Pressure is the amount of force acting on each unit area; the air around us exerts about 10^5 pascals, or 1 atmosphere. Bridgman’s piston-cylinder high-pressure apparatus could reach up to 400,000 atmospheres (~40 gigapascals), an unprecedented capability at the time. No other device in the world could keep such huge pressures steady for long periods. He invented the “Bridgman seal”, a clever combination of metals and softer materials that stopped samples from leaking and allowed simultaneous high temperatures. With this tool he systematically measured the pressures and temperatures at which ice transforms into Ice VI, Ice VII and other phases, and discovered that lead and bismuth turn from semiconductors into metals at room temperature. These data helped scientists infer what materials make up Earth’s mantle and core and guided the search for super-hard substances and superconductors. His experimental concepts later evolved into diamond-anvil cells and laser shock compression, forming the backbone of modern high-pressure science. By establishing pressure as a new experimental variable, Bridgman greatly expanded the horizons of condensed-matter physics.

Starting in the late 1910s, Bridgman refined a self-sealing piston-cylinder apparatus that exploited plastic flow in ductile metals to reinforce the gasket and maintain hydrostatic pressures exceeding 100 kbar. With this device he measured an enormous range of physical properties—electrical resistivity, thermal conductivity, compressibility, ultrasonic velocity—demonstrating quantitatively how pressure closes electronic band gaps and rearranges crystal structures. His most famous achievement was the discovery of many solid phases of water; more than half of the Ice phases labeled I–VII on today’s phase diagram were first identified by him. Modern tools such as inelastic scattering and X-ray diffraction were not yet available, so he located transition points by tracking abrupt changes in volume and electrical resistance. Bridgman cleverly used the liquid-solid transition of the pressure medium itself, employing NaCl, Na3PO4 and other salts to approximate quasi-hydrostatic conditions. His datasets were compared with theoretical compressibility models by Birch, Murnaghan and others, extending equations of state (EoS) to high pressures. Industrially, the work paved the way for cemented carbides, synthetic rubies and high-pressure growth of semiconductor crystals, bridging basic and applied research. By defining high-pressure science as an interdisciplinary field, he motivated earth scientists, planetary physicists and materials researchers to adopt pressure as an experimental variable. His monograph “The Physics of High Pressure” (1931) remains a classic reference, serving as a methodological and data compendium. These multifaceted contributions formed the basis for awarding him the 1946 Nobel Prize in Physics.

Bridgman carefully analyzed radial stress gradients in piston-cylinder systems and showed that plastic flow of gasket materials beyond the yield stress actually enhanced sealing performance. Using high-precision manganin gauges, he interpolated pressure calibration curves via the resistivity change of mercury, achieving a relative uncertainty of 10⁻³. His compressibility measurements that accounted for elastic anisotropy verified the applicability of the first-order Murnaghan equation of state up to 40 GPa. Among the earliest direct observations of electronic overlap transitions, he documented the semimetal-to-metal transformations of Bi, Sb and Pb, results that later underpinned the McWhan–Liu systematic studies. Determining the ∼2 GPa formation pressure of Ice VII linked directly to estimates of water content in Earth’s lower mantle and even to the eventual naming of the mineral “bridgmanite.” He employed induction heating for thermal control, correcting for pressure-medium thermal expansion by differential methods to calculate enthalpy changes. His techniques evolved into toroidal anvil presses and multi-anvil systems, improving hydrostaticity and pushing maximum static pressures higher. By providing the experimental infrastructure, he enabled the discovery of new phases, mapping of superconducting critical pressures, and laboratory simulation of planetary interiors.