From The National Institute of Standards and Technology
6.26.24
Stephen Eckel
In Stephen Eckel’s lab at NIST, he gets to work with some of the coldest stuff in the universe. Credit: NIST
“Temperature is probably the second most measured physical quantity in our modern world — after time.
When I wake up in the morning, the first thing I usually check is the time (to see if I should go back to sleep), but the second thing I check is the temperature outside (so that I know how to dress).
Temperature is such a common measurement that we sometimes forget how important it is. From dairy farming to rocketry, from climate science to weather prediction, so many things require an accurate knowledge of temperature.
The metric (SI) unit for temperature is called the kelvin, after Lord Kelvin, whose 200th birthday we celebrate today.
Lord Kelvin and the Early Science of Temperature
Lord Kelvin, or William Thomson, worked in what was then the emerging field of thermodynamics — transforming heat into dynamical motion. He did this both as a student at the University of Cambridge and as a young professor at the University of Glasgow. Together with his close collaborator, James Joule, he researched all sorts of problems in thermodynamics, including temperature scales.
At the time, the scientifically accepted scale for temperature was the Celsius scale, with zero temperature being the freezing point of water and 100 degrees being the boiling point of water. But after studying how gases changed volume and pressure in response to changing temperature, Thomson, Joule and other scientists realized that there was an absolute coldest temperature that could be reached.
To understand how they reached this conclusion, consider a gas in a balloon. If you cooled the balloon, the gas inside would exert less pressure against the balloon itself and against the atmosphere outside it, causing the balloon’s volume to shrink.
Don’t believe me? Inflate a balloon and stick it in your freezer. When you pull it out, you can feel the balloon expand. Now extrapolate: How cold would you have to make the balloon to make its volume go to zero (ignoring the fact that the gas inside will eventually condense into a liquid)? That must be the coldest possible temperature because the balloon cannot have a negative volume.
In 1848, Lord Kelvin used similar reasoning to accurately calculate the absolute coldest temperature as negative 273.15 Celsius (or negative 459.67 degrees Fahrenheit). It would be roughly another decade before scientists like Lord Kelvin and Ludwig Boltzmann understood that at absolute zero, the molecules in the gas stop moving.
Since 2019, all three of these scientists have been immortalized in the SI. The kelvin is our SI unit of temperature, defined through the Boltzmann constant, which relates temperature to energy, the SI unit of which is the joule.
Today, atomic physicists like myself use a technique partly pioneered at NIST called laser cooling, which uses lasers to cool clouds of between 100,000 and 1 billion atoms to temperatures of about 100 microkelvin. This temperature is 1/10,000th of a degree Celsius above absolute zero.
And we measure these ultracold temperatures in a way that would not be surprising to Lord Kelvin (although making such cold gases might be!).
We measure the average speed of the atoms in the gas. Researchers at NIST use such laser-cooled atoms for all sorts of applications, from atomic clocks to vacuum standards.
Vacuum Standard
Laser cooling atoms to near absolute zero only works inside a chamber where almost all the air has been removed by a pump to isolate the atoms from the surrounding environment. Such vacuum chambers are common and are used in industries such as semiconductor manufacturing.
Most of the components in your cellphone have been in and out of at least one vacuum chamber. The core components, like the central processing unit, have probably been through a chamber that has produced some of the best vacuums on Earth. For every trillion gas molecules that started in the chamber, all were removed but one. Such exquisite vacuums are required because leftover gas molecules can both contaminate the chip and scatter the ultraviolet light that is used to imprint the designed circuit. This can cause the chip to be ruined.
Amazingly, the current best way to measure such pure vacuums is by using what is effectively a vacuum tube. But now, the laser-cooled atoms in my lab may be the best sensor of ultralow vacuum pressures on Earth.
After the sensor atoms are cooled to near absolute zero, we hold the sensor atoms in a “trap” that is made entirely of magnetic fields. This trap is very weak, only able to hold onto the ultracold sensor atoms. The vacuum sensor works because if a cold sensor atom is struck by a leftover gas molecule, it will almost always be ejected from the weak trap. The rate at which this process occurs depends on the number of gas molecules the pump has left behind. Thus, determining the number of leftover gas molecules just involves counting the number of sensor atoms that remain after some time.
This “cold-atom vacuum standard (CAVS)” is a new way of measuring vacuum pressure, which NIST has played a crucial role in developing. We anticipate it being used to measure ultrapure vacuums in semiconductor manufacturing, quantum computers and other big science experiments, such as an experiment detecting collisions of extremely distant black holes, known as the Laser Interferometer Gravitational Wave Observatory (LIGO).
Having a standard like the CAVS that always gives the correct vacuum pressure reading will help these applications build better vacuum chambers, diagnose problems and increase both reliability and productivity.
The CAVS is the only experiment that I am aware of that needs to measure two very different temperatures at the same time: the sensor atom temperature of around 100 microkelvin (very cold!) and the temperature of the leftover gas in the vacuum chamber, near room temperature at 300 kelvin.
I think Lord Kelvin would be amazed to learn that two very different temperatures could exist at the same time, and both need to be measured for a single experiment to work.
Thermometers
Another interesting research pursuit here at NIST is trying to use atoms or molecules to build a thermometer that actually measures temperature.
You may be wondering what I mean.
After all, you probably have multiple thermometers in and around your home, and they all give you some number in either Fahrenheit or Celsius. But the truth is they all measure some other physical quantity — like the resistance of a platinum wire or the voltage generated between two dissimilar metals — that depends on temperature.
For these devices to read out a temperature in Fahrenheit or Celsius, they must be calibrated. NIST does such calibrations, and it’s more likely than not that the calibration for the thermometer in your home’s thermostat can be traced through a complicated set of steps all the way back to NIST.
But we may be able to make this whole calibration process simpler by making thermometers that directly measure temperature, using techniques that Lord Kelvin would appreciate.
For example, my colleague Daniel Barker and I are working on using lasers to measure the distribution of velocities of a gas of rubidium atoms at room temperature and above. This technique, called Doppler thermometry, gets at the very heart of how Lord Kelvin understood temperature.
Together with my colleague Eric Norrgard, I am also working on two projects trying to create a new type of infrared thermometer using atoms and molecules. If these efforts are successful, calibrating our thermometers could get much easier, and it may further other scientific advancements as well.
Keeping It (Very) Cool in the Lab
I came to NIST as a postdoctoral researcher in 2012 after finishing my graduate work at Yale University.
As a postdoc, I worked with some of the coldest stuff in the universe: Bose-Einstein condensates (BECs). Like the CAVS, BECs are also made of laser-cooled atoms, but they have been cooled even further to less than 100 billionths (!) of a degree above absolute zero.
After my postdoc, I decided to stay at NIST and try to use my experience with ultracold atoms and lasers to realize practical and useful standards, like the CAVS.
I gain a great sense of pride when I see what appear to be glowing balls of ultracold atoms — which are certainly fun to play with — used to solve real-world measurement problems. I suspect that Lord Kelvin may have felt the same sense of pride to see his measurements and theories regarding thermodynamics (which were probably also fun to work on) be applied to make more efficient steam engines.
Happy Birthday, Lord Kelvin
Lord Kelvin didn’t just calculate absolute zero. After his early work in establishing absolute temperature scales, he was instrumental in laying the first telegraph cables across the Atlantic Ocean. Lord Kelvin also invented a machine that predicted tides and a compass that helped the Royal Navy navigate the seas. While my research is not quite that varied, one of the ways I mix up my work is by working at both room temperature and temperatures near absolute zero.
One of the key things I have learned is that measuring temperature, as Lord Kelvin understood it, is almost always harder than you might think. While the ideas are straightforward, making them work in practice is the real challenge.
And this fact makes it even more impressive that Lord Kelvin accurately predicted the temperature of absolute zero … in 1848.
On his 200th birthday, I’ll take a moment to appreciate that.”
See the full article here.
Comments are invited and will be appreciated, especially if the reader finds any errors which I can correct.
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Background
The Articles of Confederation, ratified by the colonies in 1781, contained the clause, “The United States in Congress assembled shall also have the sole and exclusive right and power of regulating the alloy and value of coin struck by their own authority, or by that of the respective states—fixing the standards of weights and measures throughout the United States”. Article 1, section 8, of the Constitution of the United States (1789), transferred this power to Congress; “The Congress shall have power…To coin money, regulate the value thereof, and of foreign coin, and fix the standard of weights and measures”.
In January 1790, President George Washington, in his first annual message to Congress stated that, “Uniformity in the currency, weights, and measures of the United States is an object of great importance, and will, I am persuaded, be duly attended to”, and ordered Secretary of State Thomas Jefferson to prepare a plan for Establishing Uniformity in the Coinage, Weights, and Measures of the United States, afterwards referred to as the Jefferson report. On October 25, 1791, Washington appealed a third time to Congress, “A uniformity of the weights and measures of the country is among the important objects submitted to you by the Constitution and if it can be derived from a standard at once invariable and universal, must be no less honorable to the public council than conducive to the public convenience”, but it was not until 1838, that a uniform set of standards was worked out. In 1821, John Quincy Adams had declared “Weights and measures may be ranked among the necessities of life to every individual of human society”.
From 1830 until 1901, the role of overseeing weights and measures was carried out by the Office of Standard Weights and Measures, which was part of the U.S. Coast and Geodetic Survey in the Department of the Treasury.
Bureau of Standards
In 1901 in response to a bill proposed by Congressman James H. Southard (R- Ohio) the National Bureau of Standards was founded with the mandate to provide standard weights and measures and to serve as the national physical laboratory for the United States. (Southard had previously sponsored a bill for metric conversion of the United States.)
President Theodore Roosevelt appointed Samuel W. Stratton as the first director. The budget for the first year of operation was $40,000. The Bureau took custody of the copies of the kilogram and meter bars that were the standards for US measures, and set up a program to provide metrology services for United States scientific and commercial users. A laboratory site was constructed in Washington DC (US) and instruments were acquired from the national physical laboratories of Europe. In addition to weights and measures the Bureau developed instruments for electrical units and for measurement of light. In 1905 a meeting was called that would be the first National Conference on Weights and Measures.
Initially conceived as purely a metrology agency the Bureau of Standards was directed by Herbert Hoover to set up divisions to develop commercial standards for materials and products. Some of these standards were for products intended for government use; but product standards also affected private-sector consumption. Quality standards were developed for products including some types of clothing; automobile brake systems and headlamps; antifreeze; and electrical safety. During World War I, the Bureau worked on multiple problems related to war production even operating its own facility to produce optical glass when European supplies were cut off. Between the wars Harry Diamond of the Bureau developed a blind approach radio aircraft landing system. During World War II military research and development was carried out including development of radio propagation forecast methods; the proximity fuze and the standardized airframe used originally for Project Pigeon; and shortly afterwards the autonomously radar-guided Bat anti-ship guided bomb and the Kingfisher family of torpedo-carrying missiles.
In 1948, financed by the United States Air Force the Bureau began design and construction of SEAC: the Standards Eastern Automatic Computer. The computer went into operation in May 1950 using a combination of vacuum tubes and solid-state diode logic. About the same time the Standards Western Automatic Computer, was built at the Los Angeles office of the NBS by Harry Huskey and used for research there. A mobile version- DYSEAC- was built for the Signal Corps in 1954.
Due to a changing mission, the “National Bureau of Standards” became the “ The National Institute of Standards and Technology” in 1988.
Following September 11, 2001, NIST conducted the official investigation into the collapse of the World Trade Center buildings.
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Baldrige Performance Excellence Program, which administers the Malcolm Baldrige National Quality Award, the nation’s highest award for performance and business excellence.
NIST’s Boulder laboratories are best known for NIST‑F1 which houses an atomic clock.
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SURF III now serves as the US national standard for source-based radiometry throughout the generalized optical spectrum. All NASA-borne extreme-ultraviolet observation instruments have been calibrated at SURF since the 1970s, and SURF is used for measurement and characterization of systems for extreme ultraviolet lithography.
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Handbook 44
NIST publishes the Handbook 44 each year after the annual meeting of the National Conference on Weights and Measures (NCWM). Each edition is developed through cooperation of the Committee on Specifications and Tolerances of the NCWM and the Weights and Measures Division (WMD) of the NIST. The purpose of the book is a partial fulfillment of the statutory responsibility for “cooperation with the states in securing uniformity of weights and measures laws and methods of inspection”.
NIST has been publishing various forms of what is now the Handbook 44 since 1918 and began publication under the current name in 1949. The 2010 edition conforms to the concept of the primary use of the SI (metric) measurements recommended by the Omnibus Foreign Trade and Competitiveness Act of 1988.
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