Safety in the operation of high-speed rotors used in superchargers of high-altitude bombing planes is the goal of experimenters at General Electric rescarch laoratory, where heavy metat wheels are spun in a “vacuum bottle” until they fly apart. With virtually no air resistance to overcome, a five-pound turbine, driven by compressed air,jcan turn a 20-pound wheel at speeds up to 1,000 revolutions per second. At that tremendous speed, every pound of weight on the rim of a wheel one foot in diameter exerts a centrifugal pull of more than 100 tons, and wheels often burst with the force of an explosion, throwing fragments, which may weigh several pounds, with as much energy as the projectile of a small naval gun. In order to withstand such pounding, the vacuum chamber has massive steel walls, and it is sometimes lined with lead bricks to minimize damage to the fragments which are examined for information concerning the nature of the breaks. In making tests below the breaking point the rotor is coated with a brittle varnish that will crack at the points of stress. The cracks occur at right angles to the direction of the stress, and their number indicates the relative magnitude of the stresses.