Report on Measure for Measure Conference

Martin Vlietstra attended the St Cross Centre for the History and Philosophy of Physics (HAPP) one-day conference in Oxford on Saturday 7 June 2025 and has written a report about the conference for Metric Views.

The one-day Conference “Measure for Measure: A History of Measurement,” sponsored by the St Cross Centre for the History and Philosophy of Measurement¹ took place in the Andrew Wiles Building on 7^th June 2025. Five lectures were delivered by eminent researchers, starting with a historian who had studied units of measure used by ancient civilisations and ending with a lecture on the philosophy of measurement. Each lecture was followed by an opportunity to put questions to the speakers. The third lecture, which was devoted to the development of SI, was probably the most applicable to UKMA members with the other lectures putting SI into perspective.

One theme that came across in many of the lectures was that any unit of measure should be accessible, useful and consistent.

• The term “accessible” means that the unit could easily be verified – for example, a “foot” is accessible as most people have two feet.
• The term “useful” means that the unit must also be practicable – for example stating that the direct distance between the London Eye and the Embankment is three minutes’ walk [the author’s example!] is not very practicable as the Thames is about 250 m wide at that point!
• The term “consistent” is self-explanatory – the original definition of a foot does not produce a consistent unit of measure as my foot is probably different in length to your foot. In contrast, the length of a metre is the same worldwide.

Each of the five lectures are now described.

James Vincent:² Ancient Measures from the Body to the State

The first speaker introduced the concept of measurement as it applied to ancient civilisations where the various parts of the body were used – the foot, the finger, the palm, the hand, the cubit and the fathom to mention a few. He went on to explain that these units had been found by archaeologists on measuring sticks used by the ancient Egyptians.  He went on to explain how standardisation had crept into the Egyptian state, one of the important reasons being the remarking of boundaries after the Nile’s annual flood subsided. He also mentioned that the ancient Egyptians had two different cubits – the common cubit and the royal cubit, but the hieroglyphics did not explain how and when each was used. After standardisation had taken place, one of the “sins” for which ancient Egyptians were expected to “confess” before death was falsification of measures.

Dr Jane Wess:³ A History of Measuring Instruments

Dr Wess described many measuring instruments. The earliest devices were the measuring sticks and sundials used in Ancient Egypt. She went on to show photographs illustrating measuring devices including weighing devices, internal callipers and external callipers that had been recovered from the ruins of Pompeii. Other instruments included a variety of tools used for navigation and surveying dating from Roman times to the early 20^th century. She then moved on to various clocks showing pictures of the earliest clocks (mounted in church towers) and then to the Harrison pocket watches used by navigators in the late eighteenth century followed by a decimal clock dating from the French Revolution. Nineteenth century measuring devices included a variety of galvanometers which reminded the audience (or at any rate me) that the standardisation of electrical units of measure, which was pioneered by Maxwell and Thompson (Lord Kelvin), used metric units as their basis.

Prof Richard Brown:⁴ The Measure of All Things: The Development of the Metric System and the International System of Units

From a UKMA perspective, Professor Brown’s lecture was the most informative. In the first part of his lecture, he outlined what he considered to be the most significant events that led up to the signing of the Metre Convention. The events that he chose were:

• 1668 – John Wilkin’s Paper on a Universal Measure
• 1791-1799 – The development of the metric system during the French Revolution
• 1812-1837 – The establishment of mesures usuelles in France
• 1851, 1855 – The Great Exhibitions of London and Paris respectively
• 1867 – Geodetic standards and the Expositions Universelles (Berlin)
• 1872 – Invitations to the Metre Convention issued by the French
• 1875 – Signing of the Metre Convention on 20 May 1875
• 1889 – Acceptance of the new international prototype metres and kilograms by the CGPM

During his talk, Prof Brown highlighted the refusal of the British Government to sign up to the Metre Convention in 1875 and only finally signed up in 1884 when they wished to calibrate the standard yard against the standard metre. Prof Brown also drew to attention that it was a British company, Johnson Mathey who provided the alloys needed to cast the prototype metres and kilograms.

He then skipped over the history of the development of the metric system, picking up the thread again in 1948 when it was proposed to standardise electrical measurements. The resulting standardisation resulted in the setting up in 1960 of SI with six base units. The mole was adopted as a base unit in 1971 when the physicists and the chemists agreed to use the Carbon 12 atom as the basis for a common definition of the mole.

After describing how the definition of the metre had changed – in 1889, it was defined as the length of the international prototype metre, in 1960, it was defined in terms of the wavelength of light emitted by a Krypton-86 lamp and in 1983, in terms of the speed of light. This led to a discussion of how, in 2017, the CGPM agreed to redefine all the base units, apart from time, on physical constants of nature. Time remains tied to a substance – namely the frequency of light emitted by a specified transition of the electrons in a caesium 133 atom.

On being asked on the future of the SI, he suggested that in the 2030’s one might see a redefinition of the second though he gave no hint as to how it might be redefined.

Prof Hasok Chang⁵: ‘To see the world in a grain of sand’: Measuring unobservables through the ages.

Professor Chang’s talk centred on measuring quantities that we cannot see. The first example that he gave how Eratosthenes measured the circumference of the earth. He knew that at the equinox, the sun was directly overhead at Syene (modern day Aswan) because it shone directly down a well. At the equinox, he measured the angle of the sun’s rays at midday at Alexandria and knowing the distance between Alexandria and Syene and assuming that the earth was spherical he calculated that the earth’s circumference was about 252,000 stadia. Depending on the value of the stadium, this gave a value of width between 2.4% below and 0.6% above the currently accepted value.

He then gave a discourse on the measurement of temperature and how, when we measure temperature, we make the assumption that the material we use in the thermometer expands linearly with temperature. Can we justify this assumption? He went on to cite other similar problems and pointing out the assumptions we make along the way – for example, was Eratosthenes justified in assuming that the earth was a sphere?

Dr Jo Wolff:⁶ Can we be Realists about Measurement

Dr Wolff examined the philosophy behind the definitions and use of units of measure. In particular she asked what is meant by assigning numbers to measurements. As an example, she stated the obvious: “1+2=3”. Shen went on to point out that if we took a balance and placed a 1 lb weight and 2 lb weight alongside each other on one scale-pan, it would be balanced by a 3 lb weight in the other scale-pan. If we renamed the weights to 454 g, 907 g they would still be balanced by a weight that was (454 + 907) g, making the concept of weighing invariant with respect to choice of units. She contrasted this with the measurement of temperature. If the thermometer read 20°C, it would also read 68°F. if now we double the 20°C to get 40°C, we cannot just double the Fahrenheit reading – 40°C = 104, not 136°F (68×2 = 136).

She went on to explain the philosophical implications of such “irregularities.” Although she did not mention the Beaufort Scale or the Rockwell hardness scales by name, she devoted part of her talk to the philosophy of measurements that are based on arbitrary observations.

Prof Robert Fox:⁷ Summary

The events of the day were summarised by Professor Fox. He summarised the various historical events that were mentioned and also mentioned the Michelson–Morley experiment of 1887 in which they attempted to measure the speed of the earth relative to the “aether”. Although the experiment failed in its original objective, it was the first experiment to suggest that the speed of light is constant – something which is now assumed in the definition of the metre.

Prof Fox went on to emphasise the need for there to be agreement between nations in respect of units of measure and how the SI met this need.

Conclusions

This was certainly an interesting day for the academically inclined as is shown by the number of highly qualified academics who took part in the proceedings.

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Notes

1. University of Oxford
2. Author of “Beyond Measure: The Hidden History of Measurement”
3. Formerly University of Edinburgh
4. Director of the National Physical Laboratory, Visiting Professor at University of Surrey
5. University of Cambridge
6. University of Edinburgh
7. University of Oxford