Physicist Obtains Immense Pressures in Minute Press

A few hundred yards down the street from the cyclotron another Physics project is underway. Percy W. Bridgman '04, Higgens University Professor has succeeded in creating pressures of 1,470,000 pounds or 100,000 atmospheres, for which he was awarded the Nobel Prize in 1946.

Bridgman is currently studying the effects of this pressure on electrical resistences. Such high pressures are large enough to effect the properties of materials considerably. Except for a fraction of a percent all the matter in the universe exists under pressures of below 100,000 atmospheres. The pressure at the deepest part of the ocean is only 1,000 atmospheres and only double this in the explosion chamber of a gun.

"I got a bright idea how to get higher pressures than anyone ever heard of before," Bridgman said yesterday when recalling how he started his experiments in 1904 as graduate work for his PhD. "Before men had reached comparatively small pressures...I started with 3,000 atmospheres, then 12,000, then 30,000 and on up to 100,000 atmospheres," he added.

The high pressures are obtained in a tiny 'pressurized laboratory' which is a miniature hydraulic press. Naturally the size of the lab is restricted, because the wall must be heavy enough to withstand the pressures. Bridgman normally used a 'laboratory' that occupies a cylindrical space only half an inch in diameter and three inches long.

"For every successive increase in the pressure range, a price has to be paid in a diminution of the size of the apparatus...," he wrote describing the vessel in which he generated the pressure of 100,000 atmospheres. This chamber is only one-sixteenth of an inch in diameter and three-sixteenths of an inch long. Nevertheless volume changes "can be measured with fair precision" in this apparatus.

Even in such a small space it is possible to set up several different testing arrangements. Bridgman has conducted tests on tension, compression, bending, punching and hardness. The interior of the laboratory is connected to the outside world by seven electrically insulated leads. The leads are connected to instruments inside the laboratory for measuring the hydrostatic pressure, the force applied to the specimen, and the deformation of the specimen.

Theoretically a piston worked by a hydraulic press could exert "any desired pressure." However 20,000 atmospheres was formerly the limit, even when using heavy cylinders of heat-treated steel. Strengthening methods can raise this to 30,000 atmospheres, but the piston also reaches its limit at this pressure.

To Reach Higher Pressures

To reach higher pressures, Bridgman made "a radical change in design." To increase the strength of the vessel he made it conical on the outside and pushed it into a conical supporting sleeve; to increase the strength of the piston he made it of Carboloy, which has "a compressive strength of more than twice that of steel." This extended the pressure range to 50,000 atmospheres.

A knowledge of the properties of steel under high pressure is useful in determining the process of a projectile penetrating armour plate. The wire industry would also profit if the wire could be pulled in high pressures. Bridgman says that by stretching a wire "under pressure and releasing it at atmospheric pressure, material with strength greatly in excess of that normally possible at atmospheric pressure could be produced."

The behavior of matter under stress may help the scientist in understanding the nature of the earth's crust.