Never heard of World Metrology Day? Don’t worry, I was doing a PhD in metrology for three years before I found out we had a day for it. So what is metrology, and why does it deserve its own day?
Given how unfamiliar the term is, you could be forgiven for assuming metrology is a very niche discipline, but like electricity generation or the internet, it is an essential part of our modern civilisation. Metrology is the science of measurement, not to be confused with meteorology, the study of the weather. I understand the confusion though, and am getting used to people asking me whether it’s going to rain tomorrow.
It is concerned with the definition of internationally accepted units of measurement (kilogram, metre, etc), the practical realisation of these measurements (that is, how to actually make the measurement), and the traceability of measurements (linking a measurement you make back to a reference in a laboratory).
Image: My metrology laboratory where I work on the transmission of atomic clock signals, author supplied
Every country has a metrology institute. In Australia we have the National Measurement Institute (NMI), the UK has the National Physical Laboratory (NPL), and in the US it’s the National Institute of Standards and Technology (NIST). These institutes are responsible for the scientific, industrial, and legal aspects of measurement.
Scientific metrology focuses on the definition of the units we use to measure things, how to actually make measurements, and how to do it with ever higher degrees of accuracy and precision.
Industrial metrology is concerned with measurement in manufacturing and other technical applications such as the calibration of machinery. Industrial metrology is so important to a nation’s development that the condition of its industrial metrology program is a key indicator of a country’s economic status.
Legal metrology is about the statutory requirements of measurement for trade, taxation, and public safety. Whenever you weigh bananas at the supermarket, buy petrol, or check the time, those weights and measures are strictly legally controlled.
World Metrology Day is an international event commemorating the signing of the Metre Convention on 20 May 1875. 17 nations signed a treaty for the purpose of coordinating worldwide uniformity of measurement and the development of what we now call the metric system and the International System of Units (SI).
Metrology institutes around the world take the opportunity to celebrate and communicate the importance of metrology in our modern world. In Australia, which joined the treaty in 1947, the NMI commemorates the occasion by presenting two awards for outstanding achievement in measurement. The USA is also a signatory of the treaty, meaning that, legally speaking, America uses the metric system. Someone should probably tell them.
The measure of things
Right now is an exciting time to be a metrologist. Two big developments currently taking place in metrology involve the redefinition of two of the most basic units of measurement we use every day. The kilogram and the second.
The SI system has seven basic units of measurement, which are:
- the metre for length,
- the kilogram for mass,
- the second for time,
- the ampere for electric current,
- the Kelvin for temperature,
- the mole for amount of substance (in terms of number of atoms or molecules), and
- the candela for intensity of light.
The second is currently defined as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom”. In less technical terms, in an atomic clock, carefully controlled atoms of caesium-133 will emit microwaves with a frequency of 9,192,631,770 Hz (the clock will “tick” at 9,192,631,770 cycles per second), so adding up 9,192,631,770 of these ticks gives us one second.
Metrologists around the world have worked to produce ever more stable and precise atomic clocks. In the last few years, clocks that use atoms of ytterbium have surpassed the caesium clocks we currently use to define the second. In the next few years, we could see the second no longer defined by the over 9 billion ticks per second of caesium clocks, and instead by the 518 trillion ticks per second of ytterbium clocks. It sounds like an insignificant change, and hopefully your boss isn’t going to get angry if you’re a few trillionths of a second late to work, but more precise clocks counting more precise seconds are an important tool for scientific and industrial progress into the latter half of the 21st century.
Atomic clock at NIST. Source: NIST/Wikimedia
The second, and most of the other SI base units, are defined using physical constants, values that are fundamental properties of the universe. While a “second” is a human invention, caesium will emit microwaves at 9,192,631,770 Hz whether it is on Earth or on an alien planet in a galaxy far, far away. The metre is defined by the speed of light, another fundamental property of the universe, which is 299,792,458 metres per second.
But one of the base units, the kilogram, is not defined in terms of fundamental universal constants. The kilogram remains the only unit defined by a physical object, a platinum-iridium ingot called the international prototype kilogram (IPK) stored in a secure vault in France.
Each member nation of the Metre Convention possesses one or more copies of the IPK (Australia has copies 44 and 87). Twice since their production the copies have been re-united in France and weighed, and the results were shocking. The kilogram copies have all changed in mass, and by different amounts. Even the IPK’s six sisters, copies stored next to it in the same vault, disagree with the IPK. This has spurred metrologists to invent a new definition of the kilogram, a definition in terms of physical constants that can be measured accurately by any metrology laboratory in the world, without the need to refer to a physical object on the other side of the globe.
NIST's copy of the kilogram. Source: NIST/Wikimedia
One possible redefinition of the kilogram is being investigated by CSIRO and involves manufacturing an extremely precise sphere of silicon and counting the atoms in it. Thanks to the computer industry, the technology to produce defect-free, ultra-pure crystals of silicon is readily available. An ingot of pure silicon is cut and machined into a sphere. Special measurement techniques allow the sphere to be polished to size to an accuracy of only one layer of atoms. Mass spectrometry and X-ray measurements allow the spacing between silicon atoms to be measured and so the number of atoms in the silicon sphere to be calculated. The kilogram could then be defined in terms of the number of atoms in an object that could be reproduced by metrology laboratories from a set of instructions.
CSIRO’s prototype silicon sphere is one of the most precisely round objects in the world. If the 93.6 mm sphere was blown-up to the size of the Earth, the tallest mountain on that globe would be less than three metres high.
CSIRO's silicon sphere. Source: CSIRO/Wikimedia
However, the current front-runner in the race to redefine the kilogram is the Watt balance. The basic design of the Watt balance resembles an old fashioned set of weighing scales, with a mass on one side, and an electromagnet on the other. The electrical power (number of Watts) necessary to balance the weight can be measured very precisely, giving the mass of the kilogram in terms of amperes and volts, something that any metrology laboratory can reproduce.
You can even have a go at building a Watt balance yourself. The US NIST released plans for a desktop DIY Watt balance you can build out of LEGO. It’s about a million times less precise than NIST’s original Watt balance, but if you build it well, it will out-perform your digital kitchen scales.
Who knows, within the next few years, the bananas you buy at the supermarket could be weighed in terms of the “electric” kilogram.
So, this World Metrology Day, take a moment to reflect on how much accurate measurement affects your life, and wonder how many amps and volts the banana you had with breakfast weighs. Dig out your old LEGO sets and find out by building a Watt balance, or just grab a ruler and measure how long it is. Get the measure of things this World Metrology Day.
Want to know more?
Here are some great educational videos about metrology:
- Practical Engineering — Redefining the kilogram with the DIY Watt balance
- Veritasium — World’s roundest object
- Tom Scott — Nobody's Exactly Sure How Much A Kilogram Is Right Now
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