The aim of this contribution is to examine the major themes of Poynting's research, their unfolding during his life and (appropriately at a meeting concerned with Poynting and Lodge who kept up a long friendship built around their work) his interactions with his contemporaries, particularly important since his own letters are mostly lost, and we must look for evidence in the archives of others. My active interest in Poynting over the past decade has been more concentrated on more personal material, searching for lost letters and family reminiscences, to capture this volatile material before it evaporates. I am particularly grateful to a granddaughter, Mrs Elizabeth Ratcliffe who has placed a box file of memorabilia in the care of Birmingham University Library. Fragmentary as many items may be, they can throw useful light on more formal knowledge. There is little chance now of more learning of this kind, and one of the aims of this paper is to sketch a program for the future,
1. Chronology
We must start with significant dates in Poynting's life, from his birth in 1852. According to J.J. Thomson, a friend from Owens College days, his formal education began in his father's school. Thomas Elford Poynting was a Unitarian Minister (at Monton Church, Eccles - now Salford). It was quite usual, for both faith and economic reasons, for Unitarian Ministers to conduct schools, and John Henry and his elder brothers started their education in this way. Thomas Elford himself had a deep interest in science, but systematic study was not possible in the Unitarian College (Manchester) where his own hard-won formal education was completed and the older universities were denied him by the Test Acts*, and doubly so by the economic stringency of his situation and the demands of a substantial family. None the less, his enthusiasm was passed on to his youngest son, and a foundation laid so that in 1867 he entered Owens College, Manchester, with a Dalton Entrance Exhibition in mathematics, to prepare for London Matriculation, the key to all his future achievements. He was successful in 1869, and then embarked on a B.Sc. London Degree course in Maths and Physics at Owens, and achieved the degree in 1872, entering Trinity College Cambridge with an entrance Scholarship in October of the same year.
For the Maths Tripos he 'read with' E.J. Routh who had an enviable reputation for producing candidates near the top of the Tripos list. Routh's role resembled that of a freelance private coach rather than the supervisor of today. A list survives, in Poynting's handwriting, of the wranglerships amongst Routh's clients, but whether his genius resided in tutorial skills or a prescience in selecting high fliers is not clear. In any case in 1876 Poynting was bracketed 3rd wrangler. For the future it is significant that Routh and James Clerk Maxwell had been undergraduate "contemporaries and close friends at Trinity, and that, when Maxwell returned to Cambridge as Professor of Experimental Physics, the friendship was sustained.
For Poynting, further academic progress at Cambridge required a fellowship, which was awarded on an essay, and to sustain him during its preparation he accepted a demonstratorship at Owens College under Balfour Stuart. Significantly, a fellow demonstrator was J.J. Thomson, and the friendship established lasted throughout Poynting's life. In 1878, he returned to Trinity on a Fellowship, and joined Maxwell at the Cavendish, a collaboration which ended with Maxwell's sudden and untimely death in 1879. In the following year Poynting was appointed Professor of Physics at Mason Science College, in the same year marrying Maria Adney Cropper, the daughter of the Unitarian Minister at Stand, Lancashire.
The rest of Poynting's professional life was spent in Birmingham, and when Mason Science College became the University of Birmingham in 1900, he was appointed Dean of the Faculty of Science, a position to which he was repeatedly reappointed until 1912, when poor health compelled him to relinquish it. In March 1914, he died of influenza, his condition exacerbated by diabetes. His health had been poor for many years, and he admitted that he was unable to sustain more than six hours work per day, which makes his achievements, both in physics and in the administration of the infant university the more remarkable.
As a memorial to him, colleagues and friends subscribed to produce a volume of collected work. Delayed by World War 1, this was not published until 1920. In addition to research papers, there are discussions of education policy (remember that, in the strict sense Birmingham was the first Civic University, and that, like all the newer institutions of higher education it was entering an unknown domain). Also to be found are extra-curricular lectures to students, and more formal ones for his colleagues in other departments, as well as popular expositions, for instance, for the Enquirer, a Unitarian journal. All these are characterised by a relaxed lucid style which makes them still a pleasure to read.
2. Research Papers
Turning to the Collected Scientific Papers, we can select those papers which time has revealed to be his most enduring work, and group them under the headings of electromagnetism and measurements of the gravitational constant G.
In the first of these, we designate the items as found in the collected papers by their date and title, and accompany them by a few brief comments. In the gravity measurements, the experimental work is a continuous background to his other work, and less attempt is made to distinguish individual papers (which tend to be fewer and longer), but rather to sketch out the experimental approach as it evolved.
(i) Electromagnetism
Maxwell's progress through electromagnetic theory was rapid, and left many outposts of ignorance, surrounded rather than overcome - for example matters of energy and momentum carried by electromagnetic waves. Here we have an area which Maxwell's early death left Poynting to explore.
The rapid progress Maxwell made with his electromagnetic theory left many (perhaps most) other workers sceptical or uncomprehending: particularly over the displacement current concept. The two papers named above are examples of Poynting's exegisis. The collection of Poynting's letters to Lodge at University College London - the only archived collection - reveal Poynting offering mathematical help to Lodge, as did others. The tone of the letters indicates a close professional friendship throughout, and the slightly bantering manner of the 1888 letter might have strained the relationship, but this originally private letter to Lodge was published at Lodge's express request.
(ii) Gravity measurement
In contrast to the papers on electromagnetic theory, this work is devoted to the accurate measurement of small quantities, with the patient elimination of potentially larger errors. Maxwell is on record as stressing the importance of accurate measurement, presumably in seeking confirmation of his electromagnetic theory, but his own experimental skills and interests were of a different nature. There is no evidence of discussions between Poynting and himself on this topic, but the temptation to search for the extension of the electromagnetic theory to other interactions, of which only gravity was then known, would be real enough.
Copies of letters from Poynting to scientific instrument manufacturers have recently come to light in Manchester, so Poynting must have initiated this work before taking up his Cambridge fellowship, and certainly his gravity balance was set up in Birmingham in his earliest years there. His chosen method was, and remained, based on the chemical balance, measuring the small change in deflection brought about by bringing up a large mass near to a small mass suspended from one arm of the balance beam, measuring it with a scale and telescope via an optical lever arrangement. The experiments, starting in the 1870's, continued until 1905, and throughout great pains were devoted to the attainment of maximum accuracy, and the observations are listed and tabulated in the major papers. What emerges is a labour of love rather than a necessary chore, and even when a whole year's data were rendered useless by the gradual settling of the building's foundation it is reported with philosophical calm.
In the quest for accuracy the Oertling chemical balance was replaced by a bullion balance of larger beam though this led to further difficulties with air currents. When the flamboyant C. V. Boys developed a torsion balance with very fine drawn quartz fibres and hence a miniature construction, Poynting was ready to admit that Boys' method was inherently more accurate than his, and his gentility of character, widely commented on, was revealed in its clearest terms. At the same time, Poynting did maintain that it was important for a variety of methods for G measurement to be used and pushed to their limits of accuracy lest they were in fact measuring slightly different things - that there was richness in the nature of gravitational force going beyond the simple inverse square law relationship. This of course is exactly what Faraday had revealed beyond the rather bald concept of Coulomb's law, forming the basis of Maxwell's theory, hence in turn Poynting's starting point.
In his later gravity measurements, Poynting investigated possible anisotropies in gravitational attractions (using quartz crystal spheres rather than steel ones), and also looked for temperature effects following Faraday's lead in the exploration of dielectric properties. Disappointingly he found no measurable effects but the attempt had to be made. In a sense then, we may view Poynting's research as a single study of fundamental interactions, experimental or theoretical according to circumstances.
3. Offices and awards
The scale of institutional science in Britain in the latter part of the nineteenth century was small. The newer universities were in their infancy, with small staffs in departments heavily committed to teaching (of necessity, because student fees were the dominant source of income). As we have seen, the Cavendish Laboratory in Cambridge, the precursor of large scale academic physics research in this country, was only founded in 1872, with J C Maxwell as its director. The first government institution directly concerned with physics was the National Physical Laboratory, founded in 1899 with Glazebrook, a contemporary of Poynting's, as director. But the scattered individuals concerned were able to interact constructively through the learned societies, with the Royal Society as the most prestigious, and the British Association the widest-reaching with its annual summer meetings held sequentially in major cities, attracting large numbers and great publicity. Further, with the growing specialisation of science, new bodies, like the Physical Society, came into being.
Poynting was elected Fellow of the Royal Society in 1888, and by 1899 was elected President of Section A (Physical Sciences) of the British Association for its Dover meeting. In 1913, though a sick man, he was Vice President of the same section at the Birmingham meeting. By 1905 he was President of the Physical Society, and in 1909 he was appointed to the Council of the Royal Society, becoming its Vice President in the following year. The esteem in which he was held by the scientific community is further indicated by the award of the Adams Prize (1891) and the Hopkins Prize (1893), both by Cambridge University, and by receiving the Royal Medal of the Royal Society in 1905, for his work on radiation. Thus in terms of academic prominence and centrality of scientific administration, he played a very important role. From the standpoint of this paper all this is important since, in the absence of much of his own archive, the archives of the learned societies can be explored in the hope of reaching a more detailed view - something which has yet to be undertaken.
4. Individual Influences
Since the essence of science is communication, we may be compensated for the loss of so much of Poynting's archive by those of his contemporaries with similar scientific interests.
We can see what might be possible by examining the collection of letters from Poynting to Lodge, with some copies of Lodge's replies, which is the sole documented correspondence of Poynting's, preserved in the library of University College London. The main thrust of these letters is Poynting's elucidation of Lodge's difficulties with the mathematics of Maxwell's treatise on Electricity and Magnetism.
There must, one feels, be correspondence between Poynting and Maxwell in the Maxwell archive, likewise between Poynting and J.J. Thomson, who produced between them a definitive Textbook of Physics over the years, despite their geographical separation. Larmor, who wrote a cordial obituary for Poynting (as appreciative if less intimate than Thomson's), Heaviside and Fitzgerald also come to mind, though their interests were drawn more towards the aether in which Poynting's interest seems more marginal, as we have seen. Further afield, Hertz must be considered, though there is no evidence of any correspondence on electromagnetic theory between Poynting and any European scientist (though there is a considerable amount on G measurements.)
All these considerations show that it is too early to conclude that we shall never achieve a more rounded view of Poynting's significance in nineteenth century Physics.
*The Test Acts were various statutes making eligibility for public office conditional on professing the established religion.