Obituary: Prof Ian Mills, whose work redefined the kilogram

One of the University of Reading’s longest-serving and most influential scientists, Emeritus Professor Ian Mills OBE FRS, whose work led to a redefinition of the international standard units of measurement, has died, aged 92.

Professor Mills was a longstanding member of Reading’s Department of Chemistry, specialising in infrared spectroscopy – a technique using infrared radiation to study materials. 

His later work on the standardisation of units of measurement included his chairing the international scientific committee for standard units. This work led to a redefinition of the kilogram, from a definition based on the weight of a platinum alloy prototype, held in Paris, to one based on a mathematically-defined constant. 

Ian Mills was born in 1930 in Reading, where his father was Head of Pathology at the Royal Berkshire Hospital. Growing up in large house in Sonning, and educated at Leighton Park School, he always acknowledged that he had a privileged childhood. He was a keen hillwalker, and at 16 he and his sister walked unaccompanied across Scotland from east to west.

Computing pioneer and proud educator

Entering the University of Reading as an undergraduate, he initially studied physics, but quickly switched to chemistry as the “most interesting branch” of the physical sciences, graduating in 1951.

His love of the subject led to a PhD at St John’s, Oxford, and post-doctoral positions at the University of Minnesota in 1954, where he won the Optical Society of America Lomb medal for his work, and at Corpus Christi, Cambridge until 1957. He took a position as a lecturer in Reading, where he stayed for the rest of his career.

Ian was one of the first recipients of a personal professorship at Reading in 1966. He was a pioneer in using computational methods to calculate line frequencies in interpreting spectra, and one of the first to bring computers to Reading in general.

He led a research group building spectrometers, and was editorial advisor on several journals, including two stints as senior editor of the leading journal Molecular Physics. He co-founded and organised the biennial Colloquium on High Resolution Molecular Spectroscopy, and published more than 200 papers.

Aside from his research, he was proud of his teaching, lecturing, taking seminars and supervising 17 research students to PhD. 

His work on the experimental observation and theoretical analysis of high-resolution rotation of molecules, among other achievements, led to his election as Fellow of the Royal Society (FRS) in 1996. 

OBE appointment

But it was his later work on the standardisation of metric units that led to his appointment to OBE in 2015. From a concern about nomenclature and metrology – the study of measurement – he was recruited to be one of the authors of the first editions of the IUPAC Green Book.

He became involved in the Bureau International des Poids et Mesures and eventually being president of the Comité Consultatif des Unités from 1995 to 2013. This led to the acceptance of a scientific need to redefine the SI system of units, and the implementation of this redefinition of the SI units in terms of fundamental natural constants.

As a student at Reading he started a sailing club, and he forged friendships on the Oxford sailing team who formed his longest-lasting set of friends. He was always a practical mechanic, and as a child had a workshop in the garden where he learned to do proper woodwork and metalwork. As an adult, he enjoyed DIY and made his own furniture at home, and he relished the chance to learn glassblowing and making his own equipment at work.

Ian married Maggy, who he first met at a Scottish country dancing group in Minnesota. They had two children and two grandchildren, and one great-grandchild born in August 2022, of whom he was very proud. Maggy predeceased him by just a few weeks.

After retirement he continued to sail and walk with friends, celebrating his 80^th birthday by climbing the 1,959-ft high Haystacks in the Lake District. 

Ian continued resolutely to manage his own household well into his 90s. He died peacefully on 23 December 2022 due to heart failure, after a short illness.

• A memorial will be held at 1130hrs on Saturday 25th March in the University of Reading Chemistry Dept.
• Donations in Ian’s memory may be given to the University of Reading Chaplaincy Trust (reg. charity no. 1071305) via CAF Bank, sort code 40-52-40, account no. 00087429.