A gas centrifuge tube spins very fast to create an artificial gravity. The tubes do not spin end over end like a cheerleader's baton but around their long axis, like an axle. With the tube spinning at full speed, uranium, in the form of a gaseous compound, uranium hexafluoride, is introduced through a small feed tube running along the axis of the centrifuge tube. With these huge pseudo-gravitational fields, the heavier uranium-238 will be slightly more concentrated around the outside of the centrifuge tube than the lighter uranium-235, which will be slightly more concentrated along the central axis of the spinning tube. If the uranium hexafluoride were removed from the central axis of the tube, it would have a higher concentration of uranium-235, or be 'enriched,' compared to what went in. But the enrichment effect is small. There is, however, a simple but quite clever trick that dramatically increases the amount of enrichment possible in one centrifuge. If one end of the tube is heated, convection currents form. (Remember that in the spinning tube the centrifugal forces are such that 'up' is toward the central axis of the tube and 'down' is out toward the wall of the tube. So the heated gas 'rises,' going to the center, while the cooler gas fills in to replace it along the 'bottom,' that is, along the wall.) The lighter uranium-235 molecules will spend, on average, a little bit more time near the axis being swept along toward the cool end of the tube while the heavier uranium-238 molecules will spend a little bit more time near the wall of the tube, being swept toward the hot end. Now, rather than having the gas be slightly enriched toward the center of the tube, it is more enriched at one end of the tube. The degree of enrichment depends now on the length of the tube. Early experimental gas centrifuges had rotor tubes a meter long and the newest rotor tubes can be 10-12 meters long and get enrichment factors per stage a hundred times higher than the gas diffusion process.