The platinum-iridium cylinder (right) was the primary standard kilogram for all metric measurements in the U. S. Known as Kilogram No. 20, it was a copy of the International Prototype Kilogram, which was preserved at the  International Bureau of Weights and Measures at Sevres, France. Kilogram No. 4 (left, under double bell jar), a duplicate of No. 20, was used as a secondary standard. Noth cylinders were 39 mm in diameter and 39 mm. high; they were made of 90 percent platinum and 10 percent iridium. Re-comparison of Kilogram No. 20 with the international standard in 1937 showed that the United States standard had changed by only one part in 50 million during approximately 50 years.

A fond farewell to the humble kilo

As Le Grand K enters retirement, Simon Winchester looks back at the highly polished ball of solid metal that has defined the kilogram for over a century.

It may be straining credulity a little to imagine that anything made of so valued and costly a substance as solid platinum could ever possibly suffer feelings of melancholy.

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Except I have little doubt that one particular piece, which has been kept for the last century-and-a-quarter under three nested glass cloches in a locked and climate-controlled vault in a western suburb of Paris, is about to suffer a crisis of confidence of historic proportions. Stoic and imperturbable though the 78th element may well be, this special specimen, known informally across the world as Le Grand K, is understandably dejected, beyond all hope of consolation.

For late this coming autumn it is about to be spurned, and by everyone on the planet. And come next May, it will be formally declared to be pretty useless. Pretty, for sure. But of no further official purpose.

Le Grand K is currently the world’s standard kilogram, a highly polished little cylinder of solid metal, about the size of a Zippo lighter and so exactly made that it was never, ever to be allowed to be touched by human hand. Since September 1889 it has been the ultimate arbiter of mass, a standard against which all others on earth are measured. It stands at the head of a system of measurement that begins with the humble measuring scale, the kind of device, household, industrial or medical, that is used wherever the mass of something in the world – or in crude terms, the weight of anything, which is how mass is related to gravity – needs to be measured.

So, whether a bag of sugar in a country store in Nigeria, a brand-new jumbo jet just off the assembly-line in Seattle, or whether a jockey at a race course at Ascot – each has a measurable mass which can be and invariably is now expressed in kilograms. And the proving of the measurement of all these things and everything else, boils down, ultimately, to Le Grand K.

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For the scales that initially declare these masses to be what they are, by law and by custom, are currently checked against master copies. In these three cases they are – respectively – the standard Nigerian government kilogram kept in Lagos, the standard United States ditto maintained in a gigantic complex in Gaithersburg, Maryland, and in the case of the would-be winner of the Gold Cup, against the standard British base unit of a mass which is locked away deep inside a secure suite of government offices at the National Physical Laboratory in the west London suburb of Teddington.

Britain’s cylinder was awarded to the nation by way of a lottery held in Paris on a Saturday afternoon late in September 1889. The then fifty members of what since its founding in 1875 had come to be known as the BIPM, the Bureau Internationale des Poids et Mesures, had assembled in the grand salon of the Pavillon de Breteuil – then as now an extraterritorial building like the UN, out in the Paris suburb of Sèvres. In front of them were three copper bowls, each containing slips of paper denoting the various and newly-made precise copies of the kilogram. In alphabetical order by their French names – Allemagne first, Suisse last – the frock-coated delegates shuffled forward, picking out their slips, as for sporting event season tickets. Britain got kilogram Number 18 – and then paid a handsome 3,105 French francs for the metal of which it had been made. (The rod that was until 1960 the standard metre, and which used rather more of the platinum-iridium alloy, cost 10,151 francs, as our Treasury grumbled.)

These national measuring standards, and scores more just like them from countries around the world, have in the decades since all been checked regularly for correctness against a family, also housed in Paris, of formidably-well-guarded platinum kilogram cylinders, the so-called témoins, or witness cylinders, and which are the almost-perfectly-perfect arbiters of mass, cast at the same time in 1889 and from the self-same platinum alloy used for all the rest.

And from time to special time, these témoins are themselves measured against the super-accurate perfectly-perfect mother and father of them all, Le Grand K in Sèvres. It has only happened four times in the last 130 years, so sedulously is the grandee cared-for and protected. But by doing so, by making this comparison and thus employing the principle known as traceability – where every physical property of everything (mass being one property; length and time being others, as we shall see) can be traced back to one ultimate source of exactness – is the world’s weights and measures system, so essential to us all, kept in precise and perfect order.

Except – and here’s the reason for the dolorous mood of the moment – come November it will formally be declared that the mass of the world’s everything won’t be traced to Le Grand K, not any more, and not ever again.

Why does one kilogram of feathers have more mass than one kilogram of iron? © Getty Images

The stubby little cylinder is about to be officially demoted from ultimate arbiter to no more than a museum piece. It will suddenly have no function. It will officially and literally be past its sell-by date. Once upon a time it was a thing revered – for it was, quite literally, the kilogram, everything being compared to it, and it itself also quite literally, beyond compare. Now this elegant and beautiful artifact is destined only to gather dust, and be forgotten. Small wonder that it is enveloped in an air of metrological misery.

But more than that. The formal abandonment of the physical kilogram has implications that spread well beyond the basement of the mansion where it is secreted in Sèvres. For an era of measurement that has operated on this planet for thousands of years will come to its long-drawn-out but inevitable end.

It is all the fault of a long-dead Scotsman, the physicist and originator of the electromagnetic theory of light, James Clerk Maxwell. In a famous speech at the annual meeting of British Association for the Advancement of Science, held in Liverpool in the autumn of 1870, he challenged the wisdom of measuring things against physical objects, and by doing so he shivered the timbers of millennia of tradition.

Ever since the Egyptians, five thousand years ago, came up with the cubit, the length of a pharaoh’s forearm, so humans have used the dimensions of bits of themselves, or of their environment, as scales by which to measure things. The thumb was an inch, the foot was a foot, a thousand of their strides (the mille passus as grumbled by the Roman legions) made, more or less, a modern mile. Some were varied – a Chinese li had a length depending on whether the road went uphill or down. Some were improbable – the Koreans had a unit of time defined by how long it took a granite mountain outside Seoul to be worn to sea-level by the wings of an angel who brushed the summit with her wings once a year. The human scale, for eons, was initially everything.

But then, come more modern and rational times, the 18th century French cast romance aside and declared the only true invariable to be the Earth itself, and not the arms or fingers or wings of its inhabitants. So an intricately careful survey was made of a length of a part of the meridian between Paris and Marseilles, it was carefully extrapolated into the exact length of a full quadrant of the planet, pole to equator, the resulting number divided by ten million and lo! was born the metre. A platinum rod was fashioned of exactly that length and was declared to be the official prototype metre – and copies of it were distributed around the world, so that every bolt of cloth or width of street or length of newborn child could be known, accurately, compared to a trusted scale that in theory could be compared to this platinum rod.

But then along came James Clerk Maxwell, spoilsport-in-chief. The Earth, he declared to his Liverpool audience, was not nearly so fixed and stable as those Frenchmen who measured the meridian might suppose, whatever their hauteur. It expands and contracts, it slows down and speeds up, large volcanoes and earthquakes can change its shape, it might well one day be covered with layers of meteorites, or knocked off-balance by the collision with an asteroid. To use the mere Earth as a fixed standard for measurement was as ill-conceived a notion as using an arm or a leg or the flight of an angel. The only absolutely permanently invariables, Maxwell declared, were to be found in the world of molecular bodies – meaning that, as he put it, we must henceforward seek our standards of length, of time and of mass “in the wavelength, the period of vibration and the absolute mass of these imperishable and unalterable and perfectly similar molecules.”

It took almost sixty years for those who ran the scientific world to agree with him. The greybeards of the metrology establishment thought the old ways seemed to be quite good enough. Yet Maxwell’s reputation towered large and long. And as the demands for ever-increasing precision in measurement began to assert themselves – with masses in the 1920s being calculated to micrograms, lengths to microns, time being checked to nanoseconds, electrical currents to millivolts – so his views started to gain traction and the old ways began, if creakily, to give way.

The first to fall was the unit of length, the metre.

In 1927 it was realised that the frequency of radiation emitted by a heated sample of the silvery-blue and very poisonous metal cadmium could be determined with great exactitude – and (forgive a brief mathematical excursion here) once the radiation’s frequency was known, then, by the simple arithmetical process of dividing it into the speed of light, its wavelength could be determined also. A certain specific number of these tiny wavelengths was then formally declared to be the metre.

At first, only the scientists agreed. But after much tinkering and argumentation official recognition of this same principle was granted by governments, internationally – the only difference being a technical one, that the metal cadmium be replaced by the gas krypton. A ceremony was held in Paris on 14 October 1960, where it was officially announced that the platinum bar that had been the metre since 1889 was no longer deemed to have “sufficient precision for the needs of today’s metrology”, and consigned it to the rubbish bin of history. A bloodless and bewildering confection of numbers, all based on the frequency of the radiation of krypton gas, has ever since replaced it.

And now, fifty further years on, it is the turn of the kilogram.

It has but a few more months left of its distinguished life. In November it will be declared no longer the world’s unit of mass. In May – providing there are no objections from the nations that make up the International Bureau of Weights and Measures (BIPM) – it will be officially stood down. It will go the way of the thumb and the foot and the angelic wings. Instead the kilogram is to be determined – and one has to take a deep breath before launching into this monstrosity of a definition – “by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.” Whatever this may possibly signify.

There are reasons for all of this, of course. The demands of a world now so dominated by extreme precision are such that metres and kilograms and seconds and units of electricity and light intensity and so on have now to be measured and calibrated in concert, and measured to tolerances that were barely imaginable when the London metal smelters first cast their ingots of platinum, back in the stygian gloom of Victorian times.

I accept all these reasons for change as sensible and meet and proper and right. But in truth I do still feel a small pang of sympathy for the little platinum cylinder. It is a thing of great beauty, and once so lovingly made, yet is now to be tossed away, consigned to a glass case in some unvisited museum, unwanted and unloved for the rest of time. There is something truly pitiful about it, and all that its loss of status suggests. And so yes, I can imagine the feeling of unspeakable melancholy, deep in its little metal soul.

Exactly: How Precision Engineers Created the Modern World by Simon Winchester is out now (£25, William Collins)

The Royal Society Insight Investment Science Book Prize celebrates the best of science writing for a non-specialist audience. The winner of the 2018 Prize was announced on 1 October 2018.

Exactly: How Precision Engineers Created the Modern World by Simon Winchester (William Collins)

[This article was first published in September 2018]


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