Wilhelm Conrad Röntgen was born on March 27, 1845, at Lennep in the Lower Rhine Province of Germany, as the only child of a merchant in, and manufacturer of, cloth. His mother was Charlotte Constanze Frowein of Amsterdam, a member of an old Lennep family which had settled in Amsterdam.

When he was three years old, his family moved to Apeldoorn in The Netherlands, where he went to the Institute of Martinus Herman van Doorn, a boarding school. He did not show any special aptitude, but showed a love of nature and was fond of roaming in the open country and forests. He was especially apt at making mechanical contrivances, a characteristic which remained with him also in later life. In 1862 he entered a technical school at Utrecht, where he was however unfairly expelled, accused of having produced a caricature of one of the teachers, which was in fact done by someone else.

He then entered the University of Utrecht in 1865 to study physics. Not having attained the credentials required for a regular student, and hearing that he could enter the Polytechnic at Zurich by passing its examination, he passed this and began studies there as a student of mechanical engineering. He attended the lectures given by Clausius and also worked in the laboratory of Kundt. Both Kundt and Clausius exerted great influence on his development. In 1869 he graduated Ph.D. at the University of Zurich, was appointed assistant to Kundt and went with him to Würzburg in the same year, and three years later to Strasbourg.

In 1874 he qualified as Lecturer at Strasbourg University and in 1875 he was appointed Professor in the Academy of Agriculture at Hohenheim in Wurtemberg. In 1876 he returned to Strasbourg as Professor of Physics, but three years later he accepted the invitation to the Chair of Physics in the University of Giessen.

After having declined invitations to similar positions in the Universities of Jena (1886) and Utrecht (1888), he accepted it from the University of Würzburg (1888), where he succeeded Kohlrausch und found among his colleagues Helmholtz and Lorenz. In 1899 he declined an offer to the Chair of Physics in the University of Leipzig, but in 1900 he accepted it in the University of Munich, by special request of the Bavarian government, as successor of E. Lommel. Here he remained for the rest of his life, although he was offered, but declined, the Presidency of the Physikalisch-Technische Reichsanstalt at Berlin and the Chair of Physics of the Berlin Academy.

Röntgen's first work was published in 1870, dealing with the specific heats of gases, followed a few years later by a paper on the thermal conductivity of crystals. Among other problems he studied were the electrical and other characteristics of quartz; the influence of pressure on the refractive indices of various fluids; the modification of the planes of polarised light by electromagnetic influences; the variations in the functions of the temperature and the compressibility of water and other fluids; the phenomena accompanying the spreading of oil drops on water.

Röntgen's name, however, is chiefly associated with his discovery of the rays that he called X-rays. In 1895 he was studying the phenomena accompanying the passage of an electric current through a gas of extremely low pressure. Previous work in this field had already been carried out by J. Plucker (1801-1868), J. W. Hittorf (1824-1914), C. F. Varley (1828-1883), E. Goldstein (1850-1931), Sir William Crookes (1832-1919), H. Hertz (1857-1894) and Ph. von Lenard (1862-1947), and by the work of these scientists the properties of cathode rays - the name given by Goldstein to the electric current established in highly rarefied gases by the very high tension electricity generated by Ruhmkorff's induction coil-had become well known. Röntgen's work on cathode rays led him, however, to the discovery of a new and different kind of rays.

On the evening of November 8, 1895, he found that, if the discharge tube is enclosed in a sealed, thick black carton to exclude all light, and if he worked in a dark room, a paper plate covered on one side with barium platinocyanide placed in the path of the rays became fluorescent even when it was as far as two metres from the discharge tube. During subsequent experiments he found that objects of different thicknesses interposed in the path of the rays showed variable transparency to them when recorded on a photographic plate. When he immobilised for some moments the hand of his wife in the path of the rays over a photographic plate, he observed after development of the plate an image of his wife's hand which showed the shadows thrown by the bones of her hand and that of a ring she was wearing, surrounded by the penumbra of the flesh, which was more permeable to the rays and therefore threw a fainter shadow. This was the first "röntgenogram" ever taken. In further experiments, Röntgen showed that the new rays are produced by the impact of cathode rays on a material object. Because their nature was then unknown, he gave them the name X-rays. Later, Max von Laue and his pupils showed that they are of the same electromagnetic nature as light, but differ from it only in the higher frequency of their vibration.

Numerous honours were showered upon him. In several cities, streets were named after him, and a complete list of Prizes, Medals, honorary doctorates, honorary and corresponding memberships of learned societies in Germany as well as abroad, and other honours would fill a whole page of this book. In spite of all this, Röntgen retained the characteristic of a strikingly modest and reticent man. Throughout his life he retained his love of nature and outdoor occupations. Many vacations were spent at his summer home at Weilheim, at the foot of the Bavarian Alps, where he entertained his friends and went on many expeditions into the mountains. He was a great mountaineer and more than once got into dangerous situations. Amiable and courteous by nature, he was always understanding the views and difficulties of others. He was always shy of having an assistant, and preferred to work alone. Much of the apparatus he used was built by himself with great ingenuity and experimental skill.

Röntgen married Anna Bertha Ludwig of Zürich, whom he had met in the café run by her father. She was a niece of the poet Otto Ludwig. They married in 1872 in Apeldoorn, The Netherlands. They had no children, but in 1887 adopted Josephine Bertha Ludwig, then aged 6, daughter of Mrs. Röntgen's only brother. Four years after his wife, Röntgen died at Munich on February 10, 1923, from carcinoma of the intestine.

Antoine Henri Becquerel was born in Paris on December 15, 1852, a member of a distinguished family of scholars and scientists. His father, Alexander Edmond Becquerel, was a Professor of Applied Physics and had done research on solar radiation and on phosphorescence, while his grandfather, Antoine César, had been a Fellow of the Royal Society and the inventor of an electrolytic method for extracting metals from their ores. He entered the Polytechnic in 1872, then the government department of Pontset-Chaussees in 1874, becoming ingénieur in 1877 and being promoted to ingénieur-en-chef in 1894. In 1888 he acquired the degree of docteur-ès-sciences. From 1878 he had held an appointment as an Assistant at the Museum of Natural History, taking over from his father in the Chair of Applied Physics at the Conservatoire des Arts et Metiers. In 1892 he was appointed Professor of Applied Physics in the Department of Natural History at the Paris Museum. He became a Professor at the Polytechnic in 1895.

Becquerel's earliest work was concerned with the plane polarization of light, with the phenomenon of phosphorescence and with the absorption of light by crystals (his doctorate thesis). He also worked on the subject of terrestrial magnetism. In 1896, his previous work was overshadowed by his discovery of the phenomenon of natural radioactivity. Following a discussion with Henri Poincaré on the radiation which had recently been discovered by Röntgen (X-rays) and which was accompanied by a type of phosphorescence in the vacuum tube, Becquerel decided to investigate whether there was any connection between X-rays and naturally occurring phosphorescence. He had inherited from his father a supply of uranium salts, which phosphoresce on exposure to light. When the salts were placed near to a photographic plate covered with opaque paper, the plate was discovered to be fogged. The phenomenon was found to be common to all the uranium salts studied and was concluded to be a property of the uranium atom. Later, Becquerel showed that the rays emitted by uranium, which for a long time were named after their discoverer, caused gases to ionize and that they differed from X-rays in that they could be deflected by electric or magnetic fields. For his discovery of spontaneous radioactivity Becquerel was awarded half of the Nobel Prize for Physics in 1903, the other half being given to Pierre and Marie Curie for their study of the Becquerel radiation.

Becquerel published his findings in many papers, principally in the Annales de Physique et de Chimie and the Comptes Rendus de l'Academie des Sciences.

He was elected a member of the Academie des Sciences de France in 1889 and succeeded Berthelot as Life Secretary of that body. He was a member also of the Accademia dei Lincei and of the Royal Academy of Berlin, amongst others. He was made an Officer of the Legion of Honour in 1900.

He was married to Mlle. Janin, the daughter of a civil engineer. They had a son Jean, b. 1878, who was also a physicist: the fourth generation of scientists in the Becquerel family.

Antoine Henri Becquerel died at Le Croisic on August 25, 1908.

Pierre Curie was born in Paris, where his father was a general medical practitioner, on May 15, 1859. He received his early education at home before entering the Faculty of Sciences at the Sorbonne. He gained his Licenciateship in Physics in 1878 and continued as a demonstrator in the physics laboratory until 1882 when he was placed in charge of all practical work in the Physics and Industrial Chemistry Schools. In 1895 he obtained his Doctor of Science degree and was appointed Professor of Physics. He was promoted to Professor in the Faculty of Sciences in 1900, and in 1904 he became Titular Professor.

In his early studies on crystallography, together with his brother Jacques, Curie discovered piezoelectric effects. Later, he advanced theories of symmetry with regard to certain physical phenomena and turned his attention to magnetism. He showed that the magnetic properties of a given substance change at a certain temperature - this temperature is now known as the Curie point. To assist in his experiments he constructed several delicate pieces of apparatus - balances, electrometers, piezoelectric crystals, etc.

Curie's studies of radioactive substances were made together with his wife, whom he married in 1895. They were achieved under conditions of much hardship - barely adequate laboratory facilities and under the stress of having to do much teaching in order to earn their livelihood. They announced the discovery of radium and polonium by fractionation of pitchblende in 1898 and later they did much to elucidate the properties of radium and its transformation products. Their work in this era formed the basis for much of the subsequent research in nuclear physics and chemistry. Together they were awarded half of the Nobel Prize for Physics in 1903 on account of their study into the spontaneous radiation discovered by Becquerel, who was awarded the other half of the Prize.

Pierre Curie's work is recorded in numerous publications in the Comptes Rendus de l'Académie des Sciences, the Journal de Physique and the Annales de Physique et Chimie.

Curie was awarded the Davy Medal of the Royal Society of London in 1903 (jointly with his wife) and in 1905 he was elected to the Academy of Sciences.

His wife was formerly Marie Sklodowska, daughter of a secondary-school teacher at Warsaw, Poland. One daughter, Irene, married Frederic Joliot and they were joint recipients of the Nobel Prize for Chemistry in 1935. The younger daughter, Eve, married the American diplomat H. R. Labouisse. They have both taken lively interest in social problems, and as Director of the United Nations' Children's Fund he received on its behalf the Nobel Peace Prize in Oslo in 1965. She is the author of a famous biography of her mother, Madame Curie (Gallimard, Paris, 1938), translated into several languages.

Pierre was killed in a street accident in Paris on April 19, 1906.

Marie Curie, née Maria Sklodowska, was born in Warsaw on November 7, 1867, the daughter of a secondary-school teacher. She received a general education in local schools and some scientific training from her father. She became involved in a students' revolutionary organization and found it prudent to leave Warsaw, then in the part of Poland dominated by Russia, for Cracow, which at that time was under Austrian rule. In 1891, she went to Paris to continue her studies at the Sorbonne where she obtained Licenciateships in Physics and the Mathematical Sciences. She met Pierre Curie, Professor in the School of Physics in 1894 and in the following year they were married. She succeeded her husband as Head of the Physics Laboratory at the Sorbonne, gained her Doctor of Science degree in 1903, and following the tragic death of Pierre Curie in 1906, she took his place as Professor of General Physics in the Faculty of Sciences, the first time a woman had held this position. She was also appointed Director of the Curie Laboratory in the Radium Institute of the University of Paris, founded in 1914.

Her early researches, together with her husband, were often performed under difficult conditions, laboratory arrangements were poor and both had to undertake much teaching to earn a livelihood. The discovery of radioactivity by Henri Becquerel in 1896 inspired the Curies in their brilliant researches and analyses which led to the isolation of polonium, named after the country of Marie's birth, and radium. Mme. Curie developed methods for the separation of radium from radioactive residues in sufficient quantities to allow for its characterization and the careful study of its properties, therapeutic properties in particular.

Mme. Curie throughout her life actively promoted the use of radium to alleviate suffering and during World War I, assisted by her daughter, Irene, she personally devoted herself to this remedial work. She retained her enthusiasm for science throughout her life and did much to establish a radioactivity laboratory in her native city - in 1929 President Hoover of the United States presented her with a gift of $ 50,000, donated by American friends of science, to purchase radium for use in the laboratory in Warsaw.

Mme. Curie, quiet, dignified and unassuming, was held in high esteem and admiration by scientists throughout the world. She was a member of the Conseil du Physique Solvay from 1911 until her death and since 1922 she had been a member of the Committee of Intellectual Co-operation of the League of Nations. Her work is recorded in numerous papers in scientific journals and she is the author of Recherches sur les Substances Radioactives (1904), L'Isotopie et les Éléments Isotopes and the classic Traité' de Radioactivité (1910).

The importance of Mme. Curie's work is reflected in the numerous awards bestowed on her. She received many honorary science, medicine and law degrees and honorary memberships of learned societies throughout the world. Together with her husband, she was awarded half of the Nobel Prize for Physics in 1903, for their study into the spontaneous radiation discovered by Becquerel, who was awarded the other half of the Prize. In 1911 she received a second Nobel Prize, this time in Chemistry, in recognition of her work in radioactivity. She also received, jointly with her husband, the Davy Medal of the Royal Society in 1903 and, in 1921, President Harding of the United States, on behalf of the women of America, presented her with one gram of radium in recognition of her service to science.

For further details, cf. Biography of Pierre Curie. Mme. Curie died in Savoy, France, after a short illness, on July 4, 1934.

Guglielmo Marconi was born at Bologna, Italy, on April 25, 1874, the second son of Giuseppe Marconi, an Italian country gentleman, and Annie Jameson, daughter of Andrew Jameson of Daphne Castle in the County Wexford, Ireland. He was educated privately at Bologna, Florence and Leghorn. Even as a boy he took a keen interest in physical and electrical science and studied the works of Maxwell, Hertz, Righi, Lodge and others. In 1895 he began laboratory experiments at his father's country estate at Pontecchio where he succeeded in sending wireless signals over a distance of one and a half miles.

In 1896 Marconi took his apparatus to England where he was introduced to Mr. (later Sir) William Preece, Engineer-in-Chief of the Post Office, and later that year was granted the world's first patent for a system of wireless telegraphy. He demonstrated his system successfully in London, on Salisbury Plain and across the Bristol Channel, and in July 1897 formed The Wireless Telegraph & Signal Company Limited (in 1900 re-named Marconi's Wireless Telegraph Company Limited). In the same year he gave a demonstration to the Italian Government at Spezia where wireless signals were sent over a distance of twelve miles. In 1899 he established wireless communication between France and England across the English Channel. He erected permanent wireless stations at The Needles, Isle of Wight, at Bournemouth and later at the Haven Hotel, Poole, Dorset.

In 1900 he took out his famous patent No. 7777 for "tuned or syntonic telegraphy" and, on an historic day in December 1901, determined to prove that wireless waves were not affected by the curvature of the Earth, he used his system for transmitting the first wireless signals across the Atlantic between Poldhu, Cornwall, and St. John's, Newfoundland, a distance of 2100 miles.

Between 1902 and 1912 he patented several new inventions. In 1902, during a voyage in the American liner "Philadelphia", he first demonstrated "daylight effect" relative to wireless communication and in the same year patented his magnetic detector which then became the standard wireless receiver for many years. In December 1902 he transmitted the first complete messages to Poldhu from stations at Glace Bay, Nova Scotia, and later Cape Cod, Massachusetts, these early tests culminating in 1907 in the opening of the first transatlantic commercial service between Glace Bay and Clifden, Ireland, after the first shorter-distance public service of wireless telegraphy had been established between Bari in Italy and Avidari in Montenegro. In 1905 he patented his horizontal directional aerial and in 1912 a "timed spark" system for generating continuous waves.

In 1914 he was commissioned in the Italian Army as a Lieutenant being later promoted to Captain, and in 1916 transferred to the Navy in the rank of Commander. He was a member of the Italian Government mission to the United States in 1917 and in 1919 was appointed Italian plenipotentiary delegate to the Paris Peace Conference. He was awarded the Italian Military Medal in 1919 in recognition of his war service.

During his war service in Italy he returned to his investigation of short waves, which he had used in his first experiments. After further tests by his collaborators in England, an intensive series of trials was conducted in 1923 between experimental installations at the Poldhu Station and in Marconi's yacht "Elettra" cruising in the Atlantic and Mediterranean, and this led to the establishment of the beam system for long distance communication. Proposals to use this system as a means of Imperial communications were accepted by the British Government and the first beam station, linking England and Canada, was opened in 1926, other stations being added the following year.

In 1931 Marconi began research into the propagation characteristics of still shorter waves, resulting in the opening in 1932 of the world's first microwave radiotelephone link between the Vatican City and the Pope's summer residence at Castel Gandolfo. Two years later at Sestri Levante he demonstrated his microwave radio beacon for ship navigation and in 1935, again in Italy, gave a practical demonstration of the principles of radar, the coming of which he had first foretold in a lecture to the American Institute of Radio Engineers in New York in 1922.

He has been the recipient of honorary doctorates of several universities and many other international honours and awards, among them the Nobel Prize for Physics, which in 1909 he shared with Professor Karl Braun, the Albert Medal of the Royal Society of Arts, the John Fritz Medal and the Kelvin Medal. He was decorated by the Tsar of Russia with the Order of St. Anne, the King of Italy created him Commander of the Order of St. Maurice and St. Lazarus, and awarded him the Grand Cross of the Order of the Crown of Italy in 1902. Marconi also received the freedom of the City of Rome (1903), and was created Chevalier of the Civil Order of Savoy in 1905. Many other distinctions of this kind followed. In 1914 he was both created a Senatore in the Italian Senate and app ointed Honorary Knight Grand Cross of the Royal Victorian Order in England. He received the hereditary title of Marchese in 1929.

In 1905 he married the Hon. Beatrice O'Brien, daughter of the 14th Baron Inchiquin, the marriage being annulled in 1927, in which year he married the Countess Bezzi-Scali of Rome. He had one son and two daughters by his first and one daughter by his second wife. His recreations were hunting, cycling and motoring.

Marconi died in Rome on July 20, 1937.

Johannes Diderik van der Waals was born on November 23, 1837 in Leyden, The Netherlands, the son of Jacobus van der Waals and Elisabeth van den Burg. After having finished elementary education at his birthplace he became a schoolteacher. Although he had no knowledge of classical languages, and thus was not allowed to take academic examinations, he continued studying at Leyden University in his spare time during 1862-65. In this way he also obtained teaching certificates in mathematics and physics.

In 1864 he was appointed teacher at a secondary school at Deventer; in 1866 he moved to The Hague, first as teacher and later as Director of one of the secondary schools in that town.

New legislation whereby university students in science were exempted from the conditions concerning prior classical education enabled Van der Waals to sit for university examinations. In 1873 he obtained his doctor's degree for a thesis entitled Over de Continuïteit van den Gas - en Vloeistoftoestand (On the continuity of the gas and liquid state), which put him at once in the foremost rank of physicists. In this thesis he put forward an "Equation of State" embracing both the gaseous and the liquid state; he could demonstrate that these two states of aggregation not only merge into each other in a continuous manner, but that they are in fact of the same nature. The importance of this conclusion from Van der Waals' very first paper can be judged from the remarks made by James Clerk Maxwell in Nature, "that there can be no doubt that the name of Van der Waals will soon be among the foremost in molecular science" and "It has certainly directed the attention of more than one inquirer to the study of the Low-Dutch language in which it is written" (Maxwell probably meant to say "Low-German", which would also be incorrect, since Dutch is a language in its own right). Subsequently, numerous papers on this and related subjects were published in the Proceedings of the Royal Netherlands Academy of Sciences and in the Archives Néerlandaises, and they were also translated into other languages.

When, in 1876, the new Law on Higher Education was established which promoted the old Athenaeum Illustre of Amsterdam to university status, Van der Waals was appointed the first Professor of Physics. Together with Van't Hoff and Hugo de Vries, the geneticist, he contributed to the fame of the University, and remained faithful to it until his retirement, in spite of enticing invitations from elsewhere.

The immediate cause of Van der Waals' interest in the subject of his thesis was R. Clausius' treatise considering heat as a phenomenon of motion, which led him to look for an explanation for T. Andrews' experiments (1869) revealing the existence of "critical temperatures " in gases. It was Van der Waals' genius that made him see the necessity of taking into account the volumes of molecules and the intermolecular forces ("Van der Waals forces", as they are now generally called) in establishing the relationship between the pressure, volume and temperature of gases and liquids.

A second great discovery - arrived at after much arduous work - was published in 1880, when he enunciated the Law of Corresponding States. This showed that if pressure is expressed as a simple function of the critical pressure, volume as one of the critical volume, and temperature as one of the critical temperature, a general form of the equation of state is obtained which is applicable to all substances, since the three constants a, b, and R in the equation, which can be expressed in the critical quantities of a particular substance, will disappear. It was this law which served as a guide during experiments which ultimately led to the liquefaction of hydrogen by J. Dewar in 1898 and of helium by H. Kamerlingh Onnes in 1908. The latter, who in 1913 received the Nobel Prize for his low-temperature studies and his production of liquid helium, wrote in 1910 "that Van der Waals' studies have always been considered as a magic wand for carrying out experiments and that the Cryogenic Laboratory at Leyden has developed under the influence of his theories ".

Ten years later, in 1890, the first treatise on the "Theory of Binary Solutions" appeared in the Archives Néerlandaises - another great achievement of Van der Waals. By relating his equation of state with the Second Law of Thermodynamics, in the form first proposed by W. Gibbs in his treatises on the equilibrium of heterogeneous substances, he was able to arrive at a graphical representation of his mathematical formulations in the form of a surface which he called "Psi-surface" in honour of Gibbs, who had chosen the Greek letter Psi as a symbol for the free energy, which he realised was significant for the equilibrium. The theory of binary mixtures gave rise to numerous series of experiments, one of the first being carried out by J. P. Kuenen, who found characteristics of critical phenomena fully predictable by the theory. Lectures on this subject were subsequently assembled in the Lehrbuch der Thermodynamik (Textbook of thermodynamics) by Van der Waals and Ph. Kohnstamm.

Mention should also be made of Van der Waals' thermodynamic theory of capillarity, which in its basic form first appeared in 1893. In this, he accepted the existence of a gradual, though very rapid, change of density at the boundary layer between liquid and vapour - a view which differed from that of Gibbs, who assumed a sudden transition of the density of the fluid into that of the vapour. In contrast to Laplace, who had earlier formed a theory on these phenomena, Van der Waals also held the view that the molecules are in permanent, rapid motion. Experiments with regard to phenomena in the vicinity of the critical temperature decided in favour of Van der Waals' concepts.

Van der Waals was the recipient of numerous honours and distinctions, of which the following should be particularly mentioned. He received an honorary doctorate of the University of Cambridge; was made honorary member of the Imperial Society of Naturalists of Moscow, the Royal Irish Academy and the American Philosophical Society; corresponding member of the Institut de France and the Royal Academy of Sciences of Berlin; associate member of the Royal Academy of Sciences of Belgium; and foreign member of the Chemical Society of London, the National Academy of Sciences of the U.S.A., and of the Accademia dei Lincei of Rome.

In 1864, Van der Waals married Anna Magdalena Smit, who died early. He never married again. They had three daughters and one son. The daughters were Anne Madeleine who, after her mother's early death, ran the house and looked after her father; Jacqueline Elisabeth, who was a teacher of history and a well-known poetess; and Johanna Diderica, who was a teacher of English. The son, Johannes Diderik Jr., was Professor of Physics at Groningen University 1903-08, and subsequently succeeded his father in the Physics Chair of the University of Amsterdam.

Van der Waals' main recreations were walking, particularly in the country, and reading. He died in Amsterdam on March 8, 1923.

Max Karl Ernst Ludwig Planck was born in Kiel, Germany, on April 23, 1858, the son of Julius Wilhelm and Emma (née Patzig) Planck. His father was Professor of Constitutional Law in the University of Kiel, and later in Göttingen.

Planck studied at the Universities of Munich and Berlin, where his teachers included Kirchhoff and Helmholtz, and received his doctorate of philosophy at Munich in 1879. He was Privatdozent in Munich from 1880 to 1885, then Associate Professor of Theoretical Physics at Kiel until 1889, in which year he succeeded Kirchhoff as Professor at Berlin University, where he remained until his retirement in 1926. Afterwards he became President of the Kaiser Wilhelm Society for the Promotion of Science, a post he held until 1937. The Prussian Academy of Sciences appointed him a member in 1894 and Permanent Secretary in 1912.

Planck's earliest work was on the subject of thermodynamics, an interest he acquired from his studies under Kirchhoff, whom he greatly admired, and very considerably from reading R. Clausius' publications. He published papers on entropy, on thermoelectric ity and on the theory of dilute solutions.

At the same time also the problems of radiation processes engaged his attention and he showed that these were to be considered as electromagnetic in nature. From these studies he was led to the problem of the distribution of energy in the spectrum of full radiation. Experimental observations on the wavelength distribution of the energy emitted by a black body as a function of temperature were at variance with the predictions of classical physics. Planck was able to deduce the relationship between the ener gy and the frequency of radiation. In a paper published in 1900, he announced his derivation of the relationship: this was based on the revolutionary idea that the energy emitted by a resonator could only take on discrete values or quanta. The energy for a resonator of frequency v is hv where h is a universal constant, now called Planck's constant.

This was not only Planck's most important work but also marked a turning point in the history of physics. The importance of the discovery, with its far-reaching effect on classical physics, was not appreciated at first. However the evidence for its validi ty gradually became overwhelming as its application accounted for many discrepancies between observed phenomena and classical theory. Among these applications and developments may be mentioned Einstein's explanation of the photoelectric effect.

Planck's work on the quantum theory, as it came to be known, was published in the Annalen der Physik. His work is summarized in two books Thermodynamik (Thermodynamics) (1897) and Theorie der Wärmestrahlung (Theory of heat radiat ion) (1906).

He was elected to Foreign Membership of the Royal Society in 1926, being awarded the Society's Copley Medal in 1928.

Planck faced a troubled and tragic period in his life during the period of the Nazi government in Germany, when he felt it his duty to remain in his country but was openly opposed to some of the Government's policies, particularly as regards the persecuti on of the Jews. In the last weeks of the war he suffered great hardship after his home was destroyed by bombing.

He was revered by his colleagues not only for the importance of his discoveries but for his great personal qualities. He was also a gifted pianist and is said to have at one time considered music as a career.

Planck was twice married. Upon his appointment, in 1885, to Associate Professor in his native town Kiel he married a friend of his childhood, Marie Merck, who died in 1909. He remarried her cousin Marga von Hösslin. Three of his children died young, leaving him with two sons.

He suffered a personal tragedy when one of them was executed for his part in an unsuccessful attempt to assassinate Hitler in 1944.

He died at Göttingen on October 4, 1947.

Gustav Ludwig Hertz was born in Hamburg on July 22nd, 1887, the son of a lawyer, Dr. Gustav Hertz, and his wife Auguste, née Arning. He attended the Johanneum School in Hamburg before commencing his university education at Göttingen in 1906; he subsequently studied at the Universities of Munich and Berlin, graduating in 1911. He was appointed Research Assistant at the Physics Institute of Berlin University in 1913 but, with the onset of World War I, he was mobilized in 1914 and severely wounded in action in 1915. Hertz returned to Berlin as Privatdozent in 1917. From 1920 to 1925 he worked in the physics laboratory of the Philips Incandescent Lamp Factory at Eindhoven.

In 1925, he was elected Resident Professor and Director of the Physics Institute of the University of Halle, and in 1928 he returned to Berlin as Director of the Physics Institute in the Charlottenburg Technological University. Hertz resigned from this post for political reasons in 1935 to return to industry as director of a research laboratory of the Siemens Company. From 1945 tot 1954 he worked as the head of a research laboratory in the Soviet Union, when he was appointed Professor and Director of the Physics Institute at the Karl Marx University in Leipzig. He was made emeritus in 1961, and since then he has lived in retirement, first in Leipzig and later in Berlin.

Hertz's early researches, for his thesis, involved studies on the infrared absorption of carbon dioxide in relation to pressure and partial pressure. Together with J. Franck he began his studies on electron impact in 1913 and before his mobilization, he spent much patient work on the study and measurement of ionization potentials in various gases. He later demonstrated the quantitative relations between the series of spectral lines and the energy losses of electrons in collision with atoms corresponding to the stationary energy states of the atoms. His results were in perfect agreement with Bohr's theory of atomic structure, which included the application of Planck's quantum theory.

On his return to Berlin in 1928, it was his first task to rebuild the Physics Institute and re-establish the School, and he worked tirelessly towards this end. There he was responsible for a method of separating the isotopes of neon by means of a diffusion cascade.

Hertz has published many papers, alone, with Franck, and with Kloppers, on the quantitative exchange of energy between electrons and atoms, and on the measurement of ionization potentials. He also is the author of some papers concerning the separation of isotopes.

Gustav Hertz is Member of the German Academy of Sciences in Berlin, and Corresponding Member of the Göttingen Academy of Sciences; he is also Honorary Member of the Hungarian Academy of Sciences, Member of the Czechoslovakian Academy of Sciences, and Foreign Member of the Academy of Sciences U.S.S.R. He is recipient of the Max Planck Medal of the German Physical Society.

Professor Hertz was married in 1919, with Ellen née Dihlmann, who died in 1941. They had two sons, both physicists: Dr. Hellmuth Hertz, Professor at the Technical College in Lund, and Dr. Johannes Hertz, working at the Institute for Optics and Spectroscopy of the German Academy of Sciences in Berlin.

Since 1943, Professor Hertz is married with Charlotte, née Jollasse.

Enrico Fermi was born in Rome on 29th September, 1901, the son of Alberto Fermi, a Chief Inspector of the Ministry of Communications, and Ida de Gattis. He attended a local grammar school, and his early aptitude for mathematics and physics was recognized and encouraged by his father's colleagues, among them A. Amidei. In 1918, he won a fellowship of the Scuola Normale Superiore of Pisa. He spent four years at the University of Pisa, gaining his doctor's degree in physics in 1922, with Professor Puccianti.

Soon afterwards, in 1923, he was awarded a scholarship from the Italian Government and spent some months with Professor Max Born in Göttingen. With a Rockefeller Fellowship, in 1924, he moved to Leyden to work with P. Ehrenfest, and later that same year he returned to Italy to occupy for two years (1924-1926) the post of Lecturer in Mathematical Physics and Mechanics at the University of Florence.

In 1926, Fermi discovered the statistical laws, nowadays known as the «Fermi statistics», governing the particles subject to Pauli's exclusion principle (now referred to as «fermions», in contrast with «bosons» which obey the Bose-Einstein statistics).

In 1927, Fermi was elected Professor of Theoretical Physics at the University of Rome (a post which he retained until 1938, when he - immediately after the receipt of the Nobel Prize - emigrated to America, primarily to escape Mussolini's fascist dictatorship).

During the early years of his career in Rome he occupied himself with electrodynamic problems and with theoretical investigations on various spectroscopic phenomena. But a capital turning-point came when he directed his attention from the outer electrons towards the atomic nucleus itself. In 1934, he evolved the ß-decay theory, coalescing previous work on radiation theory with Pauli's idea of the neutrino. Following the discovery by Curie and Joliot of artificial radioactivity (1934), he demonstrated that nuclear transformation occurs in almost every element subjected to neutron bombardment. This work resulted in the discovery of slow neutrons that same year, leading to the discovery of nuclear fission and the production of elements lying beyond what was until then the Periodic Table.

In 1938, Fermi was without doubt the greatest expert on neutrons, and he continued his work on this topic on his arrival in the United States, where he was soon appointed Professor of Physics at Columbia University, N.Y. (1939-I942).

Upon the discovery of fission, by Hahn and Strassmann early in 1939, he immediately saw the possibility of emission of secondary neutrons and of a chain reaction. He proceeded to work with tremendous enthusiasm, and directed a classical series of experiments which ultimately led to the atomic pile and the first controlled nuclear chain reaction. This took place in Chicago on December 2, 1942 - on a squash court situated beneath Chicago's stadium. He subsequently played an important part in solving the problems connected with the development of the first atomic bomb (He was one of the leaders of the team of physicists on the Manhattan Project for the development of nuclear energy and the atomic bomb.)

In 1944, Fermi became American citizen, and at the end of the war (1946) he accepted a professorship at the Institute for Nuclear Studies of the University of Chicago, a position which he held until his untimely death in 1954. There he turned his attention to high-energy physics, and led investigations into the pion-nucleon interaction.

During the last years of his life Fermi occupied himself with the problem of the mysterious origin of cosmic rays, thereby developing a theory, according to which a universal magnetic field - acting as a giant accelerator - would account for the fantastic energies present in the cosmic ray particles.

Professor Fermi was the author of numerous papers both in theoretical and experimental physics. His most important contributions were:

"Sulla quantizzazione del gas perfetto monoatomico", Rend. Accad. Naz. Lincei, 1935 (also in Z. Phys., 1936), concerning the foundations of the statistics of the electronic gas and of the gases made of particles that obey the Pauli Principle.

Several papers published in Rend. Accad. Naz. Lincei, 1927-28, deal with the statistical model of the atom (Thomas-Fermi atom model) and give a semiquantitative method for the calculation of atomic properties. A resumé of this work was published by Fermi in the volume: Quantentheorie und Chemie, edited by H. Falkenhagen, Leipzig, 1928.

"Uber die magnetischen Momente der AtomKerne", Z. Phys., 1930, is a quantitative theory of the hyperfine structures of spectrum lines. The magnetic moments of some nuclei are deduced therefrom.

"Tentativo di una teoria dei raggi ß", Ricerca Scientifica, 1933 (also Z. Phys., 1934) proposes a theory of the emission of ß-rays, based on the hypothesis, first proposed by Pauli, of the existence of the neutrino.

The Nobel Prize for Physics was awarded to Fermi for his work on the artificial radioactivity produced by neutrons, and for nuclear reactions brought about by slow neutrons. The first paper on this subject "Radioattività indotta dal bombardamento di neutroni" was published by him in Ricerca Scientifica, 1934. All the work is collected in the following papers by himself and various collaborators: "Artificial radioactivity produced by neutron bombardment", Proc. Roy. Soc., 1934 and 1935; "On the absorption and diffusion of slow neutrons", Phys. Rev., 1936. The theoretical problems connected with the neutron are discussed by Fermi in the paper "Sul moto dei neutroni lenti", Ricerca Scientfica, 1936.

His Collected Papers are being published by a Committee under the Chairmanship of his friend and former pupil, Professor E. Segrè (Nobel Prize winner 1959, with O. Chamberlain, for the discovery of the antiproton).

Fermi was member of several academies and learned societies in Italy and abroad (he was early in his career, in 1929, chosen among the first 30 members of the Royal Academy of Italy).

As lecturer he was always in great demand (he has also given several courses at the University of Michigan, Ann Arbor; and Stanford University, Calif.). He was the first recipient of a special award of $50,000 - which now bears his name - for work on the atom.

Professor Fermi married Laura Capon in 1928. They had one son Giulio and one daughter Nella. His favourite pastimes were walking, mountaineering, and winter sports.

He died in Chicago on 28th November, 1954.

Wolfgang Pauli was born on April 25th, 1900 in Vienna. He received his early education in Vienna before studying at the University of Munich under Arnold Sommerfeld. He obtained his doctor's degree in 1921 and spent a year at the University of Göttingen as assistant to Max Born and a further year with Niels Bohr at Copenhagen. The years 1923-1928 were spent as a lecturer at the University of Hamburg before his appointment as Professor of Theoretical Physics at the Federal Institute of Technology in Zurich. During 1935-1936, he was visiting Professor at the Institute for Advanced Study, Princeton, New Jersey and he had similar appointments at the University of Michigan (1931 and 1941) and Purdue University (1942). He was elected to the Chair of Theoretical Physics at Princeton in 1940 but he returned to Zurich at the end of World War II.

Pauli was outstanding among the brilliant mid-twentieth century school of physicists. He was recognized as one of the leaders when, barely out of his teens and still a student, he published a masterly exposition of the theory of relativity. His exclusion principle, which is often quoted bearing his name, crystallized the existing knowledge of atomic structure at the time it was postulated and it led to the recognition of the two-valued variable required to characterize the state of an electron. Pauli was the first to recognize the existence of the neutrino, an uncharged and massless particle which carries off energy in radioactive ß-disintegration; this came at the beginning of a great decade, prior to World War II, for his centre of research in theoretical physics at Zurich.

Pauli helped to lay the foundations of the quantum theory of fields and he participated actively in the great advances made in this domain around 1945. Earlier, he had further consolidated field theory by giving proof of the relationship between spin and"statistics" of elementary particles. He has written many articles on problems of theoretical physics, mostly quantum mechanics, in scientific journals of many countries; his Theory of Relativity appears in the Enzyklopaedie der Mathematischen Wissenschaften, Volume 5, Part 2 (1920), his Quantum Theory in Handbuch der Physik, Vol. 23 (1926), and his Principles of Wave Mechanics in Handbuch der Physik, Vol. 24 (1933).

Pauli was a Foreign Member of the Royal Society of London and a member of the Swiss Physical Society, the American Physical Society and the American Association for the Advancement of Science. He was awarded the Lorentz Medal in 1930.

Wolfgang Pauli married Franciska Bertram on April 4th, 1934. He died in Zurich on December 15th, 1958.

I was born in Budapest, Hungary, on June 5, 1900, the oldest son of Bertalan Gabor, director of a mining company, and his wife Adrienne. My life-long love of physics started suddenly at the age of 15. I could not wait until I got to the university, I learned the calculus and worked through the textbook of Chwolson, the largest at that time, in the next two years. I remember how fascinated I was by Abbe's theory of the microscope and by Gabriel Lippmann's method of colour photography, which played such a great part in my work, 30 years later. Also, with my late brother George, we built up a little laboratory in our home, where we could repeat most experiments which were modern at that time, such as wireless X-rays and radioactivity. Yet, when I reached university age, I opted for engineering instead of physics. Physics was not yet a profession in Hungary, with a total of half-a-dozen university chairs - and who could have been presumptious enough to aspire to one of these?

So I acquired my degrees, (Diploma at the Technische Hochschule Berlin, 1924, Dr-Ing. in 1927), in electrical engineering, though I sneaked over from the TH as often as possible to the University of Berlin, were physics at that time was at its apogee, with Einstein, Planck, Nernst and v. Laue. Though electrical engineering remained my profession, my work was almost always in applied physics. My doctorate work was the development of one of the first high speed cathode ray oscillographs and in the course of this I made the first iron-shrouded magnetic electron lens. In 1927 I joined the Siemens & Halske AG where I made my first of my successful inventions; the high pressure quartz mercury lamp with superheated vapour and the molybdenum tape seal, since used in millions of streeet lamps. This was also my first exercise in serendipity, (the art of looking for something and finding something else), because I was not after a mercury lamp but after a cadmium lamp, and that was not a success.

In 1933, when Hitler came to power, I left Germany and after a short period in Hungary went to England. At that time, in 1934, England was still in the depths of the depression, and jobs for foreigners were very difficult. I obtained employment with the British Thomson-Houston Co., Rugby, on an inventor's agreement. The invention was a gas discharge tube with a positive characteristic, which could be operated on the mains. Unfortunately, most of its light emission was in the short ultraviolet, so that it failed to give good efficiency with the available fluorescent powders, but at least it gave me a foothold in the BTH Research Laboratory, where I remained until the end of 1948. The years after the war were the most fruitful. I wrote, among many others, my first papers on communication theory, I developed a system of stereoscopic cinematography, and in the last year, 1948 I carried out the basic experiments in holography, at that time called "wavefront reconstruction". This again was an exercise in serendipity. The original objective was an improved electron microscope, capable of resolving atomic lattices and seeing single atoms. Three year's work, 1950-53, carried out in collaboration with the AEI Research Laboratory in Aldermaston, led to some respectable results, but still far from the goal. We had started 20 years too early. Only in recent years have certain auxiliary techniques developed to the point when electron holography could become a success. On the other hand, optical holography has become a world success after the invention and introduction of the laser, and acoustical holography has now also made a promising start.

On January 1, 1949 I joined the Imperial College of Science & Technology in London, first as a Reader in Electronics, later as Professor of Applied Electron Physics, until my retirement in 1967. This was a happy time. With my young doctorands as collaborators I attacked many problems, almost always difficult ones. The first was the elucidation of Langmuirs Paradox, the inexplicably intense apparent electron interaction, in low pressure mercury arcs. The explanation was that the electrons exchanged energy not with one another, by collisions, but by interaction with an oscillating boundary layer at the wall of the discharge vessel. We made also a Wilson cloud chamber, in which the velocity of particles became measurable by impressing on them a high frequency, critical field, which produced time marks on the paths, at the points of maximum ionisation. Other developments were: a holographic microscope, a new electron-velocity spectroscope an analogue computer which was a universal, non-linear "learning" predictor, recognizer and simulator of time series, a flat thin colour television tube, and a new type of thermionic converter. Theoretical work included communication theory, plasma theory, magnetron theory and I spent several years on a scheme of fusion, in which a critical high temperature plasma would have been established by a 1000 ampere space charge-compensated ion beam, fast enough to run over the many unstable modes which arise during its formation. Fortunately the theory showed that at least one unstable mode always remained, so that no money had to be spent on its development.

After my retirement in 1967 I remained connected with the Imperial College as a Senior Research Fellow and I became Staff Scientist of CBS Laboratories, Stamford, Conn. where I have collaborated with the President, my life-long friend, Dr. Peter C. Goldmark in many new schemes of communication and display. This kept me happily occupied as an inventor, but meanwhile, ever since 1958, I have spent much time on a new interest; the future of our industrial civilisation. I became more and more convinced that a serious mismatch has developed between technology and our social institutions, and that inventive minds ought to consider social inventions as their first priority. This conviction has found expression in three books, Inventing the Future, 1963, Innovations, 1970, and The Mature Society, 1972. Though I still have much unfinished technological work on my hands, I consider this as my first priority in my remaining years.

Honours
Fellow of the Royal Society, 1956.
Hon. Member of the Hungarian Academy of Sciences, 1964.
D.Sc. Univ. of London, 1964, Hon. D.Sc. Univ. of Southampton, 1970, and Technological University Delft, 1971.
Thomas Young Medal of Physical Society London, 1967.
Cristoforo Colombo Prize of Int. Inst. Communications, Genoa, 1967.
Albert Michelson Medal of The Franklin Institute, Philadelphia, 1968. Rumford Medal of the Royal Society, 1968.
Medal of Honor of the Institution of Electrical and Electronic Engineers,1970. Prix Holweck of the French Physical Society, 1971. Commander of the Order of the British Empire, 1970.
Married since 1936 to Marjorie Louise, daughter of Joseph Kennard Butler and Louise Butler of Rugby.