#AnniversaryStory
The ‘rogues’ gallery’ of graduate students in the Cavendish Laboratory from 1897 onwards is a popular display for alumni and visitors, who enjoy seeing themselves, their colleagues and lecturers as they were in their younger days. Questions are often asked about the women in the photographs. It should be emphasised that they are all research workers, on a par with the men. But women were active in the Laboratory almost from the beginning. Among the more remarkable of these was Nora (Eleanor) Sidgwick.
John William Strutt was elected to the Cavendish Professorship in 1879, following the death of Maxwell. On the death of Strutt’s father in 1873, he had succeeded to the Baronetcy as the third Lord Rayleigh. It was not a common occurrence for a senior member of the aristocracy and major landowner to become a professional academic, but Rayleigh had already demonstrated outstanding ability in theoretical and experimental physics. He had been senior Wrangler in 1865 and first Smith's prize winner. By the time his name came forward as a candidate for the Cavendish chair, he was already known for his explanation of the colour of the sky through the process of Rayleigh scattering and he had written profusely on a very wide range of topics in the physical sciences, including experimental researches carried out at the family home at Terling Place in Essex.
In 1871 Rayleigh married Evelyn Balfour, the sister of Arthur James Balfour who was a friend of Rayleigh's at Trinity College and who was to become Prime Minister of Great Britain in 1902 – he was also responsible for the Balfour Declaration of 1917. In 1872, Rayleigh, whose health was always somewhat weak, suffered a severe bout of rheumatic fever. For convalescence, he and his wife spent the following winter in Egypt, accompanied by Eleanor (Nora) Balfour, Evelyn's sister. Nora Balfour was an outstanding mathematician and she and Rayleigh discussed physics and mathematics continually throughout his convalescence in Egypt. During this period, Rayleigh began Volume 1 of his great and influential Theory of Sound, which was published in 1877; Volume 2 appeared in the following year.
In 1876 Nora married the moral philosopher Henry Sidgwick, who with Anne Clough had founded Newnham College, in the previous year – Nora had been one of the very first students at Newnham. Henry and Nora Sidgwick were pioneers in promoting the cause of women in the University and were deeply involved in the struggle to gain the admission of women to University examinations. This they achieved in 1881, but they lost the battle to allow the degrees to be conferred. Instead, women only received a certificate confirming their success in the examinations. The admission of women to degrees only began in 1948.
Maxwell had not been sympathetic to the presence of women in the Laboratory, their attendance being restricted to the summer term when Maxwell was at his home at Glenlair in Southern Scotland. The formal opening of all physics classes to women on equal terms with the men took place in 1882. Undoubtedly, Rayleigh's decision to admit women to the experimental physics courses was influenced by the views of Henry and Nora Sidgwick. Nora was appointed Vice-Principal of Newnham College in 1880 and then Principal in 1892.
Rayleigh continued his broad range of research interests throughout his tenure of the Cavendish Chair from 1879–1884 and took the decision to continue Maxwell's programme of the determination of electrical standards, but increasing their precision by an order of magnitude or more, very much in the spirit of Maxwell's dictum that new science would come from improving the precision with which the laws of physics and the fundamental constants were known. At that time, the standard of resistance was only known to about 4% and the unit of current, as measured from the electrochemical equivalent of silver, was only known to 2%.
Nora Sidgwick was particularly involved in the determination of the ohm and the standard of current electricity. The former experiment is described by Richard Glazebrook,
"A circular disc rotates about an axis perpendicular to its plane in a magnetic field due to a concentric coil. By balancing the fall in potential between the centre and the edge of the disc against that due to the passage through a resistance of the current producing the field, the value of the resistance is given by the formula R = nM, where R is the resistance, M the coefficient of mutual inductance between the coil and the disc and n is the number of revolutions of the disc per second."
Fortunately, Rayleigh had at hand a pair of coils which had been very carefully wound by George Chrystal so that the dimensions of the coils were very precisely known (Fig.1(a) and (b)). Rayleigh and Sidgwick describe delightfully how they made use of the existing apparatus.
"… the diameter of the disc was chosen so as to be somewhat more than half that of the coils. … The disc was of brass and turned upon a solid brass rod as axle. This axle was mounted vertically in the same frame that carried the revolving coil in the experiments described in a former communication, an arrangement both economical and convenient, as it allowed the apparatus then employed for driving the disc and for observing the speed to remain almost undisturbed. The coils were supported horizontally upon wooden pieces screwed on the inner side of the three uprights of the frame."
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(a) |
(b) |
(a) Rayleigh’s rotating coil experiment. In the original experiment, the large coils were rotated using a water engine and the speed of rotation determined accurately by a stroboscopic arrangement. In Rayleigh and Sidgwick’s experiment, the rotating coil was removed and replaced by the coils wound by Chrystal (b).The coils in (b) were placed together horizontally and the copper disc rotated about the vertical axis by the water engine. (Exhibits from the Cavendish collection of scientific instruments.)
The experiment involved working out the theoretical value of M, the coefficient of mutual inductance, and great attention had to be made to the many corrections to the simple relation R = nM. The final result of their experiments was: 1 B.A. unit = 0.98677 x 109 C.G.S. units.
Two further contributions to the establishment of electrical standards were undertaken with the same meticulous care for exact measurement. The first was the determination of the absolute value of the unit of current in terms of the amount of silver deposited by electrochemical action. Again, quoting Glazebrook,
"A coil was suspended from one arm of a balance with its plane horizontal and midway between two fixed coaxial coils of larger radius. The electrodynamic attraction between the suspended and f ixed coils when carrying the same current can be balanced by weights in the opposite pan; it can also be calculated in terms of the current and the dimensions of the two coils; the current is thus measured absolutely in terms of the weight and the dimensions of the coils. If the same current also traverses a solution of nitrate of silver in a platinum bowl suitably arranged, it can also be measured in terms of the weight of silver it deposits, and thus we can express a current whose value is known in absolute units in terms of the silver deposited."
The paper by Rayleigh and Sidgwick is a very impressive achievement, considering in detail every aspect of the experiment and the problems of obtaining a precise result. The final answer was that: The number of grams of silver deposited per second by a current of 1 ampere = 0.00111794.
As part of the same experiment, Rayleigh and Sidgwick determined the electromagnetic force of the standard wet chemical cell at the time, the Clark cell, invented by Josiah Latimer Clark in 1873. The feature of the cell was that it produced a highly stable voltage, which they found to be 1.4345 volts at 15 C. This was essentially the international standard adopted in 1894.
These experiments were central to establishing the reputation of the Laboratory at the leading edge of precise measurement. These standards were of fundamental importance for industry since secondary standards could then be calibrated relative to these absolute standards. Nora Sidgwick’s contributions were central to these endeavours and an inspiration for the future generations of women research workers in the Laboratory.
This article has been written by Emeritus Jacksonian Professor Malcolm Longair and originally appeared in CavMag Issue 11, February 2014.
Main Image: Portrait of Eleanor (Nora) Sidgwick by James Jebusa Shannon, painted in 1889. © Newnham College, University of Cambridge
This article is part of our 150th Anniversary Cavendish stories.