Download The History of the Cavendish Laboratory

The History of the
Cavendish Laboratory
These notes provide a brief history of the Cavendish
Laboratory and the achievements of some of its greatest
physicists. It also provides some of the background to the
exhibits in the Cavendish Museum. The notes concentrate
on the period from the founding of the Laboratory until
1974, the centenary of its opening in 1874, and the date
when the Laboratory moved to its present site in West
Cambridge.
William Cavendish
William Cavendish, the
Seventh Duke of Devonshire
The foundation of the Natural
Sciences Tripos in 1851 set the
scene for the need to build
dedicated experimental physics
laboratories. In 1871, this was
achieved through the generosity of
the Chancellor of the University,
William Cavendish, Seventh Duke
of Devonshire, who provided
£6,300 from his own resources to
meet the costs of building and
equipping a physics laboratory, on
condition that the Colleges provided
the funding for a Professorship of
Experimental Physics.
James Clerk Maxwell
James Clerk Maxwell was elected the
first Cavendish Professor in 1871. He
was somewhat reluctant to accept the
position since he had resigned from
his post in King’s College London
some years earlier to devote his time
to his estate in Scotland and the
writing of his great Treatise on
Electricity and Magnetism. Maxwell
was responsible for the design of the
Laboratory and the equipping of its
laboratories. The plans and some of
his original apparatus are on exhibition
in the Cavendish museum.
Original Plan for the Laboratory
A original plan for the
Laboratory on the
New Museum site with
its grand entrance in
Free School Lane.
The plans show what
became known as the
Maxwell Lecture
Theatre and the Large
Laboratory.
The Original Laboratory
The entrance to the Cavendish
Laboratory in Free School Lane
in the centre of Cambridge. In
the Cavendish museum, more
plans of the laboratory are on
display as well as a number of
the pieces of apparatus
purchased by Maxwell to equip
the laboratories. In addition, the
museum contains many pieces
of apparatus and models which
he built before his return to
Cambridge in 1874.
Maxwell Lecture Theatre
The Maxwell Lecture Theatre in the Cavendish Laboratory
in Free School Lane. It is still used by the Physics
Department for 2nd year lectures. It is designed to give
students a good view of experimental demonstrations.
Maxwell’s Laboratory
The original Cavendish
Laboratory in Free School
Lane.
Maxwell did not live to see his
theories of electricity, magnetism
and statistical physics fully
confirmed by experiment. He
designed apparatus to test his
theory of the electromagnetic
field, which were carried out by
his successor, Lord Rayleigh.
Maxwell died in 1879 at the early
age of 48.
The determination of the relative strengths
of electric and magnetic forces
According to Maxwell’s theory of
electromagnetism, the speed of
light only depended upon the ratio
of the strengths of the fundamental
constants of electrostatics and
magnetostatics. The diagram
shows an example of his proposal
to achieve this. This experiment
was carried out by Maxwell’s
successor Lord Rayleigh and is on
display in the museum.
Letter from Maxwell to Kelvin.
John William Strutt
Maxwell was succeeded by John
William Strutt, Lord Rayleigh, the
author of The Theory of Sound.
He agreed to hold the chair for
only five years. His name is
associated with many physical
phenomena. He discovered the
correct expression for the
spectrum of a black-body at low
frequencies, the Rayleigh-Jeans
law. Other phenomena include
the Rayleigh criterion in optics,
the Raleigh-Taylor instability in
fluids, Rayleigh scattering, .....
Experimental Laboratories
Searle
Rayleigh was responsible
for setting up a systematic
course of instruction in
experimental physics, which
has remained at the core of
the Laboratory's teaching
programme. The photograph shows the experimental laboratories for the
training of students in 1910.
In the centre is Searle, of
Searle’s bar fame, who was
responsible for the practical
laboratories.
John Joseph (JJ) Thomson
In 1884, Rayleigh was succeeded
by the young J.J. Thomson, who
held the Cavendish Chair until
1919. His election was a surprise
since he had little experience of
experiment and had a reputation
for being clumsy with his hands.
He was, however, supported by an
outstanding group of Laboratory
assistants, pride of place going to
the chief assistant Ebenezar
Everett, who constructed the
experiments.
C.T.R. Wilson
Prof. J.J. Thomson
E. Rutherford
Students
In 1895, the University
allowed students from
other Universities to
come to Cambridge to
study for a research
degree. The first two
students to take
advantage of this were
Ernest Rutherford
from New Zealand
and John Townsend
from Dublin. This
J.S. Townsend photograph was taken
in 1897.
Changes of Direction
In 1895, Rontgen announced the discovery of X-rays and in
the following year, 1896, Becquerel discovered natural
radioactivity. Thomson and Rutherford quickly changed their
research directions, Thomson to understand the cathode rays
which produced the X-rays and Rutherford to radioactivity.
In 1897, Thomson carried out one of the great experiments of
physics when he measured the charge to mass ratio of
cathodes rays. These had been discovered in experiments
with discharge tubes at low pressures. Thomson’s most
famous experiment involved passing a beam of cathode rays
through crossed electric and magnetic fields.
The Royal
Institution Lecture of
April 1897
Charge deposited = ne
Energy deposited = 1/2 nmv2
Hence, he could can eliminate v
from the two results
e/m » 600 (e/m)Hydrogen
Thomson used only magnetic
fields. The deflection of the
beam of cathode rays in a
magnetic field enables the
quantity e/mv to be found. v
was found from the energy
and charge deposited at the
end of the tube.
The Original Thomson Tube
In the more famous experiment
of October 1897, Thomson
found the charge to mass ratio
of the cathode rays by
balancing the electric and
magnetic forces acting on the
cathode rays. The charge of
mass ratio was much less than
that of hydrogen
Thomson’s original tube.
Replica on show in the
museum.
e/m »1000 – 1800 (e/m)Hydrogen
J.J Thomson and the β particles
Ultraviolet light
In a beautiful set of experiments,
Electrons ejected Thomson showed that the β
from surface
particles were electrons. In
addition, the particles which are
ejected in the photoelectric effect,
discovered in the period 1885-7
by Heinrich Hertz, were identical
with electrons.
Hot cathode
Electrons were clearly a fundamental constituent of
atoms – the first subatomic particles to be discovered.
C.T.R. Wilson
C.T.R. Wilson was the inventor
of the Wilson Cloud Chamber.
His primary interest was in
understanding the process of
cloud formation from supersaturated water vapour. He was
inspired in these studies by the
cloud and atmospheric
phenomena he noted as an
observer at the meteorological
observatory at the summit of Ben
Nevis.
The Wilson Cloud Chamber
Wilson’s perfected
cloud chamber is on
display in the
museum, as well as
a earlier version.
In the course of his experiments, it was realised that the paths
of charged particles could be identified by the condensation
tracks they produce in the supersaturated water vapour.
Thomson’s Estimate of the
Charge of the Electron
In 1899, Thomson used one of Wilson’s early cloud
chambers to measure the charge of the electron. He
counted the total number of droplets formed and their
total charge. From these, he estimated
e = 2.2 x 10-19 C
This can be compared with the present standard value
of
e = 1.602 x 10-19 C
The technique was perfected by Millikan in his famous
oil-drop experiment. Water droplets evaporate and so
he used a heavy oil instead. He measured the charge
on the electron to about 1% accuracy.
Ernest Rutherford
In 1919, Thomson was
succeeded by Ernest
Rutherford, his former
student, as Cavendish
Professor. Much of his
famous work on
radioactivity and the
nuclear structure of the
atom was carried out at
McGill and Manchester
Universities before he
returned to Cambridge.
Rutherford’s work room in the Cavendish
Laboratory.
Nuclear
Transmutations
Rutherford included this
important pair of curves
in his coat of arms.
If the ‘radium emanation’, now
called radon, is separated out, it
decays with a short lifetime. In the
same time, the parent radium
sample recovers.
Discharge
tube
Needle
containing
radon gas
α-particles are
helium nuclei
In 1908, Rutherford demonstrated that a-particles are
actually helium nuclei. The glass
needle contains radon gas which
emits a-particles which pass
through the walls of the tube.
Helium was detected spectroscopically in the discharge tube
V. This experiment was brought
to Cambridge by Rutherford and
is in the museum.
Ernest Rutherford
Rutherford with the
apparatus with which he
demonstrated the disintegration of nuclei by
incident α−particles in 1919.
The original apparatus is in
the Cavendish Museum.
Rod with radium α
source
Zinc sulphide
screen
Gas chamber
In the experiment, αparticles were produced
by the decay of radium
nuclei. These interacted
with the nitrogen nuclei
resulting in the emission
of high energy protons
which were detected on
the luminescent screen.
The energies of the
protons were greater than
those of the incident αparticles. These tracks
was first photographed
using a Wilson Cloud
Chamber by P.M.S.
Blackett in 1925.
Blackett on the Cloud Chamber
Patrick Blackett
‘There are many decisive experiments
in the history of physics which, if they
had not been made when they were
made, would surely have been made
not much later by someone else. This
might not have been true of Wilson’s
discovery of the cloud method. In spite
of its essential simplicity, the road to its
final achievement was long and
arduous: without C.T.R. Wilson’s vision
and superb experimental skill, mankind
might have had to wait many years
before someone else found the way.’
Blackett’s Automatic Cloud
Chamber of 1928
Of 23,000 image of particle
tracks, 270,000 particle tracks
and 8 contained images of
nuclear interactions.
F.W. Aston
F.W.Aston with the mass
spectograph with which accurate
atomic masses were measured and
the isotopes of different elements
were identified. The particles were
first accelerated to a known energy
in an electric field and then their
trajectories bent by application of a
magnetic field. The perfected
instrument is in the museum.
Aston’s photographs of the
parabolic traces of different
elements, ions and molecules
James Chadwick
In 1932, Chadwick discovered the
neutron. α-particles bombard a
beryllium target, releasing neutrons.
The neutrons were allowed to collide
with a block of paraffin wax. Energetic
protons were emitted which were
detected in an ionisation chamber,
enabling the mass of the invisible
neutron will be found. Rutherford had
suggested the existence of the neutron
in 1920, but the idea had not attracted
much attention.
The First Artificial Nuclear
Disintegration
In 1932, John Cockroft and
E.T.S. Walton accelerated
protons to high energies and
induced the first artificial nuclear
disintegration by bombarding
lithium nuclei. Walton is sitting
inside the little tent, observing
the decay products on a luminescent screen. Cockcroft is on
the left. The experiment
produced definitive evidence for
Einstein’s formula E = mc2.
The Mond Laboratory
In the 1930s, the Royal
Society Mond Laboratory
was built with a particular
emphasis upon low
temperature and solid state
physics. The carving of the
crocodile on the wall of the
building by Eric Gill was
organised by Piotr Kapitsa.
"The Crocodile" was
Kapitza's pet name for
Rutherford.
Lawrence Bragg
Lawrence Bragg was
Cavendish Professor from
1938-1953. He and his
father were awarded the
Nobel prize for their
discovery the law of
diffraction of X-rays from
crystals in 1912. They
exploited the technique of
X-ray diffraction to study the
structures of all types of
materials and this gave rise
to the discipline of X-ray
crystallography.
Frances Crick and James
Watson
In the early 1950s, Francis
Crick and James Watson
worked in Bragg’s X-ray
crystallography group and
carried out their studies of the
double helix structure of DNA.
These discoveries led to the
foundation of the Laboratory for
Molecular Biology, a separate
organisation founded by the
Medical research council.
Nevill Mott
Bragg was succeeded by Nevill
Mott as Cavendish Professor in
1953. He was a specialist in
solid state physics and won the
Nobel prize for his studies of
the electric and magnetic
properties of non-crystalline
materials.
During his tenure, new research
groups made many notable
advances. These included the
radio astronomy and physics
and chemistry of solids.
The Birth of Radio Astronomy
The Cambridge efforts were led
by Martin Ryle who assembled
a brilliant team of young
physicists to attack these
problems.
After the War, a number
of University Groups
began to investigate the
nature of the cosmic
radio emission. The
principal groups
involved were at
Cambridge,
Manchester and
Sydney.
Martin Ryle and Aperture Synthesis
Martin Ryle’s contribution of genius
was the practical implementation of
Earth-rotation aperture synthesis
which resulted in high angular
resolution and high sensitivity
images of the radio sky.
The One-mile Telescope
at the Lord’s Bridge
Observatory
Radio Astronomical Discoveries
Radio astronomical observations led to a revolution in
modern astronomy. In 1963,
they led to the discovery of
quasars, the most energetic
active galactic nuclei, and in
1967 to the discovery of pulsars
by Antony Hewish and Jocelyn
Bell. Radio astronomical
observations also provide key
evidence about the evolutionary
nature of our Universe.
Antony Hewish with the low
frequency array which
discovered the pulsars – these
are identified as magnetised,
rotating neutron stars
Brian Pippard
Mott was succeeded by Brian
Pippard as Cavendish Professor
in 1970. Pippard was a
specialist in low-temperature
physics who made the first
experimental determinations of
the Fermi surface of copper.
During his tenure as Cavendish
Professor, he organised the
move of the Laboratory to West
Cambridge and the construction
of the present Laboratory.