Squeezing & Shearing

If three 240-ton locomotives were piled one on top of the other and their combined weight rested on a postage stamp, the resultant pressure would be some 1,440,000 lb. per sq. in. Such a pressure, Harvard University announced last week, has been produced in its physics laboratories by stocky, soft-spoken Percy Williams Bridgman and maintained for 15 hours on a speck of graphite as big as a pinhead.

That was a special experiment. For routine high-pressure work Dr. Bridgman uses about half the maximum, working up to 720,000 lb. and then down again in one hour. Even that squeeze has not been duplicated elsewhere. In principle the Bridgman pressure apparatus is simple, like Archimedes' theoretical lever with which the old Greek said he could move the world. It is a hydraulic press, in which is utilized the fact that a piston bearing on a small area of a confined liquid delivers its pressure against every other area of equal size in the tank. Thus if a force of 100 lb. is brought against a piston one square inch in cross section, the force transferred through the liquid to a 1,000 sq. in. piston is 100,000 lb. The catch is that to make apparatus able to stand such a stress is a delicate, costly and patience-taxing business. If the reinforcement is imperfect or if the materials are not the best in the tiny arena where the gigantic crush is finally focused, steel is likely to bulge like butter. Squeezed by 300 tons per sq. in., some of the contraction of a substance is due to a shrinkage of the atoms themselves. The complex atom of cesium shrinks most of all metals. Of 48 metals under high pressure, 39 become better conductors of electricity. Iron grows softer, glass harder. Squeezed water turns solid (''ice") in five different forms, one of which does not melt until heated to nearly 212DEGF. Under the increased pressures announced last week, two more kinds of ice are formed, one of which can be made hotter than boiling water without melting. Professor Bridgman reasoned that even more fundamental changes might occur in his materials if he could squeeze and twist them at the same time. Therefore instead of letting the vertically opposed cylinders (1 1/4 in. in diameter) compress the substance directly, he inserted between them a flat, hard steel block, ground microscopically smooth on its upper and lower sides. Between the block surfaces and the cylinder faces are placed specks (one-tenth gram or less) of the stuff to be tested. Straight pressure is applied, squeezing out some of the stuff from under the pistons, until the friction of its flow becomes so great that no more escapes. Then, as the pressure is stepped up, the steel block is rotated by a man exerting his full strength at the end of a yard-long lever, equipped with a strain gauge to measure the twisting force. When the lever is rotated through 35DEG, the distortion in the little disk of material is equal to that in a one-inch cube one of whose faces is theoretically pushed 60 inches out of position.

What happens under this treatment Dr. Bridgman recounted at length in the Physical Review. Rubber turned into a hard, translucent chip like horn. So did wood, paper, linen cloth and Duprene. Celluloid exploded, splintering the cylinder edges. Copper and sulphur together exploded, combined to form copper sulphide. Lead peroxide blew up, leaving only a film of the metal. Yellow oxide of lead left a similar film, but gave up its oxygen quietly. Ammonium nitrate, ordinarily highly explosive, refused to go off. Red phosphorus turned into black. A dye called Brom-thymol-blue did not itself change color but lost virtually all its staining properties. Some soft metals made a perfect weld with the cylinder face, had to be scraped off with a razor blade. When soft graphite was squeezed for 15 hours at the maximum pressure and then twisted, it was found to have become extremely hard and was imbedded in the steel like an abrasive. But when the pressure was released, the graphite became soft again.

This excursion of science was so new that Dr. Bridgman did not venture far in explanation, although he is now preparing for extensive microanalysis of his end products. He believes that the explosions are more of a mechanical than a chemical phenomenon. In general, the more a substance is compressed the harder it is to shear. In metals the crystal structure seems to be destroyed by the combined forces. Thus shearing is easy along the planes of crystallization, but becomes harder after the crystals crumble or realign themselves along another plane. Curves of pressure charted against shearing force show many a sharp break indicating a definite change of structure at such points.

Born 53 years ago in Cambridge, Percy Williams Bridgman has spent his entire academic life at Harvard. His black, unruly, grey-streaked hair with a bald spot on top, his firm, thin mouth, his steady gaze through spectacles, his old-fashioned collars and sombre clothes make him look a little like a rustic evangelist. He is married, lives in a small house without pets, has a daughter specializing in biology and a son studying geology, likes mountain-climbing, summers in New Hampshire. He is a member of the National Academy of Sciences, a recipient of that lordly body's Comstock Prize, awarded every five years for distinguished research. He is frequently mentioned as a candidate for the Nobel Prize and his colleagues have little doubt that sooner or later the world's top scientific honor will be his.

Percy Bridgman, as he makes clear in graceful prose, is impatient of traditionalism, mysticism, outworn modes of thought, orthodox theology, clumsy social institutions. He believes that the one passion suitable to a civilized man is that for intellectual integrity, and that this high concept is taking deeper root in the world. In this the true scientist helps. "He finds something fine," Dr. Bridgman wrote two years ago in Harper's, "in the selflessness involved in rigorously carrying through a train of thought careless of the personal implications; he feels a traitor to something deep within him if he refuses to follow out logical implications because he sees that they are going to be unpleasant; and he exults that he belongs to a race which is capable of such emotions. Intellectual honesty appears to such a worker as the last flowering of the genius of humanity. . . . The culmination of a long cultural history, and the one thing that differentiates man most notably from his biological companions. The discovery that the human animal is so constituted that it responds emotionally to the practice of intellectual honesty is just as great a discovery as ... that it responds emotionally to music.''

This file is automatically generated by a robot program, so reader's discretion is required.