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 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

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.

James Clerk Maxwell

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

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.

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.

The determination of the relative strengths of electric and magnetic forces

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 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 Searle’s bar fame, who was responsible for the practical laboratories.

Searle

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.

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.

Prof. J.J. Thomson J.S. Townsend

C.T.R. Wilson E. Rutherford

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 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

The Original Thomson Tube

Thomson’s original tube. Replica on show in the

museum.

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

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

J.J Thomson and the β particles

Ultraviolet light

Hot cathode

Electrons ejected from surface

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 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.

The Wilson Cloud Chamber

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.

Wilson’s perfected cloud chamber is on display in the museum, as well as a earlier version.

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.

Thomson’s Estimate of the Charge of the Electron

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.

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.

Nuclear Transmutations

Rutherford included this important pair of curves in his coat of arms.

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.

α-particles are helium nuclei

Needle containing radon gas

Discharge tube

Ernest Rutherford

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

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

Gas chamber

Rod with radium α source

Zinc sulphide screen

‘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 on the Cloud Chamber

Patrick Blackett

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 lumin-escent 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 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.

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 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

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.