This year [1992] sees the 150th anniversary of the birth of one of the greatest figures of nineteenth century mathematical physics, John William Strutt, Third Baron Rayleigh (fig. 1). Referred to by contemporary commentators as the last of the great Victorian polymaths, his contribution to science is exemplified by the frequency with which his name appears in the technical papers of today. Many notable scientific figures were elevated to the peerage because of their contribution to physics. There are, however, few examples of hereditary peers devoting their lives to physics with such outstanding success.

An examination of journals of applied science today shows that many of the authors refer to the original papers that Rayleigh wrote or, at least, to the terms that attribute a phenomenon to his name. This occurs less often for his contemporaries, in spite of the fact that many were influential in extending the frontiers of knowledge by their original contribution to the physical sciences. Rayleigh's work appears to have a timeless quality that makes it relevent to any age.

A recent search through the INSPEC database, using the single keyword `Rayleigh', produced the amazing result that some 15 000 papers, produced over the last 20 years, had referred to some aspect of the 3rd or, latterly, the 4th Baron Rayleigh's research

This paper aims to set out Rayleigh's `life and times', and to present some examples of the way in which GEC is currently applying the results of some of his researches.

The first direct ancestor of the Strutt family who can be reliably traced is a John Strutt I, a corn miller, who settled in the Springfield area of Chelmsford, Essex, in around 1667, working the Springfield and Moulsham mills with his son John II. This branch of the Strutts continued as millers until around 1727, when John retired to the small Essex village of Terling. John II had, earlier in 1720, purchased the farm known as Simon Collins in Terling. Thus began the Strutt's connection with that village and the farming interests which continue to this day with the Rayleigh Estates.

Upon John II's death the property passed eventually to his grandson John Strutt MP. After his marriage to Anne Goodday, the couple lived at Springfield Place in Chelmsford, which he purchased in around 1758. This was latterly owned by the Marconi Company and used as a training centre. Opportunely, Sir Matthew Featherstonhaugh, Bart, chose to sell the manor of Terling Place and the neighbouring estate of about 850 acres which John Strutt MP purchased in 1761 for the sum of £18 540 12s 6d.

John Strutt MP built Terling Place, the family seat, in around 1772, on land adjacent to the site of a former Palace of the Bishops of Norwich (fig. 2). The house is built in white brick with a central block and two wings which were added in 1820 by Joseph Holden Strutt. The west wing was to house the laboratory of John William. Col. Joseph Holden Strutt, inherited the estates on the death of John Strutt MP. In 1821 he was awarded a peerage at the Coronation of George IVth for political services and, unusually, asked that the honour be conveyed to his wife, the former Lady Charlotte Mary Gertrude Fitzgerald, a daughter of James, 1st Duke of Leinster. Thus, the Rayleigh title began with a Baroness.

2   Terling Place

John James Strutt succeeded to the Barony at the death of his mother in 1836. John James did not marry until his mid 40s and then, surprisingly, to a young woman of barely 17 years of age, Clara Latouche Vicars. It was to this couple that John William was born on the 12th of November in 1842, the first of a family of seven children.

Although John William was to hold the highest honours the world of science could bestow, early evidence of his genius was slow to appear as he was almost three years old before he could talk! He would indicate by various sounds and gestures to his mother that he wished an object of his attention to be identified, bringing forth the prophetic remark from his grandfather Col. Joseph Holden Strutt, `the child will either be very clever, or an idiot'. However by his fourth year, his Aunt Emily was complaining that he constantly pestered her with questions such as: `what becomes of the water spilt on the tablecloth after it has dried up?' - evidence of an inquisitiveness that was at the root of his genius.

He was sent to Eton when he was about 10 years of age. Although he was enthusiastic to attend school his parents showed some trepidation in sending him away so young. He was at Eton only a short while before contracting smallpox. On his recovery he returned home, shortly to contract whooping cough. His parents decided that so delicate a child should be educated at home during an extended convalescence. The services of a tutor were therefore employed to attend his early educational requirements at Terling Place. On his recovery he was sent to a Mr George Murray at Wimbledon Common. Here he obtained his groundwork in mathematics, trigonometry and statistics.

He was obviously a pleasing pupil from remarks passed by Mr Murray when he said, `it is not often a tutor has so easy and pleasant a duty to discharge as to instruct such a pupil'. A short stay at Harrow followed when he was 14. He contracted a chest infection there that left him critically ill and he never returned. Bouts of ill health were to dog John William throughout his life. On his recovery he was sent to the Rev G. T. Warner at Torquay where he remained for four years. It was Rev Warner who endeavoured to persuade John William to abandon his interests in mathematics to study classics, but fortunately for science, without success.

At 14 his scientific gifts were developing. An early interest was in electricity as evidenced by the `shocking coil' he used to good effect on his sister Clara. He was also carrying out experiments with magnets and experimenting with combinations of chemicals. His interest in photography, then in its infancy, was encouraged by his mother who assisted him in the photographic and processing operations. This laid the groundwork for his classic work on the production of diffraction gratings and his definitive paper on the pinhole camera. He was, in 1887, to describe a technique for colour photography. The technique was subsequently refined by Lippmann who was to gain the Nobel prize for his work in defining the process. In 1861, James Clerk Maxwell had devised a process for colour photography, but not with total success.

John William entered Trinity College Cambridge in October 1861 as a fellow commoner. This was customary for so-called `young men of fortune' (fig. 3). The additional fees paid for a distinctive academic gown, and as he put it, `the doubtful privilege of dining at high table with the Dons'. He had passed the entrance examinations with a considerable degree of success. The tutors had been at some pains, however, to select questions in the classics that would not tax his less-than-complete knowledge in this area of study.

3   John William Strutt as a young man (self portrait)

His mathematics tutor was the excellent Dr E. J. Routh, and it was he who was instrumental in developing John William's immense mathematical gifts that underpinned his understanding of the physical sciences. During his period at Trinity, John William attended the lectures of Sir George Stokes, himself a senior wrangler and First Smith's Prizeman, honours John William also attained. Stokes was Lucasian Professor of Mathematics, a post held some 200 years earlier by Sir Isaac Newton, and today held by Professor Stephen Hawking.

Whilst at Trinity, John William had met Arthur James Balfour, politician and philosopher. Balfour was elected to Parliament in 1874 as member for Hertford and later became Prime Minister from 1902-1905. He wrote a number of books, perhaps the most well known are, `A Defence of Philosophical Doubt' in 1879, and `Foundations of Belief', in 1894, a year incidentally in which he was President of the Society for Psychical Research. The friendship had brought John William into contact with Balfour's two elder sisters, Evelyn and Eleanor. Eleanor was subsequently to assist John William in his investigations on the development of electrical standards for the ohm and ampere during his tenure as Professor of Physics at the Cavendish Laboratory. He was to marry Evelyn in 1871.

John William graduated in 1865 gaining the distinction of Senior Wrangler, and the award of First Smith's Prizeman for his submissions the following year. Thus, at the age of 22, John William was regarded as a mathematical physicist of great promise. The scientific legacy he was to leave justified that promise in full measure

John William Strutt, 3rd Baron Rayleigh left a scientific legacy of phenomenal value.There were few areas of physics that he did not investigate.

Rayleigh's output was enormous, producing 446 papers spanning the years 1869 to 1920. The last three papers appeared after his death in 1919. The papers were published in a variety of scientific journals including the Philosophical Magazine, Nature, Proceedings of the Royal Society, and the London Mathematical Society. They were bound into six volumes latterly produced as three by Dover Publications in 1964. Rayleigh's classic textbook, `The Theory of Sound', was produced in two volumes in 1877 and 1878. The volumes were printed by Dover Publications in 1945

Rayleigh has justifiably been called the last of the great Victorian polymaths. He was to assist in the development of the great edifice of nineteenth century physics, his contribution ranking with Stokes, Kelvin and Clerk Maxwell.

Following his return to Trinity to take up his Fellowship, he began to study the scientific literature of the time. One of his professors suggested he should learn German and he used Helmholtz's `Die Lehre von den Tonempfindungen', published in 1862, as a means to study both the language and the work of the great German physicist. This stimulated his interest in acoustics and the physiology of hearing. Study of Helmholtz's papers on colour vision and the work of Young and Maxwell were also influential in leading to Rayleigh's own early experimental investigations in this area of physics. He recognized the power and applicability of the approximate methods in the development of mathematical models to study physical phenomena. Mathematical methods developed by Lagrange in which energy and momentum are expressed in terms of generalized coordinates were to prove of particular value in his study of the dynamics of elastic continua, acoustics, optics and electricity.

Rayleigh's view of the use of mathematical methods and approximations appears in the preface of his classic text `Theory of Sound' recorded here.

`In the mathematical investigations I have usually employed such methods as present themselves naturally to a physicist. The pure mathematician will complain, and (it must be confessed) sometimes with justice, of deficient rigour. But to this question there are two sides. For however it may be to maintain a uniformly high standard in pure mathematics, the physicist may do well to rest content with arguments which are fairly satisfactory and conclusive from his point of view. To his mind, exercised in a different order of ideas, the more severe procedure of the pure mathematician may appear not more but less demonstrative.'

One of the most important pieces of early research carried out at the Terling laboratory consisted of his investigations into light scattering. In 1870, John William was engaged upon experimental studies in connection with colour and colour vision, aiming to replicate research by James Clerk Maxwell. He observed during the series of experiments that different readings were obtained when the sky was blue, compared to readings obtained when the sky was overcast.

Following examination of the results from a series of experiments in which he plotted the intensities of the spectra from the red to the blue, he reasoned that small particles in the atmosphere somehow interact with, and scatter, the sunlight incident upon them. John William was then to derive mathematically the complete expression for the scattering of light, presenting his results in his classical paper of December 1870, `On the Light from the Sky - Its Polarization and Colour'. This confirmed that the intensity of scattered radiation varies inversely as the fourth power of the wavelength - a fact that Rayleigh had demonstrated earlier by dimensional analysis. This pioneering work on the scattering of light by small particles was later generalized by Gustav Mie and remains a fruitful area of scientific research to this day.

During his Fellowship period, Rayleigh had established a laboratory in the west wing at Terling Place (fig. 4). An observation dome, built by the 4th Baron for his studies of the night sky, is still in evidence today. On his accession to the title and inheritance of Terling Place in 1873, Rayleigh immediately set about improving the laboratory and facilities for experimentation, hitherto comparatively crudely appointed. He installed a private gasworks to provide gas for lighting, and for the bunsen burners and blowtorch used in the manufacture of glassware.

4   The laboratory wing at Terling Place

The laboratory complex consists of a study on the ground floor, known as the `bookroom' (fig. 5), a workshop (fig. 6), and an adjacent room fitted with a chemical bench and sink (fig. 7), part of this room being partitioned to form a photographic darkroom. The room contained a turntable on which could be placed a circular bath. The bath purchased from a local ironmonger was used in Rayleigh's experiment to measure the diameter of the oil molecule and also amongst others, to carry out observations on fluid motion - the latter in connection with Rayleigh's researches into solitary waves in the 1870s. The phenomenon of the solitary wave was first noted by Scott Russell in 1834 and is now the subject of considerable interest.

5   The study

The first floor contains the main laboratory, partitioned into a series of areas including the `black room', used for optical experiments. A heliostat was positioned on a pedestal outside the laboratory in order to direct light into the black room - and beyond - for experiments requiring a long beam of light. The laboratory was to be used as a camera when diffraction gratings were employed, the grating being placed on a shelf some 25 ft (7.5 m) from the shutter with a long focus lens in front forming an auto-collimating spectroscope, a configuration he had learned from Clerk Maxwell.

6   The workshop

Much of the experimental equipment, including glassware, was hand-made either by Rayleigh himself or George Gordon, whom Rayleigh had persuaded to join him from the Cavendish Laboratory. Rayleigh had a remarkable ability to obtain results of fundamental importance, employing extremely simple and - certainly by today's standards - crude apparatus (fig. 8). Lord Kelvin, on one of his many visits to the laboratory at Terling, is on record as saying that `everything appears to be held together with string and sealing wax' (fig. 9).

Work at the Cavendish Laboratory, Cambridge

In December 1879, Lord Rayleigh was invited to become the second Cavendish Professor of Physics, in succession to James Clerk Maxwell, and accepted a five year tenure. Rayleigh appointed R. T. Glazebrook as demonstrator, and who later became first Director at the National Physical laboratory. Rayleigh also appointed Napier Shaw as demonstrator. Shaw later became first Director at the Meteorological Office.

On the topic of electrical research, Rayleigh suggested to one of his young research assistants, J. J. Thompson, that he determine the ratio of the electrostatic unit to the electromagnetic unit. This had earlier been attempted by Weber, Maxwell and William Thomson (later Lord Kelvin) with limited success. Rayleigh designed parts of the apparatus himself as reported in one of J. J. Thomson's first papers on this topic. This phase of Thomson's work led to the discovery of the electron. During the Cavendish period, Rayleigh was at the height of his powers writing some sixty papers in the five year period, an average of one per month. In describing Rayleigh's contribution to electrical research at the Cavendish, J. J. Thomson stated that Rayleigh had changed `chaos into order'.

In December 1884 Rayleigh announced that he intended to resign his post and return to private research work at his laboratory at Terling Place. The Cavendish Laboratory became the foremost centre for Physics in the late nineteenth and early 20th century. Twenty-five Cavendish physicists have won the Nobel Prize, the first of which was awarded to Lord Rayleigh himself in 1904 (fig. 10) for the discovery of argon (described later).

10   Rayleigh's Nobel prize citation

Work on Electrical Standards for the Ohm

The ohm was defined in the mid 1860s using experimental equipment devised originally by Lord Kelvin. This comprised a circular coil that was rotated about a vertical axis, exerting thereby a magnetic force on a needle suspended at the centre of the coil. If the coil rotated fast enough the deflecting force was virtually steady permitting the angular deflection of the needle to be measured. The angle depends upon the diameter of the coil, the rate at which it spins, and the electrical resistance of the coil, in turn, controlling the current flow. The experiment had been conducted in 1863-64 at King's College London under the auspices of the British Association. The results of the committee's work was the specification of the standard resistance coil used to represent 1 ohm. Subsequent to these experiments doubt had been cast on the accuracy of the results following work by Kohlraush (1874) who had found it to be 2% too great. Work by Rowland (1878) had alternatively found it to be 1% too small. Weber also in 1878 had substantially confirmed the original value. Whilst the value of the ohm remained uncertain it was difficult to estimate the efficiency of electrical equipments.

Whilst Rayleigh was at the Cavendish he decided to re-examine the standard for the ohm using the original apparatus, devised by Lord Kelvin. A critical feature of the experiment was the speed control for the rotation of the coil. Rayleigh used an electrically-driven tuning fork and circularly-calibrated card attached to the spinning coil to confirm that the rotational speed coincided with the frequency of oscillation of the tuning fork. A secondary needle was introduced to monitor the effects of changes in the direction of the Earth's magnetic field.

Early trials were carried out in the summer of 1880. The experiments were conducted late at night to avoid magnetic and other perturbations. Rayleigh regulated the speed, Dr Schuster took the main readings and Eleanor Sidgwick (formerly Eleanor Balfour) recorded the readings of the auxiliary magnetometer. Initial errors were caused, to some extent, by the ambiguous definition of the coil dimensions in the original British Association committee specification. Most of the 1.1% discrepancy could be traced to the erroneous value of inductance that arose therefrom. The results from the series of experiments were presented in a paper published in early 1882. Rayleigh's value for the ohm was later adopted by the UK Board of Trade and, in August 1893, by an international conference in Chicago.

Early interest in the need to establish a national scientific laboratory in the United Kingdom was mooted at a meeting of the British Association for the Advancement of Science in 1868, when Alexander Strange presented a paper `On the necessity for state intervention to secure the progress of physical science'.

In his Presidential address to the British Association in 1895, Douglas Galton urged the importance of establishing such a laboratory in the United Kingdom. A committee was set up by the British Association under the Chairmanship of Galton with the `scientifically eminent and politically influential' Lord Rayleigh as one of the committee members to `consider the establishment of a National Physical Laboratory for the more accurate determination of physical constants and for other quantitative research'. A deputation of committee members, which included Lord Rayleigh, presented their deliberations to the Prime Minister of the day, Lord Salisbury (Lady Rayleigh's uncle!) The Government was subsequently to set up a committee of enquiry under the Chairmanship of Lord Rayleigh to consider and report on the desirability of establishing such a laboratory.

The committee report, dated 6th of July 1898, recommended the setting up of a National Physical Laboratory under the control of the Royal Society. Financial aid, voted by Parliament for putting the committee's conclusions into effect, amounted to £4 000 as a grant in aid of expenses and £12 000 towards the erection of suitable buildings. The council of the Royal Society appointed Lord Rayleigh as Vice Chairman of the General Board and Chairman of the Executive Committee. At a Committee meeting held on the 5th July 1899 a letter was read from Dr R. T. Glazebrook, who had expressed his willingness to undertake the Directorship of the Laboratory. Dr Glazebrook's appointment was approved by the President and Council of the Royal Society and he undertook his duties as first Director from 1st January 1900.

Rayleigh was to continue actively in the affairs of the NPL until his death in 1919.

The Royal Institution of Great Britain in Albermarle Street, London, had been founded by Count Rumford in 1799 under Royal patronage. Rumford was a colonist, soldier and a scientist of some note who had founded the Institution to bring `the improvement of life through demonstration'. Rayleigh was invited to accept the Professorship of Natural Philosophy in succession to John Tyndall in 1887, a post he was to hold until 1905. Rayleigh gave over 100 research lectures during his tenure.

It is of interest to note that Guglielmo Marconi attended a lecture presented by Rayleigh on electromagnetic propagation. Marconi wrote to Rayleigh requesting a copy of the lecture notes to which he wished to refer in preparation for his Nobel presentation (fig. 11). Marconi also wrote to Rayleigh inviting him to Cornwall to view the propagation experiments that were later to confirm Marconi's theory that electrical transmission over large distances was indeed possible.

11   Marconi's letter to Rayleigh concerning his Nobel lecture
Select image for an enlargement

Towards the end of 1880, Rayleigh's interest turned to surface physics, surface tension and capillarity. His measurement of the diameter of the olive oil molecule exemplifies his direct approach to experimental science. It was in Rayleigh's mind that the molecular diameter could be determined by the measurement of the thickness of an oil film, judged to be one molecule thick. The apparatus featured in the experiment comprised a 33 inch (82.5 cm) diameter sponge bath obtained from a local ironmongers. Having filled the bath with water a succession of oil droplets were deposited on the surface aimed at determining the volume of oil just sufficient to cover the surface of the water. The extent of the spreading film of oil was monitored by noting the activity or quiescence of particles of camphor deposited around the rim of the bath on the initially uncontaminated surface of the water. In this simple manner Rayleigh was to determine the thickness of the oil film and hence the molecular diameter of the oil molecule as 40.6 µinches (1.6 nm). Later experimenters using somewhat more elaborate methods have confirmed Rayleigh's result to within a few percent. Rayleigh's measurements were the first direct attempt to measure the dimensions of a molecule.

Rayleigh was to divide his time between the Royal Institution and his Laboratory at Terling Place. The experiments for the isolation of argon were a case in point, with the initial reduction being carried out at the Royal Institution and the final production carried out at Terling Place.

The isolation of the inert gas Argon was one of Rayleigh's most famous accomplishments and led to his award of the Nobel Prize. William Prout had earlier suggested, in 1815, that atomic weights were exact multiples of hydrogen. If Prout was right, the exact density of oxygen relative to hydrogen should have been 16 and not 15.96, the value commonly accepted at that time. Rayleigh started to examine this anomaly by conducting experiments himself on the relative densities of oxygen to hydrogen. A succession of experiments yielded the ratios 15.89, 15.882, 15.880, and 15.863.

Rayleigh then began to examine the properties of nitrogen. Using two samples of nitrogen - one prepared from ordinary air with the oxygen removed by exposure to red hot copper and a second sample obtained by the decomposition of ammonia - resulted in two different values, as detected by the weighings. In order to obtain information from research chemists he wrote a letter to Nature, dated the 29th September 1892 stating,

`I am puzzled by some results on the density of nitrogen, and shall be obliged if any of your chemical readers can offer suggestions as to the cause. According to the method of preparation I obtain two quite distinct values. The relative difference, amounting to 1/1000 part, is small in itself, but it lies entirely outside the errors of experiment, and can only be attributed to a variation of the character of the gas'.

Rayleigh then spent some two years preparing nitrogen samples by several techniques, however the nitrogen produced artificially was always lighter than nitrogen produced from the atmosphere by about 0.5 percent. The consistency of the results produced by different processes led Rayleigh to the view that the problem lay in the existence of some constituent in the atmospheric nitrogen that had remained, as yet, undetected. Professor Dewar the physical chemist, a colleague at the Royal Institution, suggested that Rayleigh refer to work, dating from 1795, by Henry Cavendish, who had himself experimented with the reduction of atmospheric nitrogen.

Henry Cavendish's experiments of some hundred years earlier had relied upon a primitive spark machine to oxidize the atmospheric nitrogen. Rayleigh was to use this technique, carrying out the initial work at the Royal Institution and completing the final reduction at the Terling laboratory.

Rayleigh had received correspondence from Professor Ramsay, a chemist at University College London, who had himself begun to examine the characteristics of the atmospheric nitrogen using magnesium as a reagent following Rayleigh's earlier publication of his initial findings. This was regarded by many at the time as a breach of scientific etiquette. Correspondence of the day indicates that Ramsay kept Rayleigh informed of his progress in the isolation of the gas. The efforts by Ramsay had put Rayleigh under some pressure to complete his experiments more rapidly than he would have preferred.

A joint paper entitled, `Argon, a New Constituent of the Atmosphere', was presented at a meeting of the Royal Society on 31st January 1895. The findings were not universally accepted by the chemical fraternity at the time. Certain criticisms pointed to the fact that so heavy an element could not possibly be a gas. Rayleigh's reply was characteristic:

For this work Rayleigh was to be awarded the Nobel Prize for Physics in 1904. In that same year, Ramsay was awarded the Nobel Prize in Chemistry for his work on the isolation of argon and helium and, with Travers, the other noble gases neon, krypton and xenon.

Rayleigh was to do fundamental work on black body radiation towards the end of the nineteenth century. This led to an expression determining the precise wavelength distribution of black body radiation as a function of absolute temperature. The expression Rayleigh derived (qualified by Jeans) fitted observation at long wavelengths. Wien alternatively had proposed a law that fitted the observations at the shorter wavelengths.

Unlike Wien's law, however, the Rayleigh-Jeans Law was based rigorously on classical physics, and involved no arbitrary constants. Its failure to describe aberrations highlighted the limits of classical theory and was not resolved until Max Planck produced his classical paper in 1901. This introduced the revolutionary idea that energy is emitted in packets or quanta, and became the foundation of quantum theory.

Rayleigh seemed reluctant, however, to bridge the gap between classical and quantum physics. Although he had read Bohr's fundamental papers on the quantum theory of atomic physics with great interest, he was reluctant to accept the radical type of thinking that contravened the known laws upon which the edifice of nineteenth century physics was based. Rayleigh refused to participate directly in the theoretical developments that led to modern quantum mechanics, although his work was influential in guiding those who were to make contributions in this area of theoretical development.

The inert gas argon of course has many applications in industry today. The inability of the gas to react with other substances makes it suitable for use in applications such as welding, where it is used as a shield to inhibit oxidation of the molten metal; in nuclear reactors, as an inert cooling agent; in electric light bulbs; and in lasers. The gas is also used in the manufacture of semiconductor crystals where an inert atmosphere plays an important part in the process.

The year 1885 saw the publication of Rayleigh's paper, `On Waves Propagating Along the Plane Surface of an Elastic Solid', in the Proceedings of the London Mathematical Society. This single paper has resulted in the development of a number of fields of application, with both industrial and defence potential. These include, for example, propagation, ultrasonic microscopy, nondestructive testing and optical guided waves.

One such application is in the development of what are known as dispersive surface acoustic wave delay lines, that have their principal application in pulse-compression radar systems. They are used both for generation of an IF chirp pulse, which drives the frequency conversion and amplification chain of the transmitter and, relatedly, for compression of the received chirp pulses into short pulses of equivalent bandwidth. The process is equivalent to a Fourier transformation and, therefore, similar devices may be used in spectrum analysis for so-called compression receivers. A further application is in the construction of nondispersive, broadband, continuously-variable delay modules that have application in target simulation.

Delay lines are normally designed to meet specific applications, as the wide range of parameters and the complexity of the specification makes it impractical to manufacture and stock standard ranges. In the majority of cases, the parameters given refer to a pair of lines consisting of, first, an expander designed to generate a down-chirp of duration equal to at least the dispersive delay quoted as a requirement, and second, a compressor to match the pulse. In all cases the input and output circuits are designed to be tuned to specific bandwidths. The sidelobe weighting is obtained from a combination of tuning, variation of aperture of the transducers and also from the chirp phase law (in the case of non-linear units).

Custom-designed surface acoustic wave devices have been produced by the GEC-Marconi Research Centre (MRC) for the GEC-Marconi product divisions (fig. 12). One such SAW device, namely a dispersive delay line, has been produced in production quantities for GEC Avionics. The filter set was custom designed for the pulse compression radar system used in the Tornado aircraft for target detection and missile delivery.

12   Surface acoustic wave (SAW) device

Design of Antenna Systems

The Rayleigh statistical distribution finds many applications in the radio propagation field. For example, deviations from a pure Rayleigh distribution in the case of the signal amplitude received via tropospheric scatter permit conclusions to be drawn about layer formation in the troposphere. The relative strengths of the Rayleigh and log-normal components in atmospheric radio noise permit the separation of the propagation conditions, for example, from lightning and other phenomena (see Beckmann).

The ultimate sensitivity of a microwave receiving system is determined by thermal noise (see Scanlan). Thermal noise is emitted by any body which is above absolute zero and not completely transparent to the wavelength of interest.

The Rayleigh-Jeans formulation for black body radiation can be used to express the radiated noise power for an antenna system for all radio frequencies and also for the lower microwave frequencies. It fails to satisfy the conditions for the shorter wavelengths. Planck's equation for black body radiation, dating from 1901, provides a relationship that satisfies the observed behaviour throughout the complete frequency range.

In measuring the radiation pattern of an antenna of, say, diameter D, it is required to establish how close a remote test source can be placed. If the range is too short, the electromagnetic field will not have developed fully to the form possessed at infinity. The transition to the far field occurs gradually and is conventionally completed at the Rayleigh Distance, R = 2D²/  , where is the wavelength. At this distance the difference in path length from the source to any point on the aperture plane has diminished to   / 16. This definition is generally adopted when designing antenna systems, although it is, in fact, four times greater than the value given in Rayleigh's 1881 paper `On images formed without reflection or refraction', based on a phase error of   / 4.

When placing the source in the so-called far-field, use is made of the image of the source in the ground, by equalizing the phases of the direct and reflected signals. For satisfactory operation of such a `ground reflection range', the smoothness of the ground should satisfy the `Rayleigh criterion' that variations in the reflected phase should not exceed   / 4 in the neighbourhood of the reflection point. The term `Rayleigh criterion' is perhaps better known as a definition of the resolving power of a telescope in terms of its aperture: two incoherent sources are considered to be resolved when their focal plane diffraction patterns are positioned such that the peak of one falls in the first null of the other. This is equivalent to an angular separation of 1.22   / D for a circular aperture. Illustrations of antenna systems and radio telescopes developed at the GEC-Marconi Research Centre are shown in figs. 13 and 14.

13   One of the eight antennas forming the Cambridge 5 km interferometer

14   The James Clerk Maxwell telescope, now located at Mauna Kea in Hawaii
and used for studying distant radio sources

Rayleigh's studies of convection in fluids are extensively cited in literature pertaining to heat transfer. The so-called `Rayleigh number', Ra, expressed in terms of the Grashof (Gr) and Prandtl numbers (Pr), Ra = Gr.Pr, provides a measure of the behaviour of the fluid in the laminar and turbulent regimes for the buoyancy-induced flows. Turbulent flow can be expected for a Rayleigh number greater than 10⁹, for example, a condition associated with heat transfer from a heated surface to a cooling fluid.

Work carried out on convective flows at MRC has been used for the assessment of the cooling of electronic packaging configurations, consisting of assemblies of printed circuit boards, and also for cooling studies of high-powered electronic devices using, in some instances, a perfluorinated liquid as the cooling medium. Convection studies have been carried out using the FloSys computational fluid dynamics program, based on the finite domain method (fig. 15). This technique reduces the governing differential equations (Navier-Stokes and momentum) to a set of algebraic equations that can be solved on the computer in terms of the fluid velocity vectors and pressures for each cell. The technique is an approximation and the order of accuracy of the solution reflects the refinement embodied within the mesh. Some numerical experimentation is required, therefore, in order to confirm the accuracy of the solution.

15   Results of FLOTHERM simulation of a power supply unit, showing temperature contours and air velocity vectors on a cross-section through the power supply
(courtesy of Flomerics Ltd.).

The characteristics of radiation that is Rayleigh scattered from atmospheric particles can be used to determine several properties of the atmosphere. In particular, work is presently being carried out at MRC to measure windshear, that is, the velocity of sudden updraughts and downdraughts (see fig. 16). These are of particular importance for aviation, since encountering this form of turbulence during take-off or landing can, in extreme circumstances, cause an aircraft to crash.

A windshear sensor operates by detecting the Doppler shift of radiation scattered by particles in the atmosphere. This gives their velocity, and a high rate-of-change of velocity with position is indicative of a dangerous downdraught.

Scattering of light from particles can strictly only be called Rayleigh scattering if the particles are small compared with the wavelength. Thus if microwave radar is used, any radiation detected will be caused by Rayleigh scattering, because atmospheric particles are typically a few microns in diameter. Although windshear detection has been demonstrated using microwave radar, there are certain advantages to be gained by using a shorter wavelength; work at MRC is based around the use of a 2.1 µm eye-safe laser and therefore involves Mie scattering. Nevertheless, the principles of a windshear sensor using Rayleigh or Mie scattering are the same.

16   Principle of forward-looking windshear sensor using solid-state laser

In 1834, Scott Russell, a Scottish engineer observed that the sudden deceleration of a canal barge set in motion a wave that remained consistent in height, wavelength and waveform for a considerable distance from the point of disturbance. The phenomenon caused some interest at the time, and both the French physicist Boussinesq, and Lord Rayleigh were, independently, to describe the phenomenon in mathematical terms. The wave could be represented as the solution to a particular class of non-linear differential equation. The equation arises in a variety of applications ranging from the transmission of light in optical cables to the motion of the Earth's continental plates. Zabusky and Kruskal were to solve the equation numerically in 1965, dubbing the solution of the solitary wave equation the soliton.

An interesting characteristic of the wave motion is the retention of the single wave characteristics following interaction between two such waves. Drazin comments in his paper that the waves appear to interact as elementary particles such as electrons or protons. The mathematical solution of the equation was proved by Kruskal et al. to have a form equivalent to that of the Schrödinger wave equation. Further applications are emerging, with applications in the description of planetary atmospheres and waves in stratified fluids.

Over recent years, as Drazin states, solitons have become important in many applications and no doubt will grow in importance as the scientific and commercial relevance of the applications become evident (see, for example, Kubota et al.).

That Terling, a small village in the English County of Essex, was the home of one of the greatest nineteenth century physicists and Nobel Prizewinner, John William Strutt, 3rd Baron Rayleigh, is known to few people. The laboratory exists in a form which is virtually unchanged in layout since 1873. An experiment set up by the 4th Baron, Robert John, to examine the behaviour of active nitrogen remains as a silent witness to the fundamental work that was carried out within the laboratory.

Lord Rayleigh died on the 30th of June 1919 from a heart attack in his 77th year. His health had begun to fail some months earlier, although his scientific output had remained undiminished. He produced five papers in 1918 and seven in 1919. Some five days before his death he had dictated, to Lady Rayleigh, the closing paragraphs of a paper on cyclones for publication in the Philosophical Magazine.

Rayleigh's funeral was at Terling, where he is buried in a corner of the chuchyard that adjoins Terling Place (fig. 17). The simple monument of red sandstone bears the inscription:

Lady Rayleigh and his two surviving sons were chief mourners. King George V sent a representative, and principal officers of Cambridge University and the Royal Society attended. Villagers lined the path to the church door. A memorial to Rayleigh was placed in Westminster Abbey, in St Andrew's Chapel. The inscription reads:

17   Rayleigh's grave

The author extends his grateful thanks to John, the Sixth Baron Rayleigh for giving his permission to photograph various artifacts at Terling Place for reproduction in this article. Sincere thanks are also due to the Hon. Guy Strutt for his kind hospitality and numerous discussions, over a number of years, pertaining to the life and times of John William Strutt, and for the loan of books and publications.

Thanks are also extended to Mrs B. J. Gruhn for research on the history of the Strutt family and making available material from lectures presented at various historical association meetings

Thanks are also gratefully made to Dr John Howard for supplying a copy of the facsimile of Paper No. 8, `On the Light from the Sky, its Polarization and Colour, Itek Corporation, in addition to the copy of the report he produced for the Rayleigh Archives Dedication Ceremony. Reference has also been made to microfilmed copies of the Rayleigh correspondence prepared by John Howard and deposited at the Lyon Playfair Library at Imperial College, London - thanks are due to the library staff at that establishment. The author gratefully acknowledges information supplied by colleagues at the GEC-Marconi Research Centre