The History of the Cavendish Laboratory

1
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.

2
William Cavendish

• 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.

William Cavendish, the Seventh Duke of Devonshire
3
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 Kings 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.

4
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.

5
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.

6
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.

7
Maxwells Laboratory

• 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 original Cavendish Laboratory in Free School
Lane.
8
The determination of the relative strengths of
electric and magnetic forces

• According to Maxwells 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 Maxwells successor Lord
  Rayleigh and is on display in the museum.

9
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, .....

10
Experimental Laboratories

• 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 photo-graph
  shows the experi-mental laboratories for the
  training of students in 1910. In the centre is
  Searle, of Searles bar fame, who was responsible
  for the practical laboratories.

Searle
11
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.

12
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 photograph was taken in 1897.

13
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. Thomsons most famous
  experiment involved passing a beam of cathode
  rays through crossed electric and magnetic
  fields.

14
The Royal Institution Lecture of April 1897

• 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.

Charge deposited ne Energy deposited 1/2
nmv2 Hence, he could can eliminate v from the
two results e/m 600 (e/m)Hydrogen
15
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 e/m 1000 1800 (e/m)Hydrogen

• Thomsons original tube. Replica on show in the
  museum.

16
J.J Thomson and the b particles

• In a beautiful set of experiments, Thomson showed
  that the b 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.

Electrons were clearly a fundamental constituent
of atoms the first subatomic particles to be
discovered.
17
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
  super-saturated 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.

18
The Wilson Cloud Chamber
Wilsons 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.

19
Thomsons Estimate of the Charge of the Electron

• In 1899, Thomson used one of Wilsons 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.

20
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.

Rutherfords work room in the Cavendish
Laboratory.
21
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.

22
a-particles are helium nuclei
Discharge tube

• In 1908, Rutherford demon-strated 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 spectro-scopically in the discharge
  tube V. This experiment was brought to Cambridge
  by Rutherford and is in the museum.

Needle containing radon gas
23
Ernest Rutherford

• Rutherford with the apparatus with which he
  demonstrated the dis-integration of nuclei by
  incident a-particles in 1919. The original
  apparatus is in the Cavendish Museum.

24

• In the experiment, a-particles 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 a-particles. These tracks was first
  photographed using a Wilson Cloud Chamber by
  P.M.S. Blackett in 1925.

25
Blackett on the Cloud Chamber
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 Wilsons 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. Wilsons vision
and superb experimental skill, mankind might have
had to wait many years before someone else found
the way.
Patrick Blackett
26
Of 23,000 image of particle tracks, 270,000
particle tracks and 8 contained images of
nuclear interactions.
Blacketts Automatic Cloud Chamber of 1928
27
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.
Astons photographs of the parabolic traces of
different elements, ions and molecules
28
James Chadwick

• In 1932, Chadwick discovered the neutron.
  a-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.

29
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 lumin-escent screen. Cockcroft is
  on the left. The experiment produced definitive
  evidence for Einsteins formula E mc2.

30
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.

31
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.

32
Frances Crick and James Watson

• In the early 1950s, Francis Crick and James
  Watson worked in Braggs 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.

33
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.

34
The Birth of Radio Astronomy

• 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.

The Cambridge efforts were led by Martin Ryle who
assembled a brilliant team of young physicists to
attack these problems.
35
Martin Ryle and Aperture Synthesis

• Martin Ryles 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 Lords Bridge
Observatory
36
Radio Astronomical Discoveries

• Radio astronomical observ-ations 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
37
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.