The History of the The History of the Cavendish LaboratoryCavendish Laboratory

These notes provide a brief history of the Cavendish These notes provide a brief history of the Cavendish Laboratory and the achievements of some of its greatest Laboratory and the achievements of some of its greatest physicists. It also provides some of the background to the physicists. It also provides some of the background to the exhibits in the Cavendish Museum. The notes concentrate exhibits in the Cavendish Museum. The notes concentrate on the period from the founding of the Laboratory until on the period from the founding of the Laboratory until 1974, the centenary of its opening in 1874, and the date 1974, the centenary of its opening in 1874, and the date when the Laboratory moved to its present site in West when the Laboratory moved to its present site in West Cambridge.Cambridge.

William CavendishWilliam CavendishThe foundation of the Natural The foundation of the Natural Sciences Tripos in 1851 set the Sciences Tripos in 1851 set the scene for the need to build scene for the need to build dedicated experimental physics dedicated experimental physics laboratories. In 1871, this was laboratories. In 1871, this was achieved through the generosity of achieved through the generosity of the Chancellor of the University, the Chancellor of the University, William Cavendish, Seventh Duke William Cavendish, Seventh Duke of Devonshire, who provided of Devonshire, who provided £6,300 from his own resources to £6,300 from his own resources to meet the costs of building and meet the costs of building and equipping a physics laboratory, on equipping a physics laboratory, on condition that the Colleges provided condition that the Colleges provided the funding for a Professorship of the funding for a Professorship of Experimental Physics. Experimental Physics.

William Cavendish, the William Cavendish, the Seventh Duke of DevonshireSeventh Duke of Devonshire

James Clerk Maxwell was elected the James Clerk Maxwell was elected the first Cavendish Professor in 1871. He first Cavendish Professor in 1871. He was somewhat reluctant to accept the was somewhat reluctant to accept the position since he had resigned from position since he had resigned from his post in King’s College London his post in King’s College London some years earlier to devote his time some years earlier to devote his time to his estate in Scotland and the to his estate in Scotland and the writing of his great Treatise on writing of his great Treatise on Electricity and Magnetism. Maxwell Electricity and Magnetism. Maxwell was responsible for the design of the was responsible for the design of the Laboratory and the equipping of its Laboratory and the equipping of its laboratories. The plans and some of laboratories. The plans and some of his original apparatus are on exhibition his original apparatus are on exhibition in the Cavendish museum.in the Cavendish museum.

James Clerk Maxwell

Original Plan for the LaboratoryOriginal Plan for the Laboratory

A original plan for the A original plan for the Laboratory on the Laboratory on the New Museum site with New Museum site with its grand entrance in its grand entrance in Free School Lane. Free School Lane. The plans show what The plans show what became known as the became known as the Maxwell Lecture Maxwell Lecture Theatre and the Large Theatre and the Large Laboratory. Laboratory.

The Original LaboratoryThe Original LaboratoryThe entrance to the Cavendish The entrance to the Cavendish Laboratory in Free School Lane Laboratory in Free School Lane in the centre of Cambridgein the centre of Cambridge. . In In the Cavendish museum, more the Cavendish museum, more plans of the laboratory are on plans of the laboratory are on display as well as a number of display as well as a number of the pieces of apparatus the pieces of apparatus purchased by Maxwell to equip purchased by Maxwell to equip the laboratories. In addition, the the laboratories. In addition, the museum contains many pieces museum contains many pieces of apparatus and models which of apparatus and models which he built before his return to he built before his return to Cambridge in 1874.Cambridge in 1874.

Maxwell Lecture TheatreMaxwell Lecture Theatre

The Maxwell Lecture Theatre in the Cavendish Laboratory The Maxwell Lecture Theatre in the Cavendish Laboratory in Free School Lane. It is still used by the Physics in Free School Lane. It is still used by the Physics Department for 2nd year lectures. It is designed to give Department for 2nd year lectures. It is designed to give students a good view of experimental demonstrations.students a good view of experimental demonstrations.

Maxwell’s LaboratoryMaxwell’s Laboratory

Maxwell did not live to see his Maxwell did not live to see his theories of electricity, magnetism theories of electricity, magnetism and statistical physics fully and statistical physics fully confirmed by experiment. He confirmed by experiment. He designed apparatus to test his designed apparatus to test his theory of the electromagnetic theory of the electromagnetic field, which were carried out by field, which were carried out by his successor, Lord Rayleigh. his successor, Lord Rayleigh. Maxwell died in 1879 at the early Maxwell died in 1879 at the early age of 48. age of 48. The original Cavendish The original Cavendish

Laboratory in Free School Laboratory in Free School Lane.Lane.

According to Maxwell’s theory of According to Maxwell’s theory of electromagnetism, the speed of electromagnetism, the speed of light only depended upon the ratio light only depended upon the ratio of the strengths of the fundamental of the strengths of the fundamental constants of electrostatics and constants of electrostatics and magnetostatics. The diagram magnetostatics. The diagram shows an example of his proposal shows an example of his proposal to achieve this. This experiment to achieve this. This experiment was carried out by Maxwell’s was carried out by Maxwell’s successor Lord Rayleigh and is on successor Lord Rayleigh and is on display in the museum. display in the museum.

The determination of the relative strengths

of electric and magnetic forces

Letter from Maxwell to Kelvin.

John William StruttJohn William StruttMaxwell was succeeded by John Maxwell was succeeded by John William Strutt, Lord Rayleigh, the William Strutt, Lord Rayleigh, the author of The Theory of Sound. author of The Theory of Sound. He agreed to hold the chair for He agreed to hold the chair for only five years. His name is only five years. His name is associated with many physical associated with many physical phenomena. He discovered the phenomena. He discovered the correct expression for the correct expression for the spectrum of a black-body at low spectrum of a black-body at low frequencies, the Rayleigh-Jeans frequencies, the Rayleigh-Jeans law. Other phenomena include law. Other phenomena include the Rayleigh criterion in optics, the Rayleigh criterion in optics, the Raleigh-Taylor instability in the Raleigh-Taylor instability in fluids, Rayleigh scattering, ..... fluids, Rayleigh scattering, .....

Experimental LaboratoriesExperimental LaboratoriesRayleigh was responsible Rayleigh was responsible for setting up a systematic for setting up a systematic course of instruction in course of instruction in experimental physics, which experimental physics, which has remained at the core of has remained at the core of the Laboratory's teaching the Laboratory's teaching programme. The photo-programme. The photo-graph shows the experi-graph shows the experi-mental laboratories for the mental laboratories for the training of students in 1910. training of students in 1910. In the centre is Searle, of In the centre is Searle, of Searle’s bar fame, who was Searle’s bar fame, who was responsible for the practical responsible for the practical laboratories.laboratories.

SearleSearle

John Joseph (JJ) ThomsonJohn Joseph (JJ) Thomson

In 1884, Rayleigh was succeeded In 1884, Rayleigh was succeeded by the young J.J. Thomson, who by the young J.J. Thomson, who held the Cavendish Chair until held the Cavendish Chair until 1919. His election was a surprise 1919. His election was a surprise since he had little experience of since he had little experience of experiment and had a reputation experiment and had a reputation for being clumsy with his hands. for being clumsy with his hands. He was, however, supported by an He was, however, supported by an outstanding group of Laboratory outstanding group of Laboratory assistants, pride of place going to assistants, pride of place going to the chief assistant Ebenezar the chief assistant Ebenezar Everett, who constructed the Everett, who constructed the experiments.experiments.

StudentsStudentsIn 1895, the University In 1895, the University allowed students from allowed students from other Universities to other Universities to come to Cambridge to come to Cambridge to study for a research study for a research degree. The first two degree. The first two students to take students to take advantage of this were advantage of this were Ernest Rutherford Ernest Rutherford from New Zealand from New Zealand and John Townsend and John Townsend from Dublin. This from Dublin. This photograph was taken photograph was taken in 1897.in 1897.

Prof. J.J. ThomsonProf. J.J. Thomson J.S. TownsendJ.S. Townsend

C.T.R. WilsonC.T.R. Wilson E. RutherfordE. Rutherford

Changes of DirectionChanges of DirectionIn 1895, Rontgen announced the discovery of X-rays and in the In 1895, Rontgen announced the discovery of X-rays and in the following year, 1896, Becquerel discovered natural radioactivity. following year, 1896, Becquerel discovered natural radioactivity. Thomson and Rutherford quickly changed their research directions, Thomson and Rutherford quickly changed their research directions, Thomson to understand the cathode rays which produced the X-rays Thomson to understand the cathode rays which produced the X-rays and Rutherford to radioactivity.and Rutherford to radioactivity.

In 1897, Thomson carried out one of the great experiments of physics In 1897, Thomson carried out one of the great experiments of physics when he measured the charge to mass ratio of cathodes rays. These when he measured the charge to mass ratio of cathodes rays. These had been discovered in experiments with discharge tubes at low had been discovered in experiments with discharge tubes at low pressures. Thomson’s most famous experiment involved passing a pressures. Thomson’s most famous experiment involved passing a beam of cathode rays through crossed electric and magnetic fields. beam of cathode rays through crossed electric and magnetic fields.

The Royal Institution Lecture of

April 1897Thomson used only magnetic Thomson used only magnetic fields. The deflection of the fields. The deflection of the beam of cathode rays in a beam of cathode rays in a magnetic field enables the magnetic field enables the quantity quantity e/mve/mv to be found. to be found. vv was found from the energy was found from the energy and charge deposited at the and charge deposited at the end of the tube. end of the tube.

Charge deposited = Charge deposited = neneEnergy deposited = Energy deposited = 1/2 nmv21/2 nmv2

Hence, he could can eliminate v Hence, he could can eliminate v from the two resultsfrom the two results

e/m » 600 (e/m)e/m » 600 (e/m)HydrogenHydrogen

The Original Thomson Tube

Thomson’s original tube. Replica Thomson’s original tube. Replica on show in the museum.on show in the museum.

In the more famous experiment In the more famous experiment of October 1897, Thomson of October 1897, Thomson found the charge to mass ratio found the charge to mass ratio of the cathode rays by of the cathode rays by balancing the electric and balancing the electric and magnetic forces acting on the magnetic forces acting on the cathode rays. The charge of cathode rays. The charge of mass ratio was much less than mass ratio was much less than that of hydrogenthat of hydrogen

e/m »1000 – 1800 (e/m)e/m »1000 – 1800 (e/m)HydrogenHydrogen

In a beautiful set of experiments, In a beautiful set of experiments, Thomson showed that theThomson showed that the particles were electrons. In particles were electrons. In addition, the particles which are addition, the particles which are ejected in the photoelectric effect, ejected in the photoelectric effect, discovered in the period 1885-7 by discovered in the period 1885-7 by Heinrich Hertz, were identical with Heinrich Hertz, were identical with electrons. electrons.

J.J Thomson and the J.J Thomson and the particles particles

Ultraviolet lightUltraviolet light

Hot cathodeHot cathode

Electrons ejected Electrons ejected from surfacefrom surface

Electrons were clearly a fundamental constituent of Electrons were clearly a fundamental constituent of atoms – the first subatomic particles to be discovered.atoms – the first subatomic particles to be discovered.

C.T.R. WilsonC.T.R. Wilson

C.T.R. Wilson was the inventor C.T.R. Wilson was the inventor of the Wilson Cloud Chamber. of the Wilson Cloud Chamber. His primary interest was in His primary interest was in understanding the process of understanding the process of cloud formation from super-cloud formation from super-saturated water vapour. He was saturated water vapour. He was inspired in these studies by the inspired in these studies by the cloud and atmospheric cloud and atmospheric phenomena he noted as an phenomena he noted as an observer at the meteorological observer at the meteorological observatory at the summit of Ben observatory at the summit of Ben Nevis.Nevis.

The Wilson Cloud ChamberThe Wilson Cloud Chamber

In the course of his experiments, it was realised that the paths In the course of his experiments, it was realised that the paths of charged particles could be identified by the condensation of charged particles could be identified by the condensation tracks they produce in the supersaturated water vapour.tracks they produce in the supersaturated water vapour.

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

In 1899, Thomson used one of Wilson’s early cloud In 1899, Thomson used one of Wilson’s early cloud chambers to measure the charge of the electron. He chambers to measure the charge of the electron. He counted the total number of droplets formed and their counted the total number of droplets formed and their total charge. From these, he estimatedtotal charge. From these, he estimated

e = 2.2 x 10-19 Ce = 2.2 x 10-19 C

This can be compared with the present standard value This can be compared with the present standard value of of

e = 1.602 x 10-19 C e = 1.602 x 10-19 C

The technique was perfected by Millikan in his famous The technique was perfected by Millikan in his famous oil-drop experiment. Water droplets evaporate and so oil-drop experiment. Water droplets evaporate and so he used a heavy oil instead. He measured the charge he used a heavy oil instead. He measured the charge on the electron to about 1% accuracy.on the electron to about 1% accuracy.

Thomson’s Estimate of the Thomson’s Estimate of the Charge of the ElectronCharge of the Electron

Ernest RutherfordErnest Rutherford

In 1919, Thomson was In 1919, Thomson was succeeded by Ernest succeeded by Ernest Rutherford, his former Rutherford, his former student, as Cavendish student, as Cavendish Professor. Much of his Professor. Much of his famous work on famous work on radioactivity and the radioactivity and the nuclear structure of the nuclear structure of the atom was carried out at atom was carried out at McGill and Manchester McGill and Manchester Universities before he Universities before he returned to Cambridge. returned to Cambridge.

Rutherford’s work room in the Cavendish Rutherford’s work room in the Cavendish Laboratory.Laboratory.

If the ‘radium emanation’, now If the ‘radium emanation’, now called radon, is separated out, it called radon, is separated out, it decays with a short lifetime. In the decays with a short lifetime. In the same time, the parent radium same time, the parent radium sample recovers. sample recovers.

Nuclear Nuclear TransmutationsTransmutations

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

In 1908, Rutherford demon-In 1908, Rutherford demon-strated that a-particles are strated that a-particles are actually helium nuclei. The glass actually helium nuclei. The glass needle contains radon gas which needle contains radon gas which emits a-particles which pass emits a-particles which pass through the walls of the tube. through the walls of the tube. Helium was detected spectro-Helium was detected spectro-scopically in the discharge tube scopically in the discharge tube V. This experiment was brought V. This experiment was brought to Cambridge by Rutherford and to Cambridge by Rutherford and is in the museum.is in the museum.

-particles are helium nuclei

Needle Needle containing containing radon gasradon gas

Discharge Discharge tubetube

Ernest RutherfordErnest Rutherford

Rutherford with the apparatus Rutherford with the apparatus with which he demonstrated with which he demonstrated the dis-integration of nuclei the dis-integration of nuclei by incident by incident particles particles inin1919.1919.The original The original apparatus is in the Cavendish apparatus is in the Cavendish Museum. Museum.

In the experiment, In the experiment, --particles were produced particles were produced by the decay of radium by the decay of radium nuclei. These interacted nuclei. These interacted with the nitrogen nuclei with the nitrogen nuclei resulting in the emission resulting in the emission of high energy protons of high energy protons which were detected on which were detected on the luminescent screen. the luminescent screen. The energies of the The energies of the protons were greater than protons were greater than those of the incident those of the incident --particles. These tracks particles. These tracks was first photographed was first photographed using a Wilson Cloud using a Wilson Cloud Chamber by P.M.S. Chamber by P.M.S. Blackett in 1925. Blackett in 1925.

Gas chamberGas chamber

Rod with radium Rod with radium sourcesource

Zinc sulphide Zinc sulphide screenscreen

‘‘There are many decisive experiments There are many decisive experiments in the history of physics which, if they in the history of physics which, if they had not been made when they were had not been made when they were made, would surely have been made made, would surely have been made not much later by someone else. This not much later by someone else. This might not have been true of Wilson’s might not have been true of Wilson’s discovery of the cloud method. In spite discovery of the cloud method. In spite of its essential simplicity, the road to its of its essential simplicity, the road to its final achievement was long and final achievement was long and arduous: without C.T.R. Wilson’s vision arduous: without C.T.R. Wilson’s vision and superb experimental skill, mankind and superb experimental skill, mankind might have had to wait many years might have had to wait many years before someone else found the way.’before someone else found the way.’

Blackett on the Cloud ChamberBlackett on the Cloud Chamber

Patrick BlackettPatrick Blackett

Blackett’s Automatic Cloud Blackett’s Automatic Cloud Chamber of 1928Chamber of 1928

Of 23,000 image of particle Of 23,000 image of particle tracks, 270,000 particle tracks tracks, 270,000 particle tracks

and 8 contained images of and 8 contained images of nuclear interactions.nuclear interactions.

F.W. AstonF.W. AstonF.W.Aston with the mass F.W.Aston with the mass spectograph with which accurate spectograph with which accurate atomic masses were measured and atomic masses were measured and the isotopes of different elements the isotopes of different elements were identified. The particles were were identified. The particles were first accelerated to a known energy first accelerated to a known energy in an electric field and then their in an electric field and then their trajectories bent by application of a trajectories bent by application of a magnetic field. The perfected magnetic field. The perfected instrument is in the museum. instrument is in the museum.

Aston’s photographs of the Aston’s photographs of the parabolic traces of different parabolic traces of different elements, ions and moleculeselements, ions and molecules

James ChadwickJames Chadwick

In 1932, Chadwick discovered the In 1932, Chadwick discovered the neutron. neutron. -particles bombard a -particles bombard a beryllium target, releasing neutrons. beryllium target, releasing neutrons. The neutrons were allowed to collide The neutrons were allowed to collide with a block of paraffin wax. Energetic with a block of paraffin wax. Energetic protons were emitted which were protons were emitted which were detected in an ionisation chamber, detected in an ionisation chamber, enabling the mass of the invisible enabling the mass of the invisible neutron will be found. Rutherford had neutron will be found. Rutherford had suggested the existence of the neutron suggested the existence of the neutron in 1920, but the idea had not attracted in 1920, but the idea had not attracted much attention. much attention.

The First Artificial Nuclear The First Artificial Nuclear DisintegrationDisintegration

In 1932, John Cockroft and In 1932, John Cockroft and E.T.S. Walton accelerated E.T.S. Walton accelerated protons to high energies and protons to high energies and induced the first artificial nuclear induced the first artificial nuclear disintegration by bombarding disintegration by bombarding lithium nuclei. Walton is sitting lithium nuclei. Walton is sitting inside the little tent, observing the inside the little tent, observing the decay products on a lumin-decay products on a lumin-escent screen. Cockcroft is on escent screen. Cockcroft is on the left. The experiment the left. The experiment produced definitive evidence for produced definitive evidence for Einstein’s formula Einstein’s formula E = mcE = mc22..

The Mond LaboratoryThe Mond Laboratory

In the 1930s, the Royal In the 1930s, the Royal Society Mond Laboratory Society Mond Laboratory was built with a particular was built with a particular emphasis upon low emphasis upon low temperature and solid state temperature and solid state physics. The carving of the physics. The carving of the crocodile on the wall of the crocodile on the wall of the building by Eric Gill was building by Eric Gill was organised by Piotr Kapitsa. organised by Piotr Kapitsa. "The Crocodile" was "The Crocodile" was Kapitza's pet name for Kapitza's pet name for Rutherford. Rutherford.

Lawrence BraggLawrence BraggLawrence Bragg was Lawrence Bragg was Cavendish Professor from Cavendish Professor from 1938-1953. He and his 1938-1953. He and his father were awarded the father were awarded the Nobel prize for their Nobel prize for their discovery the law of discovery the law of diffraction of X-rays from diffraction of X-rays from crystals in 1912. They crystals in 1912. They exploited the technique of exploited the technique of X-ray diffraction to study the X-ray diffraction to study the structures of all types of structures of all types of materials and this gave rise materials and this gave rise to the discipline of X-ray to the discipline of X-ray crystallography. crystallography.

Frances Crick and James Frances Crick and James WatsonWatson

In the early 1950s, Francis In the early 1950s, Francis Crick and James Watson Crick and James Watson worked in Bragg’s X-ray worked in Bragg’s X-ray crystallography group and crystallography group and carried out their studies of the carried out their studies of the double helix structure of DNA. double helix structure of DNA. These discoveries led to the These discoveries led to the foundation of the Laboratory for foundation of the Laboratory for Molecular Biology, a separate Molecular Biology, a separate organisation founded by the organisation founded by the Medical research council.Medical research council.

Nevill MottNevill MottBragg was succeeded by Nevill Bragg was succeeded by Nevill Mott as Cavendish Professor in Mott as Cavendish Professor in 1953. He was a specialist in 1953. He was a specialist in solid state physics and won the solid state physics and won the Nobel prize for his studies of Nobel prize for his studies of the electric and magnetic the electric and magnetic properties of non-crystalline properties of non-crystalline materials.materials.

During his tenure, new research During his tenure, new research groups made many notable groups made many notable advances. These included the advances. These included the radio astronomy and physics radio astronomy and physics and chemistry of solids.and chemistry of solids.

The Birth of Radio AstronomyThe Birth of Radio Astronomy

After the War, a After the War, a number of University number of University Groups began to Groups began to investigate the nature investigate the nature of the cosmic radio of the cosmic radio emission. The principal emission. The principal groups involved were at groups involved were at Cambridge, Manchester Cambridge, Manchester and Sydney.and Sydney.

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

Martin Ryle and Aperture SynthesisMartin Ryle and Aperture Synthesis

Martin Ryle’s contribution of genius Martin Ryle’s contribution of genius was the practical implementation of was the practical implementation of Earth-rotation aperture synthesis Earth-rotation aperture synthesis which resulted in high angular which resulted in high angular resolution and high sensitivity resolution and high sensitivity images of the radio sky.images of the radio sky.

The One-mile Telescope The One-mile Telescope at the Lord’s Bridge at the Lord’s Bridge ObservatoryObservatory

Radio Astronomical DiscoveriesRadio Astronomical Discoveries

Radio astronomical observ-Radio astronomical observ-ations led to a revolution in ations led to a revolution in modern astronomy. In 1963, modern astronomy. In 1963, they led to the discovery of they led to the discovery of quasars, the most energetic quasars, the most energetic active galactic nuclei, and in active galactic nuclei, and in 1967 to the discovery of pulsars 1967 to the discovery of pulsars by Antony Hewish and Jocelyn by Antony Hewish and Jocelyn Bell. Radio astronomical Bell. Radio astronomical observations also provide key observations also provide key evidence about the evolutionary evidence about the evolutionary nature of our Universe.nature of our Universe.

Antony Hewish with the low Antony Hewish with the low frequency array which frequency array which discovered the pulsars – these discovered the pulsars – these are identified as magnetised, are identified as magnetised, rotating neutron starsrotating neutron stars

Brian PippardBrian PippardMott was succeeded by Brian Mott was succeeded by Brian Pippard as Cavendish Professor Pippard as Cavendish Professor in 1970. Pippard was a in 1970. Pippard was a specialist in low-temperature specialist in low-temperature physics who made the first physics who made the first experimental determinations of experimental determinations of the Fermi surface of copper.the Fermi surface of copper.

During his tenure as Cavendish During his tenure as Cavendish Professor, he organised the Professor, he organised the move of the Laboratory to West move of the Laboratory to West Cambridge and the construction Cambridge and the construction of the present Laboratory.of the present Laboratory.