Only forty million miles from the sun's surface, its sunward side is raised nearly to red heat by the blazing radiation. And the shadow side, naturally, is down to the utter cold of space. There is a temperature drop of nearly seven hundred degrees between the two sides, and Williamson cells turn that heat-difference into electric current, with an efficiency of 99 percent. Then the big Dugald tubes - they are twenty feet long on the Power Planet - transform it into the beam which is focused on the Earth and delivers something over a billion horsepower to the various receivers that have been erected. The space station itself is ten miles across, and it rotates at a carefully calculated speed so that the centrifugal force at its outer edge is very nearly equal to the normal gravity of Earth. So that the nearer its center one goes, of course, the less is that force, and also the less impression of weight one has.