The Nobel Prize in Physics 1946
Percy W. Bridgman
Presentation Speech by Professor A.E. Lindh, member of the Nobel Committee for Physics
Your Majesty, Your Royal Highnesses, Ladies
and Gentlemen.
The earliest known attempts to attain high pressures and to study
various properties of matter under the influence of these
pressures date from the beginning and middle of the 17th century.
The experiments, which were carried out by extremely primitive
methods, aimed in the first place at throwing light on the
compressibility of liquids. These investigations did not become
of a more scientific and systematic nature until the beginning of
the last century, although at first they had the same limited
aims as before. However, they were gradually extended to other
fields. As an example may be mentioned attempts to compress a
number of gases at high pressure, when great divergencies from
Boyle's law revealed themselves, and further, investigations of
the effect of pressure on the refractivity of water, the effect
of pressure on the resistance of an electric conductor. Most
important of all was in 1861 Andrews' discovery of the critical
phenomena in gases.
A period of intensive research began after this last discovery, a
period which lasted until the beginning of the 1890's. During
this period the leading research workers in this field were the
two French physicists, Cailletet and Amagat, of whom the former
was active during an earlier phase of the period mentioned. These
two scientists, especially the latter, made important
contributions towards improving the technique for attaining high
pressures and worked out reliable methods for measuring them.
Amagat developed a special technique for ensuring effective
scalings or packings, a fundamental problem when working on high
pressures. Thanks to this technique, Amagat succeeded in
obtaining constant pressures of 3,000 kg/cm2 and more.
His contributions were of the greatest importance for further
work, and subsequently a large number of research workers in
different countries devoted themselves to the study of high
pressures. Although very comprehensive work in this field was
done, no considerable progress was made in the matter of an
improved technique. The limit remained at 3,000
kg/cm2, and it was not until after 1905 that it began
to make gigantic leaps upwards. The credit for this is due in the
first place to Professor P. W. Bridgman, who is today to receive
the 1946 Nobel Prize for Physics for his invention of an
apparatus for obtaining extremely high pressures, and for the
discoveries he has made with it within that field of
physics.
According to Professor Bridgman himself, it was by a mere chance
that he came to devote his activities to high pressures. At about
the year 1905, he began to study certain optical phenomena under
the influence of pressure. During his experiments the apparatus,
parts of which were made of glass, exploded, and an essential
part of it was destroyed and had to be replaced. In the interval,
Bridgman tried to find another use for the actual pressure
apparatus, and while working out a sealing device for the
pressure chamber, he found that the sealing device he had
constructed functioned far better than he had at first imagined,
for the efficiency of the sealing proved to increase as the
pressure increased, and there was no perceptible leakage. A new
pressure range had presented itself, a range which was not, as in
Amagat's experiments, limited by leakage, but only by the
strength of the material of which the pressure apparatus was
made. After the problem of leakage had been solved, an advance
towards higher pressures came to be essentially a question of
materials.
Even in his earliest experiments Bridgman succeeded in arriving
at pressures of 20,000 kg/cm2. In the earlier investigations,
however, the pressure was kept at a lower value, in order to
avoid deformation of the material then used. Considerable time
was devoted both to investigating the material and to different
methods of making accurate determinations of the pressures. From
his first successful attempts to pass Amagat's pressure limit of
3,000 kg/cm2, Bridgman has step by step, by means of
his brilliant apparatus and skilful use of the resources of
modern technics, extended the pressure range, and has made
pressures up to 100,000 kg/cm2 available for research
work. In certain cases pressures of between 400,000 and 500,000
kg/cm2 have been attained.
The essential features of the Bridgman pressure apparatus are two
containers, communicating with each other by means of strong
connecting channels. The whole system is filled with an
appropriate fluid. In one of the containers (the pressure chamber
itself) the pressure fluid is subjected, by means of a movable
piston, to a great pressure, which is transmitted by the fluid to
the other container, the actual experimental chamber. This
last-mentioned part varies in accordance with the nature of the
projected investigations.
Working on the principle that the resistance of a vessel which is
subjected to internal pressure is increased if at the same time
it is subjected to external pressure, Bridgman used double
high-pressure vessels for ranges from cat 30,000 to 100,000
kg/cm2. The internal pressure vessel with an external
conical surface is fitted into a corresponding bore in a
reinforcement cylinder and equipped with a cylindrical channel,
where the material which is to be subjected to pressure is placed
between two pistons working from opposite ends. As material for
the high-pressure vessel in the range from 50,000 to 100,000
kg/cm2, Bridgman used the extremely hard tungsten
carbide, or, as it is called, carboloy, which is subject to the
least possible deformation.
The investigations with the highest pressures, between 400,000
and 500,000 kg/cm2, have been made with the help of
carboloy pistons, the effective surface being 3 mm in diameter.
This small size is a necessary result of the expansion of the
pressure area under these high pressures, and therefore the
amount of matter which can be compressed is extremely small. With
a pressure of 425,000 kg/cm2 Bridgman obtained
compressed material in the form of small thin flakes. An X-ray
analysis of these showed that, under the influence of the
pressure, the structure had passed from crystalline to
noncrystalline form, i.e. the substance had become
amorphous.
Bridgman's research work has not been directed only towards
attaining record high pressures. The gradual advance towards
higher and ever higher pressures was immediately taken advantage
of for investigations in fields where the impossibility of
attaining higher pressures had previously put a stop to research
work. Bridgman's important discoveries in the course of this work
are so many that here it is only possible to touch upon them
briefly.
The majority of Bridgman's earlier investigations were carried
out within the range up to 12,000 kg/cm2. The first
more comprehensive investigations had to do with the solid and
fluid conditions, and these investigations were subsequently
extended to ranges up to 50,000 kg/cm2, new
modifications of different substances being discovered, inter
alia, of both ordinary and heavy water in solid form, and
altogether seven different mod)fications of ice are known.
Further, two new modifications of phosphorus have been
discovered, one stable form- the so-called black phosphorus - and
one unstable form. By means of investigations of compressibility
at pressures right up to 100,000 kg/cm2, a large
number of polymorphous substances have been discovered. A great
deal of work has been devoted to meticulous investigations of the
pressure effect on electric resistance, and here, inter
alia, the existence of a resistance minimum for certain
metals at very high pressures has been established. Bridgman's
interest has also been directed towards other spheres. Thus,
investigations have been made into the pressure effect on
thermoelectric phenomena, on the conduction of heat in gases, on
the viscosity of fluids, which have led to discoveries,
significant both scientifically and technically. This applies
also to his work on the effect of pressure on the elastic
properties of solid bodies. These contributions were all made in
fields which had previously not attracted much interest.
Attention should also be called to the extensive and difficult
investigations of materials, which were a necessary precondition
for the successful advances towards higher pressures,
investigations which are of the greatest importance for further
work in the field of high pressures.
Finally, attention should be called to the immense scientific
value of the impressive collection of data regarding the
properties of matter at high pressures which Bridgman has
assembled during his long and important research activities in
the field of high pressure physics.
Professor Bridgman. In awarding you this
year's Nobel Prize for Physics, The Royal Swedish Academy of
Sciences desires to express its unreserved acknowledgement of
your outstanding pioneer work in the field of high-pressure
physics. By means of your ingenious apparatus, combined with a
brilliant experimental technique, you have, by your intense
research work and the resulting manifold and remarkable
discoveries, very greatly enriched our knowledge of the
properties of matter at high pressures.
On behalf of The Royal Swedish Academy of Sciences, I
congratulate you on your important and successful work in the
service of science, and I now ask you to receive your Nobel Prize
from the hands of His Majesty the King.
From Nobel Lectures, Physics 1942-1962, Elsevier Publishing Company, Amsterdam, 1964
Copyright © The Nobel Foundation 1946
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