The most important three pieces of metal in New Zealand, explained – Stuff.co.nz

Posted: September 24, 2021 at 11:26 am

On May 20, 2019, the world changed forever. And you probably didnt even notice. So significant was the change that the US National Institute of Standards and Technology branded it a turning point for humanity.

No, Im not talking about the calamitous Game of Thrones finale. I am talking about the evolution of an often unnoticed international framework. Its called the International System of Units (SI) and it's why one kilogram of milk powder weighs the same in New Zealand as it does in China.

The SI, which is derived from the French metric system, structures how we measure the world around us. This global apparatus underpins, for example, the 20,000 weighing scales in supermarkets around Aotearoa. It ensures each of these scales are precisely calibrated, so you pay for the right measure of chia seeds.

Measurement allows us to describe the mechanisms of life in a purely objective and necessary manner. The concept of a kilogram is unmoved by politics, disinterested in social media chatter a dispassionate, unbending foundational truth.

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In New Zealand, an organisation called the Measurement Standards Laboratory (MSL) is the caretaker of not only the kilogram but five other units of measurement, including the metre and the second.

In a fire-resistant safe in its lab in Lower Hutt there are three pieces of metal. They are New Zealands primary kilograms. They are our ludicrously precise gold standards for weight (made out of stainless steel) and they define weight here in Aotearoa. Lets explain how they came to be.

MSL

The three NZ primary kilograms that are kept in double bell jars.

This particular system provides a consistent unified framework for how we measure stuff. Centuries ago, individual towns or groups of people would have taken an idiosyncratic approach to measurements. They might have, lets say, used a piece of metal lodged in the square to define their towns unit of length.

As Dr Peter Saunders of MSL explains in this video, in the Croatian town of Dubrovnik there was a statue called Orlandos Column. In that town, the standard length used by traders was the length of Orlandos forearm. In Bremen in Germany, there was another statue of Orlando. There the standard length was the distance between the knees.

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In the Croatian town of Dubrovnik there was a statue called Orlandos Column. In that town, the standard length used by traders was the length of Orlandos forearm.

At one stage in Europe, he says, there were about 27,000 different standards to measure volume.

The SI framework is relatively new, born from the metric system, which emerged from the chaos of the French Revolution. Even so, there was metric system hesitancy at the time. Napoleon abandoned it and according to this BBC piece, the Parisian authorities even once used cops to enforce the thing.

(By the way, the UK has announced plans to let shops, once again, sell items in pounds and ounces.)

But ultimately along came the industrial revolution and with it the absolute necessity to measure properly. Ultimately the Treaty of the Metre was signed by 17 nations in 1875 with New Zealand formally signing up in 1991.

The treaty founded the International Bureau of Weights and Measures (BIPM), whose headquarters are in France. (The site is actually considered international territory.) This organisation is responsible for keeping measurement consistent across the world.

Its mission is based on seven founding units of measurement including the second, the metre and the kilogram. You can see the full list below.

Its probably a good idea to think of these as primary colours. You can mix and match the seven to create other measures these are called derived units, akin to secondary colours. For example, measures like velocity or concentration are worked out off those base units.

The metric system was designed to be for all times, for all people so it made sense to base it on the world around us. This made for some cool but fairly unsustainable measuring standards.

In the 1790s the metre was defined as one ten millionth of the distance from the North Pole to the equator (passing through Paris, of course). The kilogram was defined as the mass of a litre of water at 4C.

These measures later manifested in physical artefacts a ruler or bar defined the metre and in 1889, a platinum-iridium cylinder of metal (iridium makes the metal more stable) a touch larger than a golf ball came to be. This was the worlds kilogram the one to rule them all and defined what a kilogram was. Its name: the International Prototype Kilogram (IPK); it also went by the nickname the Big K or Le Grand K.

Supplied

A replica of the prototype of the kilogram in Paris.

Le Grand K sits in controlled conditions in a vault inside the BIPMs headquarters with a handful of official replicas. Copies were distributed around the world, used by member states, including New Zealand, to define what a kilogram is.

But scientists would sometimes still need to fly off to Paris to ensure their equipment and subsequently their own nations primary kilogram was the same as Le Grand K.

Thats right. A single piece of metal defined what everything weighed. But there were problems. Four times since 1989, the worlds kilogram was a fraction off, not matching up with its copies.

What caused this? Well, it could be anything from a dust particle or two landing on the metal, to a minute chemical reaction causing a tiny change.

Yet, despite this, it still technically defined what a kilogram was. It was basically infallible, unquestionable, correct no matter what a kind of metric Emperor with no clothes. This may sound amusing, but it ultimately had all sorts of downstream effects. A kilogram defines other stuff pressure and energy. And if what we thought of as a kilogram was off, so was everything else.

This is far from ideal. Sure, it might not really matter for people buying a kilogram of cheese or coffee, but for pharmaceutical manufacturing, for example, things have to be precise.

All this meant the definition of base measures defined by objects needed to change. In 2018, it was agreed that the seven underlying measurements needed to instead be informed by a series of constants fixed in nature, rather than manufactured physical rulers or weights.

Therefore, the definition of a metre became: the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second. This is based on the speed of light in a vacuum being exactly 299,792,458 metres per second (m/s). Thats what light does. It doesnt change.

While this may sound much more fiddly and certainly more complicated than a nice ruler, it ensures consistency.

The kilogram is now defined by something called Plancks constant, a number that helps illuminate quantum mechanics a branch of science which informs our understanding of the almost infinitesimally small particles that make up matter basically the building blocks of everything.

The new definition of a kilogram is tough going. If you want to read it below, you can. If you want to skip this sentence, I really would not blame you.

The kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 x 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 (the speed of light) and VCs (the caesium frequency).

I know, right ... The key takeaway is the fact the agreed Plancks constant is now 6.62607015 10-34 m2 kg/s. This number alone gives scientists around the world a starting point to build a wholly consistent definition of the kilogram.

There are three primary kilograms in New Zealand, each made of stainless steel and stored in double bell jars. Think of these as close relatives to Le Grand K in Paris.

The primary kilograms are actually rarely handled. Doing so risks corrupting them in some way and subsequently warping the definition of a kilogram. Theyre like the good cutlery, too precious to touch.

Yin Hsien Fung, who is one of the lead scientists in the Mass Quantities team at MSL, told Stuff that he and his team typically work with another set of weights called the working standards. Every two and a half years, scientists take the primary kilograms out and compare the two sets of weights to make sure they match up.

MSL

The fire-resistant cabinet safe where we store all our mass standards. The 3 primary kilograms are on the left side of the second rung.

The primary kilograms also need to be regularly compared to Le Grand K to ensure they are consistent.

So once every five years, another weight, called the Transfer Standard, is sent off to France.

Greg Reid, a MSL laboratory technician, organises the whole trip a kind of travel agent for a very important lump of metal.

He explains: Before sending the transfer standard to Paris, we run the transfer standard through a series of comparisons with the primary kilograms, which took 20 nights of measurements. The transfer standard is then wrapped around with a specially cleaned chamois cloth and tied with coloured strings, before packaging it and sealing it inside an aluminium container, where it will sit during transit.

The transfer standard even has its own passport of sorts, which means it can pass safely into Paris along with a letter telling New Zealand and French customs officers how precious it is and what to do if they need to inspect it.

The letter also states that this is a property of the New Zealand government and the past history and future usefulness of this weight depends on it remaining absolutely clean. It went to Paris last September and returned in November.

One thing we need to be clear on is that the physical artefacts werent tossed out the window in May 2019. Theyre still being used. New Zealand is essentially in a transitional period as it builds a device called the Kibble balance.

The workings of a Kibble balance are complex, based on gravitational and electromagnetic forces, but because the Planck number is constant, it will basically serve up insanely accurate measurements. Its why in several years the travel standard wont need to be jetted off to France.

The end goal is for New Zealand to be able to derive, or realise, the kilogram right here, which offers up much more security, Fung explains.

MSL

The Transfer Standard as it was about to be wrapped around with the chamois cloth before sending it off.

Our three primary kilograms essentially sit at the top of a vast pyramid, defining all that sit below.

They inform other calibration labs. And they, in turn, ensure equipment used by industries across New Zealand is accurate. It all flows downwards from MSL's lab (which is informed by Le Grand K), rippling out across the country all the way to basic weighing scales.

In May 2020, a Wellington student called Te Aomania Te Koha with the support of MSL produced a report exploring Mori measurement before the arrival of European settlers.

The findings of the report (you can read it here) are primarily based on the work of a scholar called Elsdon Best, who wrote that in the 1830s and 1840s Mori used a system based on the length of different parts of the body in construction work. Many cultures, such as the Native Americans and Egyptians, used similar methods.

The report outlines that one person would be selected, usually the chief of that tribe or someone of high status, and their body measurements would be taken and marked onto either a cord or a rod.

The measuring-rod, which was usually for measurements of a more important individual, is referred to as a rauru.

In the table above, there is a single measurement that stands out the Kumi. Te Koha notes in the study: The importance of this should be emphasised because this could be considered the first step towards producing a scientific system of measurement; that is, a table of units in which one unit represents a certain number of a preceding one.

This suggested Mori use of something called the base-10 counting system or decimal system, which is used throughout the world today.

There were limitations to the study, though, because Mori history was oral, passed down, but not necessarily written down, which is why the author made a strong case for further research, which is likely to go ahead.

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The most important three pieces of metal in New Zealand, explained - Stuff.co.nz

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