 reaches its saturation pressure, which in turn limits the rotation speed, because condensation must be avoided. In the so-called countercurrent centrifuge, the bottom of the gaseous mix can be heated, producing convection currents. But the countercurrent is usually stimulated mechanically by the scoop collecting the enriched fraction. In such a way, the enrichment in each horizontal layer is repeated (and thus multiplied) in the next layer, similarly as in column distillation. One scoop is behind a perforated baffle that rotates with the centrifuge; it collects the 238UF6-rich fraction. The other scoop is without baffle. It slows down the gas rotation and thus increases the pressure towards the inside, so that also the 235UF6-rich fraction can be collected without pumping. Each centrifuge has one inlet on the axis and two output lines, one collecting the gas at he bottom and one at the top. Quantitatively, the radial pressure (or density) distribution can be given by where p is the pressure, r the variable radius and R its maximum, M the molecular mass, ω the angular velocity, k the Boltzmann constant and T the temperature. (This equation is similar to the barometric formula.) Writing this equation for oth isotopes and dividing, gives the (r-dependent) isotope ratio. It only contains ΔM (not the relative mass difference ΔM/M) in the exponent. The radial enrichment factor then results by dividing through the initial isotope ratio. To calculate the total enrichment in a countercurrent centrifuge of height H, one has to add a factor of H/(R√2) in the exponent. According to Glaser, early centrifuges had rotor diameters of 7.4 to 15 cm and lengths of 0.3 to 3.2 m, and the peripheral speed was 350 to 500 m/s. The modern centrifuge TC-21 of Urenco has a diameter of 20 cm and a length of more than 5 m, spinning with 770 m/s. Centrus (formerly Usec) plans a centrifuge