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Sisir Kumar Mitra
A Pioneer in Radio Physics
Dr Subodh Mahanti


Sisir Kumar Mitra’s decision to change over to radio research was undoubtedly a bold one. Radio was then a new born science still in its teething stage. It had not found place in the curriculum of any of the Indian universities, not to speak of research facilities in the subject. Mitra recognized this difficulty but was not deterred.

J.N. Bhar in Biographical Memoirs of Fellows of the National Institute of
Sciences of India, Vol.1, 1966

Sisir Kumar Mitra is the doyen of radio science in India. He is known for his seminal work on ionosphere. The ionosphere, that extends from about 60 km to several thousand kilometres high in the atmosphere, plays a major role in long distance radio communications. The air in the ionosphere is ionized. The name `ionosphere’ was proposed by Robert Alexander Watson Watt, (1892-1973) a British engineer. The first experimental evidence for the existence of ionosphere was provided by Edward Victor Appleton (1832-1965) and Samuel Jackson Barnett (1873-1956) in 1925. The ionosphere has been characterized by different layers like F, E, D, and C in order of decreasing attitude. The existence of E-layer was first predicted by Oliver Heaviside (1850-1925) and Arthur Edwin Kennely (1861-1989) and the F-layer was discovered by Appleton. The first experimental evidence of E-region of the ionosphere was obtained by Mitra and his coworkers in 1930. Mitra and his co-worker P Syam announced the reception of regular echoes from heights around 55 km and they called it D-layer, the existence of which was originally reported by Appleton in 1928. Mitra’s group detected echoes from as low as 20 km. The very low level reflections were believed to come from a hitherto unsuspected layer. Mitra called this layer C-layer. One of the most important works of Mitra was his explanation for the Appleton ionization anomaly.

Mitra introduced `wireless’ to the post-graduate course in physics at the Calcutta University. This marked the beginning of radio science teaching in India. He established a strong ionospheric research school at Calcutta. It was under Mitra’s leadership a full-fledged Department of Radio Physics and Electronics was created in the Calcutta University. This was the first teaching and research department in India in Radio Physics, which later became the Institute of Radio Physics and Electronics. India’s first entry into organized international scientific research was made possible by Mitra, when his laboratory participated in the second international polar year (IPY2-1932), for specific investigations on different aspects of the ionosphere. He established the first ionospheric field station at Haringhata, about 45 km north of Kolkata (then Calcutta) for ionospheric investigations in 1950. Mitra played a pioneering role in radio broadcasting in India.

To his students, Mira was a great source of inspiration. He used to emphasise that only with sustained efforts one can be successful as a researcher. He was against the concept of rigidly specified task and he never exercised rigid control over his research students. He believed that one could be a successful teacher in higher education only if one had oneself made some contribution to the subject.

Sisir Kumar Mitra was born in Kolkata on 24 October 1890. His parental home was at Konnagar in Hoogly district. His father Joykrishna Mitra was a school teacher. Joykrishna married Saratkumari, who came from a Brahmo family, in 1878 against the wishes of his parents. As a result he was disinherited from his parental property and he had to leave his parental house. The newly married couple moved to Midnapur, the hometown of Saratkumari, where they lived about a decade. In 1889, they moved to Kolkata, where Joykrishna started teaching in a school. At the time of Sisir Kumar’s birth, his mother was a student in the Campbell Medical School. After passing the final examination in 1892, Saratkumari got an appointment in Lady Dufferin Medical Hospital in Bhagalpore in Bihar and the whole family moved there. Joykrishna also managed to get an appointment as clerk in the local municipal office.

Mitra started his school education in the Bhagalpore Zilla (District) School. After passing the entrance examination from the Zilla School Mitra joined the T.N.J. College, Bhagalpore, from where he passed his FA Examination. He returned to Kolkata in 1908 to join the Presidency College as a B.Sc. student where he came into contact with J.C. Bose and P.C. Ray, the two pioneers of modern scientific research in India. He used to be fascinated by instruments designed and constructed by J.C. Bose for studying the properties of microwave and to detect various responses of plants. In 1912, he passed out of Presidency College taking his M.Sc. degree in physics and topping the list of successful candidates.

The environment of the Presidency College had a strong influence on Mitra. He had made up his mind to dedicate himself to scientific research. And he also got an opportunity to work under Jagadis Chandra Bose in the Presidency College immediately after his MSc results were out. In those days it was not easy to get a research fellowship. His widowed mother, who was supporting his education at Kolkata, badly needed some financial help to run the family. Her own income was not sufficient. So when Mitra got an appointment as Lecturer in the T. N. J. College at Bhagalpore, from where he passed his FA Examination, he left Kolkata to join the same. From Bhagalpore he shifted to Bankura in West Bengal on being appointed a lecturer in the Christian College. He spent four years teaching in colleges but at the same time harbored a burning desire in his mind to pursue a research career. Of course, there was no scope for carrying out researches in these mufassil colleges. But then a person like Mitra could not sit idle. So he started devising ingenuous demonstration experiments with whatever he could collect from the impoverished college laboratory, for illustrating his class lectures. He also started writing popular science articles in Bengali.

Mitra finally got an opportunity to realize his dream of carrying out research. While Mitra was teaching in College, Sir Asutosh Mookerjee, the then Vice-Chancellor of the Calcutta University was trying to initiate postgraduate teaching and research in science. With the untiring efforts of Sir Asutosh, the University College of Science was established in 1916. Mitra was one among those who were invited by Mookerjee to join the newly created Department of Physics in the newly created University college of Science. Mitra, who was then twent-six years old, joined the Department as Lecturer. This marked the beginning of Mitra’s outstanding scientific career. C. V. Raman joined the Department as Palit Professor. Mitra started working under Raman’s guidance on interference and diffraction of light. Mitra worked in the laboratory of C.V. Raman in the Indian Association for the Cultivation of Science. It took him only three years to complete his doctorate thesis which enabled him to get a D.Sc. from Calcutta University in 1919.

In 1920 Mitra went to the University of Sorbonne in Paris where he joined the research group of Charles Fabry (1867-1945), who had discovered ozone in the upper atmosphere in 1913. At Fabry’s Laboratory Mitra worked on the determination of wavelength standards in the region 2000-2300 angstrom of the copper spectrum. Based on this work he got his second doctorate in 1923. From Sorbonne he went to work under Marie Curie at the Institute of Radium. However, he did not stay there long. He went to work with Prof. Gutton, who was working on radio valve circuits in the Institute of Physics at the Universtiy of Nancy, Paris. Though he spent only a few months at Gutton’s Laboratory but it was here that he finally made up his mind to make his career in radio research.

Why did he choose to shift to radio research though he was successful in spectroscopic research? “May be”, as J. N. Bhar wrote, “he had been so deeply influenced by Bose’s experiment on wireless waves that a desire for doing research in this line lay hidden in his subconscious mind”. It may be noted that Mitra witnessed Bose’s experiments when he was a student in Presidency College. Whatever might be the reason it was certainly a bold decision. When Mitra took his decision to pursue a career in radio science, it was still in its infancy. Radio science was not a part of the curriculum of any university in India and so there was no question of availability of any research facility in the subject. Without being deterred by these obstacles he wrote to Sir Asutosh Mookerjee about his decision. Sir Asutosh in his reply dated 10th May 1923 wrote: “I am glad to receive your letter dated 18th April and to hear that you have been so successful in your work. The course of investigation you suggest as to signals by wireless telegraphy is very attractive. Do please draw up a scheme and make it as inexpensive as possible. I shall see what we can do. But you must rest assured that there will be plenty of opposition. That need not frighten us; we shall have to fight our way through.”

Mitra returned from France at the end of 1923 and he was appointed Khaira Professor of Physics. He became a part of the team consisting of C. V. Raman, D. M. Bose and others who organised the post-graduate teaching in physics in the Calcutta University. Mitra’s task was to establish the `wireless section’ for teaching and carrying out research in the new subject. With the active support of Sir Asutosh Mookerjee, Mitra was able to start for the first time postgraduate teaching and research on wireless in India.

To have an understanding of the significance of Mitra’s work one should have an idea of ionosphere and what was the status of its understanding when Mitra started his work. The presence of ionosphere is vital for long distance radio communication. Ionosphere is a region of the upper atmosphere that reflects short radio waves enabling transmission to be made round the curved surface of the earth by sky waves. All regions of our atmosphere are defined in terms of ionization, temperature and composition. The region which extends form 60 km to several thousand kilometers above the earth is called ionosphere. The existence of an ionized layer in the atmosphere or the ionosphere was first suspected in 1882 by the Scottish physicist Balfour Stewart. He proposed the existence of a layer of air capable of conducting electricity in the upper atmosphere, and whose presence, Stewart thought, was the reason for the continuous minor variation in Earth’s magnetic field. However, Stewart’s prediction remained mostly unnoticed till 1901 when Guglielmo Marconi (1874-1937) succeeded transmitting wireless signals across the Atlantic Ocean, a distance of about 2800 kilometres. For physicists this was an unexpected achievement. Because like light, radio waves also travel in straight lines and so their detection on the earth’s surface should not be possible beyond the line of sight. To explain the propagation of radio waves the existence of an electrically conducting air layer, as proposed by Stewart, was again invoked. This was in 1902. And this time this was proposed independently by Oliver Heavside and Arthur Kennelly. They proposed that such a layer situating about at a height of 80 km in the atmosphere would act as a repeater station by reflecting radio waves at a considerable distance beyond the horizon. The proposed layer came to be known as Kennelly-Heaveside layer, the existence of which was experimentally proved in 1924 by Edward V. Appleton and Samuel Jackson Barnett. They devised two alternative methods –the angle of incidence method and the frequency change method -- to determine experimentally the location of the ionized layer which vary in behavior with the position of the sun and with sunspot cycle.

G. Breit and M. A. Tuve in the United States devised a technique for determining the height of the reflecting region. Radio waves travel with the speed of light. Thus the height of the reflecting region can be caculated if one can measure the time taken by the transmitted radio waves back to the earth. Breit and Tuve transmitted a short pulse of radio waves upward and then they determined the time taken for the pulse to be reflected back to the receiver, a few kilometers away, by means of an oscilloscope. In this way they found the altitude of the Kennely-Heaviside layer in the range of 80-100km.

Appleton, in the course of his investigation with radio waves of shorter wavelengths, another reflecting layer at the height of roughly 200 to 400 km. This layer was called Appleton layer. The names of these two layer were changed by Appleton as E (Kennelly-Heaveside layer) and F (Appleton layer) layers. This is the nomenclature now universally followed. Subsequently another layer, a layer below the E layer, was discovered. This layer was called D-layer. The D-layer which is located between 50 and 90 km altitude disappears during the day. The F-layer splits into two different regions, namely F1 and F2. The F1 region which exists only in daytime, has a peak density around 200km. In the F2 region, the altitude of the peak density occurs at about 300km in daytime and it shifts to higher altitude in the night. The E-region is between 90 and 160 km and the F-region (sometimes called the Appleton layer) is from 160 km up to about 400 km. These early experiments carried out by Appleton and his co-workers attracted the attention of Mitra and he decided to conduct similar investigations in his newly established laboratory. He could motivate a small team of young and enthusiastic scientists to take up this challenging work.

Mitra’s group could measure the heights of the different layers of the ionosphere by an instrument designed and built indigenously. The investigations carried out by Mitra’s laboratory not only provided the first general picture of the ionospheric condition in a sub-tropical region of low altitude like Calcutta but experimental results obtained by Mitra and his coworkers also threw considerable light on the effect of thunderstorm, magnetic storm and meteoric shower on upper atmospheric ionization. Mitra also gave a theory of the D-layer which was first reported by Appleton in 1928. D-layer is an absorbing layer formed during the daytime just below the E-layer, the echo from this layer is only occasionally observed. Mitra and his co-workers conclusively established the existence of this layer. The experimental evidence for the E-layer of the ionosphere was obtained for the first time by Mitra’s group. H. Rakshit working under Mitra developed the first indigenous equipment for ionospheric studies by the angle of incidence method. The Calcutta Station of the Indian State Brodcasting Service made available its medium wave transmitter to Mitra’s group. They often broadcast transmissions outside regular programme period. Utilising these facilities Mitra’s group experimentally proved the existence of the E-layer.

Mitra’s group detected reflections from as low as as 20 km. Mitra proposed that such low-level reflections were coming from a hitherto unsuspected layer. Mitra called this layer C-layer. At the beginning the discovery was received with certain amount of skepticism by the western scientists, but later it was corroborated by many observers in England and America. The echoes originating from such a low layer may be due to reflection from the temperature discontinuities and from moisture layer in the lower atmosphere.

While developing teaching and research facilities at the Calcutta University Mitra got involved in the development of broadcasting in India. A private company called the Indian States and Eastern Agency had installed the country’s first broadcast transmitter in Kolkata in 1923. The Radio Club of Bengal, a registered society, under the Chairmanship of J.R. Stapleton, published a journal called “Radio“ for creating interest in this newly found subject by providing relevant information. In 1926, before the establishment of the Indian Broadcasting Company in 1927, Radio Club’s transmitters at Dalhousie Square broadcast regular programme every evening. Mitra and his coworkers constructed another transmitter and installed in the newly established Wireless Laboratory in the University College of Science in Calcutta. Its call sign was 2 CZ. It is worthwhile to note that there was a time when Mitra’s transmitter in his Wireless Laboratory and the Radio club’s transmitter at Dalhousie Square were the only ones broadcasting regular programme in the Eastern region of India.

The wireless laboratory established by Mitra started working on the measurement of the atmospherics and for this purpose a huge aerial connected to a valve amplifier and an automatic recording devise was set up.

With the financial assistance from the Council of Scientific and Industrial Research, Mitra established a field station for ionospheric work at Haringhata- a village about 50 km from Calcutta following a suitable capital and recurring grant sanctioned by the council of Scientific and Industrial Research in 1949. An ionosounde, donated by the Commonwealth Scientific and Industrial Research Organization of Australia was installed in the station. This was the first ionosphere field station of its kind in India. In 1955, the station started round the clock observations of the ionosphered.

Mitra’s well-know treatise, The Upper Atmosphere, was his major scientific contribution. The idea of writing such a book arose in Mitra’s mind in 1935. In that year the National Institute of Sciences of India (later renamed as Indian National Science Academy) organised a symposium on ionosphere. Mitra was invited to open the symposium, which was held in August 1935. His opening address was titled `Report on the Present State of our Knowledge of the Ionosphere’. The Report, which was a comprehensive account of the existing information on the ionosphere, was welcomed by scientists working on ionosphere all over the world. This prompted Mitra to think of writing a treatise on the upper atmosphere, of which the ionosphere was only a part. It took ten years to finalise the manuscript. This was the `First heroic attempt’, as Sir Edward Appleton remarked, to put together in one volume the results of investigations in many different fields all concerned directly or indirectly with the upper atmosphere. It is worthwhile to quote here A. P. Mitra, one of Sisir Kumar Mitra’s illustrious students on the significance of The Upper Atmosphere. A. P. Mitra wrote: “The Upper Atmosphere was a milestone in atmospheric science. It considered for the first time the atmospheric environment as a whole, neutral and ionized, its thermal structure and distribution of constituents, its motions, the interaction of solar radiation and particle stream with these gaseous constituents, and the mechanism of airglow. The ionosphere was treated as only a part of this vast panorama and interlinked the Sun, the Earth and the atmosphere. This was then an entirely new concept. Secondly, deviating from the then existing practice of studying the ionosphere from the point of view of propagation of radio waves, Mitra viewed the exploring radio waves as a remote sensing tool, sensing levels which could not be reached were balloons and with only beginning to be explored by rockets…Even in this vastly changed (and greatly expanded) canvas, much of the information given in The Upper Atmosphere and the results of his scientific investigation stand as benchmark.” There were other books of high repute on ionosphere like S. Champan and D J Bartel’s Geomagnetism and J.A. Fleming’s Terrestrial Magnetism and Electricity, but they dealt with specialized topics. So Mitra’s book became indispensable as a reference book for all those who were interested in upper atmospheric research. However, the publication of the treatise was not a smooth affair. Mitra and his colleagues who worked very hard to finalise the manuscript wanted to get it published by a reputed foreign publisher. Accordingly Mitra wrote to a few reputed publishers giving the list of contents. But none came forward to undertake its publication. It is interesting to quote the letter that he got from one of the publishers rejecting his manuscript. The publishers wrote: “We have given very careful consideration to your letter of May 16th. Your name, of course, is well-known to us, and we naturally are assured that your treatise on the Upper atmosphere is an admirable work. We are sorry to say, however, that for various reasons we do not think it would be a practicable proposition for us to undertake its publication.

In the first place, from what you say, it is a very large book and would be extremely expensive to produce even in India. Of course, it would be better to have it printed in this country, but that would be even more expensive. From previous experience of books of this nature we feel very doubtful whether it would have a large enough sale to cover the expense of publication; in fact we anticipate that it would involve us in a considerable financial loss.

Books of this kind really ought to be published by the University Presses who exist largely in order to publish learned works whose appeal is very limited. A further consideration is that even in the small field covered by your book it would have to compete with Chapman & Bartel’s Geomagnetism and works by Sir Napier Shaw.

On the whole, therefore, we are very sorry to say that we can only thank you and regret that we feel unable to publish the work. We are returning the list of contents which you kindly sent us.”

There is no wonder that such a reception from the publishers disappointed Mitra and his groups. At this juncture Meghnad Saha came in their rescue. At that time Saha was the President of the Asiatic Society of Bengal. Saha decided that the Asiatic Society would undertake the publication of the Mitra’s manuscript, however costly the publication might be. Thus the Asiatic Society undertook its publication as one of its Memoirs and the first edition of The Upper Atmosphere came out in 1947. The book was widely appreciated and the first edition of 2000 copies was sold within three years. This was certainly a great achievement. In 1952 the Asiatic Society brought out a revised edition of the book The entire book was translated in Russian and was published by the foreign Language Publishing House, Moscow in 1955.

Another scientific problem in which Mitra got involved was that of active nitrogen which he thought would solve the problem of the night sky luminescence. He believed that the faint glow in the night sky was due to mutual neutralisaiton of ions and electrons in the upper atmosphere. According to the theory propounded by Mitra in 1943, the after glow is emitted in the act of neutralisaton of N2+ ions by recombination with electrons which is a three-body collision process. The persistence of the glow was due to the fact that the third body is rarely found in the upper atmosphere and so the recombination process was delayed. In 1945 Mitra published a book titled “Active Nitrogen: A New Theory”. Mitra’s theory was criticized by pointing out that the presence of N2+ ions in the glowing gas could not be experimentally detected. He modified his theory by proposing that active nitrogen is a mixture of nitrogen atoms in the ground state and metastable state, a product of dissociative recombination of N2+ and electrons. So N2+ ions are not the active substance but they together with electrons are the parent bodies of the active substances.

In the late 30s Mitra felt the necessity of having an independent postgraduate department of electronics and radiophysics and in 1945, he submitted a proposal to the Calcutta University for creating such a department. Though the University accepted the proposal, it could not be implemented due to financial constraints. The Department came into being in 1947 with the availability of funds from Ministry of Education, Government of India. The Department was transformed into the Institute of Radio Physics and Electronics, which was later selected by the University Grants Commission, as one of the first five centers of advanced study under a scheme sponsored jointly with UNESCO.

It was Mitra who felt the need of an all India Radio Research Organisation. As a first step towards realization of this goal Mitra decided to win the support of the prominent British scientists for his proposal. In his lecture before the Maxwell Society in 1936 Mitra not only highlighted the need of greater cooperation between the ionospheric scientists working in different countries but he also raised the question of establishing a co-ordinating body in India on the lines of Radio Research Board of England. For discussing the desirability of creating such a Board in India he invited leading British scientists to a dinner on May 05, 1936. The guests included Sir E. V. Appleton, Sydney Chapman (1888-1970), Sir Robert Alexander Watson-Watt (1892-1973), Edward Neville da Costa Andrade (1887-1971), R. A. Gregory, the distinguished editor of Nature and many others. All the assembled guests strongly favoured the idea mooted by Mitra. Gregory thought it fit to write an editorial on it in Nature. He wrote: “The time would now appear to be very opportune for considering the establishment of a similar Radio Research Board in India, where fundamental research in radio communication has so far been limited to the activities of quite small bands of workers in different universities, notably those under Prof. S. K. Mitra at Calcutta and under Prof. M. N. Saha at Allahabad…It is surely time that India was able to take its place in a such a world-wide scheme (of radio research), and it is to be hoped that those in a position to do so will foster the inauguration of a suitable Radio Research Board and provide the necessary funds to initiate its work. The research already carried out in India indicates that the Universities are ready to provide a programme of problems of a fundamental nature, and even the nucleus of a staff of trained personnel, keen and enthusiastic to continue their investigations which are at present being limited through lack of resources.” After coming back to India Mitra launched a strong campaign for establishing a Radio Research Board. But while his proposal was strongly supported by the British scientists but that did not have much impact on the authorities in India. Meghnad Saha, who was then at Allahabad, lent strong support to Mitra’s cause. And when Saha came back to Kolkata in 1938 they together renewed their campaign with greater vigour. Finally the Radio Research Committee was formed in 1942 under the Department of Scientific and Industrial Research, then functioning under the Department of Commerce, Government of India. Mitra was appointed as the Chairman of the Committee. This was undoubtedly a major step in promoting radio research in the country. After the appointment of the Radio Research Committee a number of radio research centers grew up in the country.

Mitra’s concern for industrial development in the country is well known. While referring to the creation of the Council of