Bridgman, Percy Williams

(b. Cambridge, Massachusetts, 21 April 1882; d. Randolph, New Hampshire, 20 August 1961)

physics, philosophy of science.

Bridgman was the only son of Raymond Landon Bridgman, a newspaper correspondent and the author of a number of books on public affairs, and Ann Maria Williams Bridgman. The family moved to Newton, Massachusetts, where Percy attended the public schools until he entered Harvard College in 1900. He graduated with a B.A., summa cum laude, in 1904, with rigorous training in physics and mathematics. He remained at Harvard for his M.A. (1905) and Ph.D. (1908) in physics, whereupon he was immediately appointed research fellow in the department of physics, then instructor in 1910. In 1912 Bridgman married Olive Ware. The couple had two children. Bridgman was appointed assistant professor in 1913, professor in 1919, Hollis professor of mathematics and natural philosophy in 1926, Higgins university professor in 1950, and professor emeritus in 1954.

Percy Bridgman’s penetrating analytical thought and physical intuition, fertile imagination for mechanical detail, and exceptional dexterity in manipulating equipment defined a clear channel of activity into which he threw himself with untiring energy and singleness of purpose. He was an individualist of the most determined stamp, and refused to be diverted by faculty business, by the demands of society, or by any personal weakness from his main interest: his scholarly activity as experimenter, teacher, and critic of the basic concepts of physical science.

While avoiding almost all university committees, Bridgman was an active member of the American Academy of Arts and Sciences, and served on the board of editors of its journal, Daedalus. He was fond of music and also pursued a number of other avocations—all with concentration and perfectionism, whether it was chess, handball, gardening, mountain climbing, or photography. But play was never allowed to interfere with the main business of his life; his unremitting activity was reflected in the high and steady output of papers on physics and philosophy of science. Bridgman wrote about six papers a year, many with such titles as “The Resistance of 72 Elements, Alloys, and Compounds to 100,000 kg/cm².” His lifetime total was over 260 papers, in addition to thirteen books that were largely the products of his summers at Randolph, New Hampshire. All of his writing is remarkably personal and often in the first person singular; whether the subject is the polymorphism of bismuth or the duties of intelligent individuals in an unintelligent society, the characteristic Bridgman tone and quality are immediately evident.

Bridgman, a man of generosity and integrity, was regarded with affection and admiration by his associates. His honors included the Rumford Medal of the American Academy of Arts and Sciences, the Cresson Medal of the Franklin Institute, the Roozeboom Medal of the Royal Academy of Sciences of Amsterdam, the Bingham Medal of the Society of Rheology, the Comstock Prize of the National Academy of Sciences, the New York Award of the Research Corporation of America, and, “for the invention of an apparatus to produce extremely high pressures, and for the discoveries he made therewith in the field of high-pressure physics,” the Nobel Prize in physics for 1946. He was president of the American Physical Society in 1942, a member of the National Academy of Sciences, a fellow of the American Academy of Arts and Sciences, a foreign member of academies of science in England, Mexico, and India, and the holder of honorary degrees from six universities.

Bridgman’s early papers give no explanation of the origins of his interest in high pressures. He may have been influenced by Theodore Richards, who had measured the compressibility of elements, or by Wallace Sabine, with whom he took a research course in heat and light for four years.

His first three papers, published in the Proceedings of the American Academy of Arts and Sciences (Vol. 44 , 1908–1909), laid the foundation for most of his later work. The maximum pressure attained—6,500 atmospheres—was not much higher than was currently used by other investigators, and was inefficiently produced with a screw compressor turned with a six-foot wrench. Bridgman’s first concern appears to have been the establishment of an adequate pressure scale rather than the production of drastically higher pressures. He developed the free-piston gauge, or pressure balance, used earlier by Amagat, and introduced a more convenient secondary gauge based on the effect of pressure upon the electrical resistance of mercury (the subject of his Ph.D. thesis).

The new design of a leakproof pressure seal or “packing”—later called the “unsupported area seal,” and the key to so much subsequent achievement—appears in the discussion of the free-piston gauge, with scarcely a suggestion of its importance. Indeed, Bridgman later explained (American Scientist, Vol. 31 , 1943) that the self-sealing feature of his first highpressure packing was incidental to the design of a closure for the pressure vessel that could be rapidly assembled or taken apart; the basic advantages of the scheme were realized only afterward. In his brief autobiographical remarks in a questionnaire filed with the National Academy of Sciences, under the heading “Discoveries Which You Regard as Most Important,” Bridgman wrote: “Doubtless the most influential single discovery was that of a method of producing high hydrostatic pressure without leak. The discovery of the method had a strong element of accident.”

In principle, the construction insures that the sealing gasket, of rubber or soft metal, is restrained on the upper, or low-pressure, side of the vessel by a

fixed surface the area of which is somewhat smaller than that acting on the other side of the packing. Hence, the latter is always compressed to a pressure higher than that to be confined inside the vessel (Fig. 1); the high pressure itself is used to tighten the packing; and the ultimate limitation becomes the strength of the metal parts. It was mainly this advance that allowed Bridgman to open up a virgin field for experimental exploration.

The third paper of the early series gives new measurements of the compressibility of steel, mercury, and glass. We recognize already the characteristic Bridgman style: the evident pleasure in the manipulations of shop and laboratory; the meticulous pursuit of the numerous corrections; the experiments with homely mixtures of mercury, molasses, glycerine, and marine glue. None of these early measurements proved to be definitive; the absolute gauge was soon improved, the mercury gauge was discarded in favor of a manganin wire gauge, and the compressibilities were revised. But his rapid succession of publications quickly transformed the field of high-pressure research.

By 1910 the equipment had been completely redesigned. The screw compressor was replaced by a hydraulic ram, and the new packing was systematically exploited. For the first time, pressures of the order of 20,000 atmospheres and more are reported. Bridgman remarks: “The magnitude of the fluid pressure mentioned here requires brief comment, because without a word of explanation it may seem so large as to cast discredit on the accuracy of all the data.” The techniques to be used for the next twenty years had been substantially perfected and were described more fully in the paper “The Technique of High Pressure Experimenting” (1914).

Bridgman had the good fortune to begin his experiments at a time when metallurgical advances were providing steels of unprecedentedly high strength; his achievement of still higher pressures in the 1930’s was made possible by the development of the cobaltbonded tungsten carbides. The leakproof packing would have been of little value with Amagat’s steel, but the new alloys permitted spectacular increases of the useful pressure range. Bridgman settled on an electric-furnace chrome-vanadium steel (equivalent to the present AISI 6150) for most of his pressure vessels and connecting tubes. It is not a deep-hardening steel, and in pressure vessels the size of Bridgman’s, four or five inches in diameter, the interior remains relatively soft. This condition is advantageous in pressure vessels because the elastic limit is reached first in the ductile material near the bore, which can be stretched appreciably without rupture; at the same time, the expansion of the inner part transmits the load to the strong outer parts, which are inefficiently stressed so long as the whole cylinder remains in the elastic range. Thus Bridgman found that pressures far in excess of predictions based on simple elastic criteria could be contained.

The maximum fluid pressure for routine measurements of the mechanical, electrical, and thermal properties of matter was gradually raised to 30,000 atmospheres. Still higher pressures, to an estimated 400,000 atmospheres, were finally obtained in quasifluid systems.

At pressures above 3,000 atmospheres, Bridgman was in a realm of physical conditions new to the physicist; the instruments for measuring pressure had to be devised and calibrated, and novel methods developed for making other kinds of physical measurements. Bridgman’s fifty years of concentrated effort, characterized both by the magnitude of the pressures employed and by the range of phenomena investigated, have provided a large part of all the measurements now used in this field and form the basis for most of the recent advances in high-pressure technology. His The Physics of High Pressure (1931) has remained the basic work in this field.

Much of Bridgman’s work was done while the theory of the solid state was still in its infancy, and the interpretation of many of his measurements has become possible only in recent years. The massive treasure of data that he left has proved invaluable for the development of solid state physics. Also among Bridgman’s achievements were an early method of refining by zone melting, the discovery of polymorphism of many materials at high pressures (including ice at high temperature), and a new electrical effect (internal Peltier heat) in metal crystals. His investigations had great geophysical significance, for they proved that drastic alterations in the physical properties and crystal structure of rock material must take place under the high pressures that prevail in the earth’s interior. He lived to see the artificial production of many natural high-pressure mineral forms, such as diamond, coesite, and jadeite, by techniques based on his discoveries.

From the beginning, Bridgman, working alone or with his long-time machinist Charles Chase and research assistant Leonard Abbott, made much of his own apparatus. In almost any weather, he would arrive on his bicycle as soon as the workshop opened at 8:00 a.m. His papers contain many useful bits of shop lore and throw light on the amount of labor and persistence underlying his studies: “It is easy, if all precautions are observed, to drill a hole… seventeen inches long in from seven to eight hours.” They also helped greatly in the adaptation of his techniques all over the world when, after 1945, there was a great rise of interest in experimental high-pressure work.

One may surmise that Bridgman thoroughly enjoyed the complete personal control he exercised over his equipment. The manipulations took him from pumps to measuring apparatus—usually a set of direct current electrical bridges or potentiometers requiring telescopic observation of galvanometer deflections—to the notebook in which notations were made in his private shorthand, and back to the pumps for a new cycle. All of this was performed as fast as the various thermal and pressure lags would permit, sometimes on a fixed schedule that covered several hours. Much philosophical debate has taken place over the meaning of his term operational, but his original meaning must have been closely related to the manifold physical activities of his laboratory, with every adjustment and every measurement dictated by his own mind and controlled by his own muscles.

This desire for full personal involvement in the experiment probably also accounted for Bridgman’s reluctance to do joint research or to take on thesis students. He rarely had more than two at one time; the record shows fourteen doctoral theses on high-pressure topics, in addition to several on other subjects that he supervised. He was usually most pleased when least consulted, but was always willing to listen to interesting findings or to put his mind to real difficulties.

Bridgman’s mechanical genius was reflected in the essential simplicity of his apparatus and his mastery of manipulative techniques. Whether machining a miniature mechanical part, blowing glass, preparing samples of intractable materials, drawing wires, sealing off volatile liquids, purifying chemicals, or growing simple crystals of unprecedented size, he accomplished his purpose with rapidity, a minimum of equipment, and a remarkably low budget.

During World War I, Bridgman moved with his family to New London, Connecticut, where he engaged in the development of sound-detection systems for antisubmarine warfare. He also developed an application of his high-pressure laboratory technique for the prestraining of one-piece gun barrels. Just before World War II, Bridgman’s libertarian outlook and his fear of “the misuse of scientific information” caused him to close his laboratory to “citizens of any totalitarian state.” After the war broke out, he undertook a series of studies for the Watertown Arsenal on the plastic flow of steel under high pressure, a consideration related to the problem of the strength of armor plate. For the Manhattan Project he measured the compressibility of uranium and plutonium.

Bridgman’s lectures were at first baffling to many students. He spoke quickly, and in spurts, with little regard for clear enunciation. Nevertheless, his basic lucidity of thought and his way of coming to grips with the subject forced students to think deeply for themselves. This was reinforced by problem sets of legendary brevity and difficulty.

One of Bridgman’s early teaching assignments—giving two advanced courses in electrodynamics, suddenly thrust upon him in 1914 by the death of B. O. Peirce—turned out to be the genesis of his active interest in philosophy of science. Years later he commented on the obscurity of the underlying conceptual situation that he found in electrodynamics, and the intellectual distress that it caused him. His efforts to meet the logical problems in this area led to a critical examination of the logical structure of physics, until he could say, “I was able to think the situation through to my own satisfaction.”

Bridgman’s first publication in this area (1916) dealt with dimensional analysis, and he returned to this subject in his first book, Dimensional Analysis (1922). It was the first systematic and critical exposition of the principles involved and was characterized by a rigorous analysis of the mental operations involved in dimensional reasoning; the analysis was based on his demand that the equations of physics be given unambiguous meanings by interpreting the letter symbols for the different physical quantities as placeholders for the numbers that form the measure-values of the physical quantities, rather than as place-holders for “physical quantities” formed by multiplying each measure-value by the corresponding physical unit.

Bridgman’s success in thinking his way through the confusions of dimensional analysis encouraged him to turn his attention to the larger task of eliminating similar confusions in the broader field of physics proper. He was deeply impressed by Einstein’s demonstration of the meaninglessness of the conception of absolute simultaneity between events at different places, noting that the proof involved an analysis of the operations of synchronization of clocks at different locations. That the basic concepts of time and space had been seriously misconceived, that sloppy thinking and uncritical usage of language were revealed at the core of physics, seemed to Bridgman to call for a critical reexamination of the conceptual structure of physics as a whole. In order to circumvent the word traps of ordinary speech, he proposed to use physical and mental operations as the measure of meaning.

The resulting “operational” point of view was brilliantly argued, in simple but stark statements, in The Logic of Modern Physics (1927): “In general, we mean by any concept nothing more than a set of operations; the concept is synonymous with the corresponding set of operations…. If a specific question has meaning, it must be possible to find operations by which answers may be given to it. It will be found in many cases that the operations cannot exist, and the question therefore has no meaning.”

This volume was of immense value to a generation of scientists then facing the apparent paradoxes of a new world of atoms and quanta that flatly refused to follow the rules of common sense. In due course The Logic of Modern Physics was followed by other books and papers extending and deepening Bridgman’s critical examination of the concepts and theories of physics. His Princeton University lectures were published as The Nature of Physical Theory (1936). The Nature of Thermodynamics followed (1941), and A Sophisticate’s Primer of Relativity appeared posthumously (1962). In these studies Bridgman drew attention away from the apparent precision of the mathematical equations of physics and the seemingly rigorous logic of axiomatically constructed theories, and turned it to the matrix of crude observations and approximate verbal explanations from which the symbols and equations derive their significance. His relentless probing exposed a surprising penumbra of uncertainty regarding the interpretation of the symbols in thermodynamics (for example, in different physical situations) and the limits of applicability of its concepts. Through these books, and through papers on the application of his ideas to other areas of science and even to the social sciences, Bridgman’s influence spread far beyond the field of physics. His philosophic point of view is usually classified with the positivism of Stallo, Mach, Charles Peirce, William James, and the Vienna Circle; but it derived from his own experience and maintained its individual line.

Bridgman’s influence was strongest among scientists, who found his point of view congenial; and much of what he had to say is commonly accepted among them today. His philosophic writing was, to be sure, always iconoclastic and stimulated a good deal of controversy, especially among some philosophers of science. Operational analysis, he had to explain, was proposed as an aid to clear thinking, and not as a solution for all the problems of philosophy. Again and again he expressed his dislike of the word “operationalism,” with its implication of an associated dogma. Similarly, he gave repeated evidence in operational terms of the seriousness with which he advocated almost ruthless intellectual integrity. One case in point was the publication in 1959 of his own appraisal of The Logic of Modern Physics.

A final affirmation of this ideal is to be found in the circumstances of his death. In the essay “The Struggle for Intellectual Integrity” (1933), he had dealt with the choice of death if one’s probable future is irreversibly and predominantly painful. This choice came to him with suddenness in his eightieth year, and he was not a man to think one thing and do another. After careful diagnosis by undoubted authorities, he found that, in his own words,

…the disease [Paget’s disease] has run its normal course, and has now turned into a well-developed cancer for which apparently nothing can be done…. In the meantime there is considerable pain, and the doctors here do not offer much prospect that it can be made better…. I would like to take advantage of the situation in which I find myself to establish a general principle, namely, that when the ultimate end is as inevitable as it now appears to be, the individual has a right to ask his doctor to end it for him.

Unable to make such arrangements, and finding that his limbs were rapidly losing mobility, Bridgman felt obliged to take action himself. He left behind a two-sentence note:

It isn’t decent for Society to make a man do this thing himself. Probably this is the last day I will be able to do it myself. P.W.B.

A day after his death, Harvard University Press received one of the last things Bridgman must have written, the index for the collection of his complete scientific papers. His ashes were buried in the garden of his beloved summer home.

BIBLIOGRAPHY

I. Original Works. Among Bridgman’s writings are Dimensional Analysis (New Haven, 1922; rev. ed., 1931); A Condensed Collection of Thermodynamic Formulas (Cambridge, Mass., 1925); The Logic of Modern Physics (New York, 1927); The Physics of High Pressure (New York, 1931; new impression with suppl., London, 1949); The Thermodynamics of Electrical Phenomena in Metals (New York, 1934); The Nature of Physical Theory (Princeton, 1936); The Intelligent Individual and Society (New York, 1938); The Nature of Thermodynamics (Cambridge, Mass., 1941); Reflections of a Physicist (New York, 1950, 1955); The Nature of Some of Our Physical Concepts (New York, 1952); Studies in Large Plastic Flow and Fracture, With Special Emphasis on the Effects of Hydrostatic Pressure (New York, 1952); The Way Things Are (Cambridge, Mass., 1959); The Thermodynamics of Electrical Phenomena in Metals and a Condensed Collection of Thermodynamic Formulas, rev. ed. (New York, 1961); A Sophisticate’s Primer of Relativity (Middletown, Conn., 1962); and Collected Experimental Papers of P. W. Bridgman, 7 vols. (Cambridge, Mass., 1964).

Bridgman’s books on philosophy of science and his Collected Experimental Papers contain reprints of almost all his published papers. A fairly complete listing of individual titles is also contained in the obituary note of the Royal Society of London (1962). Further biographical details are in a booklet of essays presented at the memorial meeting at Harvard University, 24 October 1961, and in an obituary volume of the National Academy of Sciences (in press). Much of the material in this article was drawn from these sources and Birch et al. Bridgman’s documentary Nachlass is largely at the Harvard University Archives; some materials (e.g., scientific data and laboratory books) are kept in Lyman Laboratory, Harvard University, and at the Center for History and Philosophy of Science, American Institute of Physics, New York City. Much of his equipment is still in use in research laboratories at Harvard, but some items are at the Smithsonian Institution, Washington, D.C.

II. Secondary Literature. Among the works on Bridgman are Francis Birch, Roger Hickman, Gerald Holton, and Edwin C. Kemble, “Percy Williams Bridgman,” in Faculty of Arts and Sciences, Harvard University Gazette (31 March 1962); and Philipp Frank, The Validation of Scientific Theories (Boston, 1956), ch. 2.

Edwin C. Kemble

Francis Birch

Gerald Holton

Percy Williams Bridgman

The American experimental physicist Percy Williams Bridgman (1882-1961) was a pioneer in investigating the effects of enormous pressures on the behavior of matter—solid, liquid, and gas.

Percy Bridgman was born in Cambridge, Mass., on April 21, 1882, the son of Raymond Landon and Mary Ann Maria Williams Bridgman. At high school in Newton, Mass., he was led into the field of science by the influence of one of his teachers.

Bridgman received his doctorate from Harvard University in 1908 and remained there as a research fellow in physics. He married Olive Ware in 1912, with whom he had a daughter and a son. By 1919 he rose to a full professorship, and 7 years later the university appointed him Hollis professor of mathematics and natural philosophy.

In 1946 Bridgman received the Nobel Prize in physics. He was a fellow of the American Academy of Arts and Sciences and at one time served as president of the American Physical Society. He continued to work at Harvard several years after his official retirement, until he died on Aug. 20, 1961.

Bridgman's major work dealt with the building of apparatus for the investigation of the effects of high pressures, apparatus that would not burst under pressures never reached before. Quite by accident he discovered that a packed plug automatically became tighter as more pressure was applied. This proved a key to his further experimentation. Using the steel alloy Carboloy and new methods of construction and immersing the vessel itself in a fluid maintained at a pressure of approximately 450,000 pounds per square inch (psi), which Bridgman later increased to more than 1,500,000 psi, he reached, inside the vessel, 6,000,000 psi by 1950. To measure such hitherto unattainable pressures, Bridgman invented new measuring methods.

The most striking effect of these enormous pressures was the change in the melting point of many substances. Bridgman also found different crystalline forms of matter which are stable under very high pressure but unstable under low pressure. Ordinary ice, for example, becomes unstable at pressures above about 29,000 psi and is replaced by stable forms. One of these forms is stable under a pressure of 290,000 psi at a temperature as high as 180°F. This "hot ice" is more dense than ordinary ice and sinks completely in water.

In 1955 the General Electric Company announced the production of synthetic diamonds, which their scientists, working on methods and information derived from Bridgman's work, had produced from ordinary carbon subjected to extremely high pressures and temperatures.

Further Reading

Reflections of a Physicist (1950; 2d ed. 1955) is a collection of Bridgman's nontechnical writings on science. A detailed biography of Bridgman is in National Academy of Sciences, Biographical Memoirs, vol. 41 (1970). Niels H. de V. Heathcote, Nobel Prize Winners in Physics: 1901-1950 (1954), contains a chapter on Bridgman. He is included in Royal Society, Biographical Memoirs of Fellows of the Royal Society, vol. 8 (1962), and in National Academy of Sciences, Biographical Memoirs, vol. 12 (1970).

Additional Sources

Walter, Maila L., Science and cultural crisis: an intellectual biography of Percy Williams Bridgman (1882-1961), Stanford, Calif.: Stanford University Press, 1990. □

Percy Williams Bridgman

1882-1961

American physicist who was awarded the 1946 Nobel Prize for Physics for work in high-pressure physics. Bridgman's self-tightening joint allowed him to extend the range of pressures under which substances could be studied from 3,000 to 100,000 atmospheres. His work later served as the basis for General Electric's development of synthetic diamonds. Bridgman is also known for his "operational" philosophy of scientific methodology, according to which science should restrict itself to concepts definable by specific physical operations.