NIST Calibrates All US Customary Weights to Metric Kilogram Prototype

Is America Actually Metric? — Veritasium

Veritasium

American customary units possess no independent physical standards and legally function as arbitrary mathematical conversions layered over metric prototypes.

AI-assisted summary of Is America Actually Metric? — original source. Check the original for full context.

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Key ideas

  • Despite cultural reliance on customary units like pounds and gallons, American measurements have been legally derived from the metric system since the US signed the 1875 Treaty of the Metre. (0:00)
  • US national mass standards trace back to K20, an original 1889 platinum-iridium cylinder stored at the National Institute of Standards and Technology (NIST). (1:27)
  • Calibrating stainless steel secondary standards against platinum-iridium prototypes requires precise air buoyancy corrections due to an approximate 110-milligram lift discrepancy caused by differing material densities. (3:54)
  • Over a century of comparisons revealed an approximate 50-microgram relative mass divergence between the International Prototype Kilogram and its national copies, prompting the 2018 redefinition of the kilogram based on the Planck constant. (5:21)

Detailed notes

## Background Published on YouTube by Veritasium (hosted by Derek Muller) on May 10, 2017, this video explores the surprising reality of the United States' measurement system. The description explains that the US signed the 1875 Metre Convention, legally defining all customary units in terms of SI standards traceable back to the K20 national prototype kilogram housed at NIST.

## TL;DR Derek Muller visits the National Institute of Standards and Technology (NIST) to investigate the little-known fact that the United States is officially and legally on the metric system, converting metric values into customary units like pounds and inches purely by convention. Physicist Patrick Abbott displays the national prototype kilogram cylinders K20 and K92, detailing how calibration cascades down to commercial scales while metrologists account for complications like air buoyancy on stainless steel secondary standards. The discussion also recounts the analog reality of hand-carrying priceless platinum-iridium artifacts in camera bags to Paris for recalibration. Ultimately, the video reveals the underlying metrological crisis: a 50-microgram drift between the International Prototype Kilogram and its global copies, which forced the international scientific community to prepare for redefining the kilogram via Planck's constant.

## Atmosphere & style The video carries an engaging, playfully irreverent, yet deeply inquisitive atmosphere as Derek Muller uncovers the counterintuitive absurdities of American metrology. The dynamic between Muller and NIST physicist Patrick Abbott is candid, informative, and humorous, punctuated by Abbott frankly admitting that converting back and forth between SI and customary units is 'stupid.' Watching it feels like getting an exclusive behind-the-scenes pass into a high-security vault, balancing nerdy scientific intricacies like buoyancy corrections and surface roughness with high-stakes, almost comical real-world anecdotes like smuggling irreplaceable platinum cylinders through airport security in ordinary camera bags.

## Bulletin - [00:00](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=0s) Despite cultural reliance on customary units like pounds and gallons, American measurements have been legally derived from the metric system since the US signed the 1875 Treaty of the Metre. - [01:27](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=87s) US national mass standards trace back to K20, an original 1889 platinum-iridium cylinder stored at the National Institute of Standards and Technology (NIST). - [03:54](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=234s) Calibrating stainless steel secondary standards against platinum-iridium prototypes requires precise air buoyancy corrections due to an approximate 110-milligram lift discrepancy caused by differing material densities. - [05:21](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=321s) Over a century of comparisons revealed an approximate 50-microgram relative mass divergence between the International Prototype Kilogram and its national copies, prompting the 2018 redefinition of the kilogram based on the Planck constant.

## Key takeaways - [00:00](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=0s) The United States was an original signatory of the 1875 Treaty of the Metre, committing to the international metric framework. - [00:00](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=0s) Customary US units such as pounds, feet, and gallons carry no independent physical standards and are mathematically defined through SI metric equivalents. - [00:00](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=0s) In 1889, France delivered two primary prototype kilogram cylinders—designated K4 and K20—to the United States. - [01:27](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=87s) Primary mass standard K20 is constructed from 90 percent platinum and 10 percent iridium to achieve high density, hardness, and resistance to oxidation. - [01:27](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=87s) Surface roughness on mass artifacts increases surface area, significantly raising the risk of airborne contaminant adhesion that alters total mass. - [01:27](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=87s) Manufacturing a modern platinum-iridium prototype costs around $100,000, while historic primary prototypes remain irreplaceable. - [03:54](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=234s) NIST metrologists transfer mass calibrations from K20 to stainless steel working standards to avoid wear on the primary national prototype. - [03:54](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=234s) Because stainless steel is roughly one-third as dense as platinum-iridium, an equal-mass steel standard displaces more air, experiencing roughly 110 milligrams more buoyant force at atmospheric pressure. - [03:54](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=234s) Mass comparisons must occur inside sealed comparator chambers to strictly account for fluctuations in temperature, relative humidity, and barometric pressure. - [05:21](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=321s) National mass prototypes sent to Paris for recalibration are transported as hand luggage in padded camera bags rather than checked baggage or cargo. - [05:21](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=321s) Periodic international weigh-ins revealed a roughly 50-microgram divergence between the primary International Prototype Kilogram (IPK) and its worldwide sister copies. - [05:21](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=321s) Because the IPK was defined by law as exactly one kilogram, metrologists could measure relative divergence among copies but could never determine absolute drift. - [05:21](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=321s) The persistent drift and vulnerability of physical artifact standards led the international metrology community to redefine the kilogram using the Planck constant in 2018.

## Detailed walkthrough ### [00:00](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=0s) The Treaty of the Metre and America's Metric Secret

At the National Institute of Standards and Technology (NIST), host Derek Muller meets with physicist Patrick Abbott to examine the original artifacts that underpin American measurements. Abbott explains that in the 1880s, forty kilogram cylinders were manufactured alongside the International Prototype of the Kilogram (IPK) and distributed to signatories of the 1875 Treaty of the Metre. The United States signed the convention—effectively pledging to adopt the metric system—and received two official prototypes designated K4 and K20.

Despite the widespread cultural assumption that the United States operates entirely on its own customary system, American units like feet, gallons, and pounds are legally defined by SI standards. Patrick Abbott reveals the counterintuitive reality of American metrology: “So it's just a little translation that we do here but our country is actually on the metric system.” Derek points out the absurdity of basing national measurements on metric standards only to slap on an arbitrary conversion factor that workers and scientists must continually reverse. When Derek asks if that seems insane, Abbott agrees candidly: “Yes, it's stupid.”

- **Treaty of the Metre (1875):** Formed the foundation of the modern metric system; the US was an original signatory. - **US Allotment:** Received two platinum-iridium prototypes, designated **K4** and **K20**, delivered in 1889. - **Legal definition:** Customary units (pounds, inches, gallons) possess no independent physical standards and are mathematically tied to SI units.

---

[01:27](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=87s) Entering the Vault: K20 and K92

Gaining access to the mass standards requires strict contamination controls and physical security protocols. After donning protective shoe covers using a mechanical “BootieButler” dispenser and passing through keyed security layers, Abbott presents K20 alongside a modern counterpart, K92. For over 130 years, virtually every accurate mass determination made in the United States has been traceable back to K20, an unvarnished cylinder composed of 90 percent platinum and 10 percent iridium. The iridium confers essential hardness to the otherwise soft platinum, and both elements offer exceptional density along with high resistance to chemical oxidation.

```mermaid flowchart TD accTitle: Mass Standard Traceability Flow accDescr: Diagram illustrating how mass standard definitions flow from the International Prototype Kilogram in Paris down to everyday American commercial scales.

subgraph Paris ["Sèvres, France (BIPM)"] IPK["International Prototype Kilogram (IPK)"] end

subgraph NIST ["NIST Vault (Gaithersburg, MD)"] K20["Primary Standard K20<br/>(90% Pt / 10% Ir, 21.5 g/cm³)"] SS["Secondary Standards<br/>(Stainless Steel, 8 g/cm³)"] end

subgraph Field ["US Commerce & Industry"] State["State Metrology Labs"] Scales["Commercial & Scientific Scales<br/>(Pounds, Ounces, Grams)"] end

IPK -->|"Periodic international recalibration"| K20 K20 -->|"Mass comparator transfer (buoyancy corrected)"| SS SS -->|"Disseminated calibration"| State State -->|"Traceable calibration factor"| Scales ```

Looking closely at K20 reveals machining swirls and a slightly rough texture typical of late nineteenth-century fabrication, whereas the newer K92 gleams with a mirror polish. Abbott explains that a pristine finish is not purely aesthetic: “Well you want it not to be very rough because roughness increases surface area, surface area increases the probability that you will get contaminants on that will change the mass of the entire thing.” While fabricating a new platinum-iridium prototype costs roughly $100,000, historic national standards like K4 and K20 are irreplaceable cultural and scientific treasures.

- **Alloy composition:** 90% platinum, 10% iridium (selected for hardness, density, and oxidation resistance). - **Surface finish effect:** Microscopic surface roughness expands contact area, trapping ambient hydrocarbons and particulates that alter physical mass. - **Replacement cost:** A newly manufactured prototype cylinder costs roughly $100,000; the historical originals are priceless.

---

[03:54](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=234s) Secondary Standards and Buoyancy Calibration

To preserve K20 from physical wear and airborne contamination, NIST metrologists do not loan it out directly; instead, they transfer its calibrated value to secondary working standards made of stainless steel. This process introduces a significant aerodynamic complication. Stainless steel has a density of approximately 8 g/cm³, whereas platinum-iridium reaches roughly 21.5 g/cm³—making the stainless steel weights nearly three times larger in physical volume for the same mass.

Because the volumes diverge so dramatically, air buoyancy exerts an unequal upward force equal to the mass of the air displaced by each cylinder. At atmospheric pressure, a stainless steel standard experiences around 110 milligrams more buoyant lift than K20. To prevent barometric pressure swings, relative humidity, and temperature variations from distorting measurements, the comparison takes place inside a tightly isolated mass comparator chamber. Meterological fluctuations—chief among them barometric pressure—alter the local density of air and must be factored into every high-precision comparison.

- **Stainless steel density:** ~8.0 g/cm³ - **Platinum-iridium density:** ~21.5 g/cm³ (almost 3× denser than steel) - **Buoyancy offset:** Up to ~110 mg difference in upward buoyant force between equal-mass steel and platinum-iridium cylinders. - **Mass comparator environment:** Enclosed in an isolated chamber to stabilize temperature, humidity, and barometric pressure fluctuations.

---

[05:21](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=321s) Hand-Carrying Mass Standards and the Drift Crisis

Periodically, national prototype kilograms must travel back to the International Bureau of Weights and Measures (BIPM) outside Paris for calibration against the primary IPK. Transporting an irreplaceable national artifact involves surprisingly analog logistics: the cylinder is secured in its protective container, wrapped in bubble wrap, stashed in an ordinary camera bag, and hand-carried like a diplomatic pouch. Abbott notes that couriers never let the bag out of their sight or stash it in airplane overhead bins, treating it with the care of a nuclear briefcase. This manual transit once triggered an airport customs scare: when Abbott stated the artifact was made of platinum and iridium, an official mistook iridium for a radioactive threat, requiring Abbott to defuse the situation before boarding.

The far greater scientific crisis was uncovered during the periodic weigh-ins in Paris. When all 40 sister prototypes were compared against the IPK, the data indicated that the IPK appeared to have grown roughly 50 micrograms heavier than its siblings over the course of a century—or conversely, that the national prototypes were losing mass. Intriguingly, measurements gathered at the end of 2013 showed that the IPK stopped gaining mass, while the various national prototypes continued to diverge in different directions.

Abbott emphasizes the inherent vulnerability of relying on an artifact whose value is circular: “All that matters is the difference between them and the International prototype because the International prototype is the only thing you need the only thing that you really know because it's defined as one kilogram.” Because metrologists could only measure relative differences, there was no independent benchmark to prove whether the IPK was accumulating surface contaminants or whether the copies were shedding mass through wear. This fundamental instability prompted the international metrology community to prepare a complete overhaul for 2018, abandoning physical cylinders to redefine the kilogram using the Planck constant.

- **Transport security:** Hand-carried in a bubble-wrapped camera bag; never separated from the metrologist or placed in aircraft overhead storage. - **Customs incident:** An airport screener confused the non-radioactive transition metal *iridium* with a radiological hazard. - **Drift magnitude:** A relative discrepancy of approximately 50 micrograms accumulated across a century of comparisons. - **The metrological limitation:** Because the IPK was defined as exactly one kilogram by decree, artifact-based metrology could detect divergence between copies, but could never determine absolute drift. - **2018 redefinition:** The upcoming revision of the International System of Units retired physical artifact prototypes in favor of fundamental physical constants.

## Mentioned Derek Muller, Patrick Abbott, Ben Stein, National Institute of Standards and Technology (NIST), Treaty of the Metre, International Prototype of the Kilogram (IPK), K4, K20, K92, International Bureau of Weights and Measures (BIPM), Planck constant

## Video description The US signed the metre convention and bases all customary units on SI standards. As an aside, the Utah constitution from 1895 required the metric system to be taught in schools. This requirement was repealed in 1987. ¯\_(ツ)_/¯

Huge thanks to NIST, Ben Stein and Patrick Abbott. https://www.nist.gov/ https://www.nist.gov/pml/weights-and-measures/si-units-mass

Special thanks to Patreon Supporters: Tony Fadell, Donal Botkin, Jeff Straathof, Zach Mueller, Ron Neal, Nathan Hansen

Back in 1875 The US signed the Metre Convention, which basically committed the country to use the metric system. In return, French scientists sent two platinum-iridium cylinders that weigh 1kg to the US in 1889 (known by their designations K4 and K20 from a set of 40 identical objects that were produced and sent around the world). So even though everything you see and buy in the US is usually reported in pounds, all weights are traceable back to the K20 kilogram (by applying a conversion factor to get to pounds).

When I was in DC a few weeks ago, I visited the National Institute of Standards and Technology (NIST) and got up close with K20, which is still kept there and used to calibrate all mass standards in this country. I thought it was pretty cool.

Edited by Bill Connor

[Source](https://www.youtube.com/watch?v=SmSJXC6_qQ8)

中文简报

NIST将所有美国常用度量衡校准至公制千克原器 美国传统单位没有独立的实物基准，在法律层面上只是基于公制原器人为设定的数学换算。 Derek Muller走访了美国国家标准与技术研究院（NIST），探寻一个鲜为人知的事实：美国在官方和法律层面上其实一直采用公制系统，磅和英寸等常用单位纯粹是按约定由公制数值换算而来。物理学家Patrick Abbott展示了国家原型千克圆柱体K20和K92，详细介绍了校准如何逐级传递至商业秤具，以及计量学家如何消除不锈钢二级标准件受到的空气浮力等干扰。探讨中还提及了一段真实的模拟时代经历——人们曾用相机包装着价值连城的铂铱合金实物，亲手护送至巴黎重新校准。该视频最终揭示了深层的计量学危机：国际千克原器与其全球各副本之间出现了50微克的漂移，这迫使国际科学界着手准备利用普朗克常数重新定义千克。 尽管文化上依然依赖磅和加仑等传统单位，但自美国签署1875年《米制公约》以来，美国的度量衡在法律层面上均源自公制系统。 (0:00) 美国国家质量标准可追溯至K20，这是保存在美国国家标准与技术研究院（NIST）的一件1889年原始铂铱合金圆柱体。 (1:27) 由于材料密度不同产生约110毫克的浮力差，以铂铱合金原器校准不锈钢二级标准件时，必须进行精确的空气浮力修正。 (3:54) 历经一个多世纪的比对，国际千克原器与其各国副本之间出现了约50微克的相对质量偏差，这促成了2018年基于普朗克常数对千克的重新定义。 (5:21)

Reading summary

Despite the widespread public belief that the United States uses an independent customary measurement system, American units are legally defined by and anchored to the metric system through physical mass prototypes like K20. The Metric Foundation of US Customary Units (0:00) The United States was an original signatory of the 1875 Treaty of the Metre, an international agreement that established the modern metric system. In 1889, the US received two platinum-iridium kilogram cylinders, designated K4 and K20, from an allotment of forty copies manufactured alongside the International Prototype of the Kilogram (IPK). According to National Institute of Standards and Technology (NIST) physicist Patrick Abbott, American units such as pounds, feet, and gallons maintain no independent physical reference standards; they are legally defined through direct mathematical conversions to International System of Units (SI) standards. This administrative reality means American metrology functions entirely on the metric system, applying conversion factors for commercial and public use that metrologists and scientists must continually calculate. Host Derek Muller and Abbott characterize this dual system of defining national weights and measures in SI units only to convert them into customary units as an unnecessary and counterintuitive complication. Physical Preservation and Buoyancy Calibration Challenges (3:54) For more than 130 years, nearly all high-precision mass determinations in the United States have traced back to K20, which is kept under strict physical security and contamination protocols at NIST. The prototype is cast from 90 percent platinum and 10 percent iridium—an alloy selected because iridium provides structural hardness to platinum, while both metals offer extreme density and high resistance to oxidation. Modern replacements like K92 feature a mirror polish, which Abbott explains is critical because microscopic surface roughness expands the surface area capable of trapping ambient hydrocarbons and dust that alter an artifact's mass. While fabricating a new replacement cylinder costs approximately $100,000, historic national standards are irreplaceable. To shield K20 from physical wear and environmental contamination, NIST metrologists do not calibrate commercial scales directly against it. Instead, they calibrate secondary working standards made of stainless steel. This transfer introduces significant physical discrepancies: stainless steel has a density of about 8.0 g/cm³, whereas platinum-iridium reaches roughly 21.5 g/cm³. Because the equal-mass steel cylinders displace nearly three times the volume of air, atmospheric buoyancy exerts roughly 110 milligrams more upward force on the steel weights. Metrologists must conduct comparisons inside sealed mass comparator chambers to measure and correct for local air density fluctuations caused by shifts in barometric pressure, temperature, and humidity. Artifact Divergence and the Planck Redefinition (5:21) National prototypes historically required periodic transit to the International Bureau of Weights and Measures (BIPM) outside Paris for direct comparison against the primary IPK. Transporting these cylinders involved rudimentary physical security: couriers carried the prototypes in bubble wrap inside ordinary camera bags without ever placing them in overhead compartments. During one transfer, an airport customs screener even mistook the prototype's iridium content for a radiological hazard. These international comparisons exposed a fundamental flaw in artifact-based metrology: over the course of a century, the IPK diverged by approximately 50 micrograms relative to its sister copies. Because the IPK was defined by law as exactly one kilogram, metrologists had no independent reference to prove whether the primary prototype was gaining mass via surface contamination or whether the distributed national copies were losing mass through handling. Measurements taken at the end of 2013 showed that the IPK's mass shift had halted while the national prototypes continued to drift in various directions. This inherent circularity and instability led the international metrology community to retire physical prototypes in 2018 in favor of a definition grounded in the Planck constant. Artifact-based metrology was fundamentally compromised because comparing physical cylinders can only measure relative divergence, never absolute drift. This vulnerability confirmed that national standards—even when rigorously safeguarded—must ultimately rely on invariant fundamental constants rather than manufactured objects.

精炼阅读摘要

尽管公众普遍认为美国长期沿用独立的美制计量体系，但美国国家标准与技术研究院（NIST）的物理解析表明，磅、英尺与加仑等传统单位在法律和物理基准上完全锚定于国际单位制（SI）。美国早在1875年签署《米制公约》后便以公制作为国家测量的底层核心，其日常生活中的非公制单位实质上仅是对底层公制数据进行的换算叠加。 美国度量衡背后的公制法定事实 (0:00) 虽然美国大众文化中普遍使用英尺、加仑和磅等美制单位，但这些度量单位在物理现实中并不存在任何独立的国家物理实物标准。物理学家Patrick Abbott向主持人Derek Muller证实，美国自1875年作为创始签约国加入《米制公约》（Treaty of the Metre）以来，便在法律层面上承诺采纳公制，并于1889年正式接收了编号为K4和K20的两具铂铱合金千克基准原器。美国现行的传统单位在法律定义上完全是通过固定的数学换算因子直接绑定至公制SI单位。 这种表面与底层脱节的计量架构引发了制度理性上的争议。Derek指出，国家测量系统全盘建立在公制标准之上，却又在应用端人为增加一层繁琐的换算系数，迫使科研人员与行业从业者必须反复进行反向换算，这显得十分反常。Patrick Abbott对此坦率认同，表示美国虽然在实际运作中执行这层换算，但从本质来看整个国家早已运行在公制系统之上，而人为保留这种转换机制确实显得不合逻辑。 国家基准K20的材料工艺与防污控制 (1:27) 保管于NIST地下安全金库中的K20圆柱体是过去130多年来美国几乎所有高精度质量测量与校准溯源的物理源头。为了防止外部污染，研究人员进入存放区域必须执行严格的安全验证并穿戴由机械鞋套机分发的专用鞋套。K20本身由90%的铂与10%的铱合金铸造而成，其中铱的加入为质地偏软的纯铂提供了关键的结构硬度，同时两种高密度贵金属组合赋予了原器极高的抗化学氧化能力。目前制造一枚同规格铂铱合金圆柱体的成本约为10万美元，但K20这类承载国家计量史的历史原器被视为无价之宝。 物理原器的表面加工精度对其质量稳定性具有决定性影响。与十九世纪末制造、表面残留微观车削纹理的K20相比，现代制造的K92原器表面打磨得如同镜面般光滑。Patrick Abbott解释称，极度光滑的镜面并非出于美观考量，而是为了尽可能缩减材料的微观表面积。物体表面若存在微观粗糙度，会显著扩大与外界空气的接触面积，从而增加吸附环境中碳氢化合物和悬浮颗粒物的概率，进而对整具原器的物理质量产生微克级的不可逆扰动。 次级标准质量传递中的空气浮力校正 (3:54) 为防止K20在日常检定中遭受机械磨损或空气污染，NIST计量人员绝不会将其直接出借或频繁移动，而是将其精密校准的质量基准传递至由不锈钢制造的日常工作用次级标准器。然而，这一物理传递过程引入了显著的空气动力学与流体力学修正难题。铂铱合金的密度高达约21.5 g/cm³，而不锈钢的密度仅约为8.0 g/cm³。这意味着在质量完全相同的条件下，不锈钢圆柱体的物理体积几乎是铂铱合金原器的三倍。 体积的巨大差异导致二者在大气中所排开的空气质量截然不同，进而产生不相等的向上浮力。在标准大气压环境下，不锈钢标准器所承受的空气浮力比同等质量的K20高出约110毫克。由于气压波动、相对湿度和环境温度变化会直接改变局部空气的实际密度，微小的气象扰动都会严重扭曲高精度天平的测量结果。因此，质量比较过程必须被安置在高度隔离的质量比较仪测试舱内进行，以严格控制并数学扣除环境浮力引发的误差。 人工原器的漂移困境与量子常数重构 (5:21) 为了维持国家度量衡的国际一致性，各国持有的千克基准原器必须定期送往位于法国塞夫勒的国际计量局（BIPM），与国际千克原器（IPK）进行直接对比校准。这一跨国运输流程高度依赖人工操作：原器在包裹气泡膜后被装入普通相机包，由计量人员全程贴身手提护送，严禁脱离视线或存放于客舱行李架上，其安保要求堪比核按钮手提箱。Patrick Abbott回忆，这种实体运输曾在机场安检环节引发险情，当时安检人员误将非放射性过渡金属“铱”（iridium）当成具有放射性危险的管制材料，需要专业人员当场澄清化解。 然而，人工物理原器最根本的危机在于其内生的漂移不可测性。巴黎百年间的定期比对数据显示，IPK与其40枚姐妹原器之间累计出现了约50微克的相对质量漂移，显示IPK比其复制品增重，或者副本相对变轻；至2013年底的测量又显示IPK停止增重，而各国原器则向不同方向各自漂移。Abbott强调，由于IPK在定义上被绝对规定为一千克，计量学界只能测量它与副本之间的相对差值，根本无法独立判定究竟是IPK因吸附表面污染物而变重，还是副本文物因摩擦磨损而变轻。这种基于物理实物循环定义的内在脆弱性，促使国际计量学界在2018年彻底废除实物原器，转为基于普朗克常数重新定义千克。 实物度量衡时代的落幕揭示了计量学的根本矛盾：依托人造实物的基准必然面临微观污染、机械损耗与相对漂移的物理极限。2018年国际单位制利用普朗克常数重构千克定义，彻底消除了对单个物理圆柱体的脆弱依赖；但在制度应用层，美国日常度量衡与底层公制标准之间的双轨换算脱节仍作为一种历史惯性继续存在。

Detailed notes — zh

## Background 该视频由知名科普频道 Veritasium 于 2017 年 5 月 10 日发布在 YouTube 平台上。视频简介指出，美国早在 1875 年就签署了《米制公约》，其现行所有英制单位在法定标准上均锚定于国际公制（SI）标准，主持人通过实地探访美国国家标准与技术研究院（NIST），近距离记录了支撑全美质量度量基准的 K20 千克原器。 ## TL;DR 视频揭示了美国度量衡鲜为人知的真相：表面上通行的英制单位，其底层法定基准早已完全依赖公制系统进行换算。主持人深入 NIST 实验室探访了拥有 130 年历史的国家千克原器 K20，详细解析了铂铱合金选材与表面加工对防止污染、保持质量恒定的科学原理。内容还展示了传递质量基准时必须克服的空气浮力干扰，并回顾了原器由专人跨国护送时遭遇的海关惊魂。最后，视频指出了实物原器在百年比对中发生无法解释的微克级质量漂移，进而引出了 2018 年国际计量界放弃实物圆柱体、转向基本物理常数重新定义千克的历史背景。 ## Atmosphere & style 全片洋溢着好奇探索与严谨科学交织的探秘氛围，主持人充满感染力的提问与计量专家直率幽默的解答形成了生动有趣的互动。从穿戴防护装备进入高安保洁净室的仪式感，到近距离凝视无价国家原器时的敬畏，镜头极具视觉带入感。在探讨严肃计量物理的同时穿插了专人携包护送、海关误认辐射等荒诞趣闻，在轻松诙谐中自然过渡到实物基准不可靠的深刻计量学困境，节奏明快且极富启发性。 ## Bulletin - [00:00](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=0s) 美国日常生活虽使用磅、英尺等英制单位，但法定标准早在1875年签署《米制公约》后就已全部基于公制单位，英制读数仅是数学换算的结果。 - [01:27](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=87s) 美国全境的质量标准溯源自编号为K20的铂铱合金千克原器，该原器由90%铂和10%铱制成，保存于美国国家标准与技术研究院（NIST）。 - [04:02](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=242s) 由于铂铱合金与不锈钢二级工作标准器的密度相差近三倍，空气浮力差异可在质量比对中引起约110毫克的读数偏差，校准必须在严密控温控湿的密闭室内进行。 - [06:49](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=409s) 跨越一个世纪的巴黎原器比对显示各国家原器与国际千克原器（IPK）出现约50微克的离散漂移，促使国际计量界决定自2018年起彻底废除实物原器、改用物理常数重新定义千克。 ## Key takeaways - [00:00](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=0s) 1880年代共制造了40个铂铱合金千克原器副本，美国作为《米制公约》签署国分得了编号为K4和K20的两个官方国家原器。 - [00:00](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=0s) 美国现行法定度量衡体系实际上完全建立在公制系统之上，日常使用的英制单位仅是通过中间换算系数表达的公制单位。 - [01:27](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=87s) K20由90%铂和10%铱合金制成，掺铱能大幅提升硬度，同时该合金具备极高的密度与卓越的抗氧化性。 - [01:27](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=87s) 千克原器对表面光洁度要求极高，现代制造（如K92）追求镜面光洁度是为了降低微观表面积，减少吸附空气污染物导致的质量改变。 - [04:02](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=242s) 铂铱合金原器密度（约21.5 g/cm³）约为不锈钢（约8 g/cm³）的近三倍，相同质量下不锈钢体积明显大得多。 - [04:02](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=242s) 不锈钢标准器与铂铱原器之间的巨大体积差导致了约110毫克的空气浮力偏差，浮力受温度、气压和湿度影响，因此比对必须在恒温恒湿密闭室进行。 - [05:21](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=321s) 各国的国家原器需要定期送回法国巴黎进行集体校准比对，运输过程由专人装入普通相机包并全程抱在怀中随身护送。 - [05:21](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=321s) 护送K20过海关时，海关人员曾因将合金名称中的“铱”（Iridium）误认为放射性物质而引发虚惊。 - [06:49](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=409s) 百年间的定期比对显示，国际千克原器（IPK）与其他40个国家原器之间出现了约50微克的漂移偏差，且各原器质量变化幅度各不相同。 - [06:49](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=409s) 基于实物圆柱体的基准完全依赖相对测量，在缺乏外部绝对物理基准的情况下，科学家无法确定具体哪一个原器发生了真实质量变化。 - [06:49](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=409s) 实物千克原器无法根除的质量漂移缺陷，直接推动了2018年国际计量界利用普朗克常数等基本物理常数对千克进行历史性重定义。 ## Detailed walkthrough ### [00:00](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=0s) 美国隐秘的公制底色与米制公约 主持人来到美国国家标准与技术研究院（NIST），准备亲眼见证最初的千克基准原器。接待他的专家 Patrick Abbott 证实了这一点，并介绍这些最初的原器制造于 19 世纪 80 年代。当时一共打造了 40 个与国际千克原器（IPK）互为“孪生兄弟”的圆柱体，并分发给签署《米制公约》（Treaty of the Meter）的各缔约国；其中美国分得了两个，编号分别为 K4 和 K20。当主持人问及《米制公约》究竟是什么时，Patrick 指出它就是现代公制体系的奠基石。 面对主持人关于“美国当年签署公约难道是打算采用公制吗”的疑问，Patrick 揭开了一个少有人知的事实：美国其实早已在公制体系上运作。美国日常生活和商业中普遍使用的英制单位（如英尺、加仑、磅等），其法定定义实际上全部基于国际公制单位转换而来，平日里的英制读数仅仅是一层换算而已。主持人对此忍不住吐槽，指出把所有度量衡都锚定在公制系统上、中间硬塞一个转换系数、之后某些领域又得换算回去的操作简直荒唐，Patrick 也当场笑着直率回应：“Yes, it's stupid.”（“是的，这很愚蠢。”） - 1880 年代共制造了 40 个铂铱合金千克原器副本，分发给《米制公约》签署国。 - 美国分得的两个官方国家原器编号分别为 K4 和 K20。 - 美国看似使用英制单位，但法定标准完全建立在公制定义之上，日常英制只是数学换算。 --- ### [01:27](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=87s) 进入实验室与一睹 K20 原器真容 为了亲眼见到存放在 NIST 内部的千克原器，两人必须通过严格的安保与洁净规程。在进入高安全级别区域前，主持人使用自动鞋套机（BootieButler）穿上防护鞋套，随后通过层层防护门并输入密码。Patrick 向主持人展示了两个原器：一个是拥有约 130 年历史的原始千克原器 K20，另一个则是现代制造的 K92。 能够如此近距离面对 K20 让主持人倍感震撼。在过去 130 年间，美国境内所有经过精准测量的质量数据，追根溯源都来自这块由 90% 铂和 10% 铱组成的金属块。选用铂铱合金是因为加入铱后硬度远高于纯铂，且二者均具有极高的密度与卓越的抗氧化性。K20 的制造工艺与时间均与存放于巴黎郊区地下保管库中的国际千克原器完全一致，而那件法国的原器在当时依然是全球“千克”的法定实物基准。 仔细俯视观察可以发现，由于早期加工技术的局限，K20 的表面抛光程度相对粗糙，留有明显的加工痕迹；相比之下，现代生产的 K92 表面光洁如镜，毫无旋纹。Patrick 解释，追求极高光洁度绝非为了美观，而是因为表面越粗糙，微观表面积就越大，附着空气污染物从而改变物体质量的概率也会随之增加。当谈及价值时，Patrick 透露购买一个现代全新原器大约需要 10 万美元，而承载了 130 年国家度量衡历史的 K20 和 K4 则是无法用金钱衡量的无价之宝。 ```mermaid flowchart TD accTitle: 原器精度要求与材料逻辑 accDescr: 展示千克原器的材料选择原因以及表面光洁度对质量稳定性的影响链条 subgraph Material["材料特性 (90% Pt + 10% Ir)"] M1["高密度与抗氧化性"] M2["掺铱提高合金硬度"] end subgraph Surface["表面光洁度工艺"] S1["现代制造技术 (如 K92)"] --> S2["极高光洁度 / 无旋纹"] S2 --> S3["降低微观比表面积"] S3 --> S4["减少空气污染物附着"] S4 --> S5["确保千克基准质量长期恒定"] end ``` --- ### [04:02](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=242s) 浮力效应与基准传递室的环境控制 该实验室的核心职责并非频繁搬动 K20，而是在不直接接触 K20 的前提下，将其精准的质量基准传递给需要校准的机构。技术人员通过质量比对仪，将铂铱合金原器的质量传递到不锈钢制成的二级工作标准器上。肉眼可见不锈钢标准器的体积要比铂铱合金原器大得多。这是由于两者的密度差异悬殊：不锈钢的密度约为 8 g/cm³，而铂铱合金高达约 21.5 g/cm³，后者的密度接近前者的三倍，因此相同质量下不锈钢体积要大近三倍。 体积差异带来了严峻的物理挑战——空气浮力。浸没在空气中的任何物体都会受到等于其排开空气重量的向上浮力。由于两类砝码体积差异巨大，在空气中比对与 K20 质量几乎相同的不锈钢标准器时，浮力差可导致读数产生约 110 毫克的偏差。浮力的大小取决于空气的温度、压力和湿度，其中气压对空气密度的影响最为剧烈。为此，整个高精度质量比对仪必须安置在与外界严密隔离的恒温恒湿密闭室内，以杜绝气象波动对校准精度的干扰。 --- ### [05:21](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=321s) 人肉护送原器与海关辐射虚惊 千克标准面临的最大危机不仅是日常天气波动，而是原器自身质量的漂移。为了重新校准，各国的千克原器都需要定期送回法国巴黎进行“大比武”式的集体称重比对，K20 也不例外。其运输方式出人意料地原始：完全依靠专人随身手提护送。原器被移出玻璃罩后放入专用固定盒，层层包裹气泡垫，塞入普通的相机包中，由专人斜挎在肩上，一刻也不能离开视线。 在乘坐飞机跨国运输时，护送人员绝不会把包放进头顶行李舱，而是像携带“核按钮手提箱”一样全程紧抱在怀中。Patrick 分享了一次险情：虽然从未被迫开箱检查，但曾有海关人员盘问其材质，当听到“铂铱合金”（Platinum-iridium）中的“铱”（Iridium）字时，对方误将其与放射性物质混淆，当场大惊失色、情绪激动。Patrick 花费了很大力气向海关人员科普和安抚，保证该物体绝对没有放射性、毫无安全威胁，才得以顺利过关。 --- ### [06:49](https://www.youtube.com/watch?v=SmSJXC6_qQ8&t=409s) 原器质量漂移与 2018 年千克重定义前夕 然而，比安检更严峻的真正危机是千克实物基准本身的不可靠性。在巴黎国际度量衡局的比对中，所有国家原器都会与国际千克原器（IPK）进行精确校准。绘制历史数据后科学家们震惊地发现，在一个世纪的跨度中，各原器与国际千克原器之间出现了明显的偏差，IPK 看起来比其他原器重了大约 50 微克。更令人困惑的是，2013 年底测得的最新数据显示，这种质量增长趋势似乎突然停滞了。 Patrick 指出，更令人费解的是这 40 个原器在比对中呈现出“分道扬镳”的离散态势：有的质量变化很大，有的变化较小，有的几乎没有变化。由于测量是相对的，人类唯一确知的是它们彼此之间的差值，而国际千克原器被法定定义为绝对的“1 千克”，因此无法得知到底是 IPK 单独发生了变化，还是所有砝码都在以不同速率增重或失重。Patrick 强调：“It's a relative measurement and that's the weakness of the system.”（“这是一种相对测量，而这正是该体系的致命弱点。”）正是由于实物圆柱体存在无法根除的漂移风险，国际计量界决定从 2018 年起彻底废除金属圆柱体定义，转向利用基本物理常数来重新定义千克。 - 历经百年的比对显示，各国家原器与国际千克原器之间产生了约 50 微克的离散漂移。 - 相对测量机制使得科学家无法在不借助外部绝对物理常数的情况下，厘清具体哪一个原器在物理上发生了质量改变。 - 这一根本缺陷直接促成了 2018 年国际单位制（SI）对千克定义的历史性重构。 ## Mentioned Veritasium, Patrick Abbott, Ben Stein, National Institute of Standards and Technology (NIST), 国际千克原器 (IPK), K4, K20, K92, 米制公约 (Treaty of the Meter), 国际度量衡局 (BIPM), BootieButler ## Video description The US signed the metre convention and bases all customary units on SI standards. As an aside, the Utah constitution from 1895 required the metric system to be taught in schools. This requirement was repealed in 1987. ¯\_(ツ)_/¯ Huge thanks to NIST, Ben Stein and Patrick Abbott. https://www.nist.gov/ https://www.nist.gov/pml/weights-and-measures/si-units-mass Special thanks to Patreon Supporters: Tony Fadell, Donal Botkin, Jeff Straathof, Zach Mueller, Ron Neal, Nathan Hansen Back in 1875 The US signed the Metre Convention, which basically committed the country to use the metric system. In return, French scientists sent two platinum-iridium cylinders that weigh 1kg to the US in 1889 (known by their designations K4 and K20 from a set of 40 identical objects that were produced and sent around the world). So even though everything you see and buy in the US is usually reported in pounds, all weights are traceable back to the K20 kilogram (by applying a conversion factor to get to pounds). When I was in DC a few weeks ago, I visited the National Institute of Standards and Technology (NIST) and got up close with K20, which is still kept there and used to calibrate all mass standards in this country. I thought it was pretty cool. Edited by Bill Connor [Source](https://www.youtube.com/watch?v=SmSJXC6_qQ8)

Original transcript

[0:00] I'm here at the National Institute for Standards and Technology, and [0:03] I'm about to see some of the original kilogram standards. Is that right? [0:08] Patrick: You are, you are. [0:09] When were these made? [0:11] Well the originals were made in the 1880s. [0:15] There were 40 of them that were brethren of the [0:18] International prototype kilogram [0:20] and these 40 were distributed to the signatories [0:23] of the Treaty of the meter and the [0:26] United States was given two, K4 and K20. [0:30] What was this meter agreement? [0:33] The treaty of the meter? [0:34] Yeah, what is that? [0:35] It's the modern-day foundation of the metric system. [0:38] So the U.S. signed that? [0:40] The U.S. signed it. [0:41] As if they were going to become metric? [0:44] Yes. [0:45] A little known secret and I'll tell you something else [0:48] that all the units that we commonly use [0:50] like feet and gallons and so on are [0:55] actually defined in terms of metric units. [0:58] So it's just a little translation [1:00] that we do here but our country is [1:02] actually on the metric system. [1:05] [Laughter] [1:07] Doesn't that seem crazy? [1:09] Yes! [1:09] Isn't that insane that like you base all of these measurements on the metric [1:13] system and then you add a conversion [1:15] factor and then later some people have [1:18] to convert back. [1:18] Yes, it's stupid. [1:20] [Laughter] [1:22] Are you allowed to say that? [1:24] It's true. [1:27] So can we see it? [1:27] sure sure [1:28] What do we have to do? [1:29] This requires that we go through [1:31] some high-security gear and also that [1:33] would be somewhat clean in what we do [1:36] so I'll have to ask you to put on some [1:38] booties here. [1:39] Let me see if I can do this [1:40] all right so, did I do it? [1:43] Yep, there you go. [1:44] straight back, all the way there you go. [1:46] This is the first [1:47] BootieButler that I've ever... [1:50] I'm a big fan. [1:54] This is the first layer. [2:00] Secret code [2:05] I'm going to show you two I'm going to [2:07] show you one of the originals and I'm [2:08] going to show you a more modern version [2:12] This is K 20. [2:14] Oh my goodness that is like the [2:17] original kilogram mass standard of the US. [2:19] That's right. [2:22] I never thought I would [2:23] get this close to it [2:25] virtually every mass that has been [2:26] accurately weighed in the US over the [2:28] past hundred and thirty years can trace [2:30] its measurement back to this one [2:32] kilogram hunk of 90 percent platinum 10 percent iridium. [2:35] Iridium makes the alloy much [2:37] harder than pure platinum and both [2:39] elements were selected for their high [2:40] density and resistance to oxidation this [2:43] kilogram was created in the same way at [2:45] around the same time as an object which [2:48] to this day remains the definition of [2:50] the kilogram an identical cylinder [2:52] stored in a basement vault on the [2:54] outskirts of Paris. [2:55] If you look straight [2:57] down from the top you can maybe see K 20 [3:01] doesn't have a very nice finish on it [3:03] compared to the more modern prototypes [3:06] here's K 92 so it's got a much higher [3:09] polish on it different manufacturing [3:12] techniques absolutely no swirls it is a [3:15] really like beautiful looking specimen [3:17] yes it is I mean is there a reason why [3:20] you want it to look so beautiful? [3:22] well you want it not to be very rough [3:26] because roughness increases surface area [3:28] surface area increases the probability [3:31] that you will get contaminants on that [3:33] will change the mass of the entire thing [3:35] how much is it worth [3:37] well monetarily the new ones are about [3:42] $100,000 a piece if you were to buy one [3:44] oh my goodness so but if you think about [3:47] K 20 K for with about 130 years of [3:51] history they're priceless you could [3:54] never replace them the purpose of this [3:55] room then is to share the precise mass [3:58] of K 20 with anyone who wants to make a [3:59] measurement without sharing K 20 itself [4:02] what we do in this suite here is we [4:06] transfer the definition of the kilogram [4:08] from the platinum-iridium prototypes to [4:10] stainless steel secondary standards and [4:12] you can tell that they're a lot bigger [4:16] and the platinum-iridium prototypes and [4:19] that's because of the relative [4:21] difference in density the density of [4:23] stainless steel is about 8 grams per [4:27] cubic centimeter whereas the density of [4:30] platinum iridium is about 21.5 grams per [4:34] cubic centimeter so there's all of the [4:36] three times difference in density which [4:38] tells you why this is so much bigger in [4:40] volume and that creates a problem [4:42] ordinarily we don't worry much about the [4:44] buoyant force that is the upward force [4:46] on every object in the atmosphere equal [4:48] to the weight of air it displaces but [4:50] since the volumes of these masses are so [4:52] different a stainless steel object that [4:54] has around the same mass as k20 can have [4:56] its weight reduced due to buoyancy by [4:58] around 110 milligrams [5:00] the precise amount depends on the [5:02] temperature pressure and humidity of the [5:04] air that's why the mass comparator [5:06] itself is inside of a chamber here [5:09] that's isolated from the outside world [5:11] so that the temperature remains [5:12] relatively stable I'm the humidity the [5:14] same pressure is by far the biggest [5:16] contributor to air density so we don't [5:19] want it changing all over the place with [5:21] the weather the problem scientists are [5:22] having with the kilogram now is much [5:24] bigger than weather fluctuations it's [5:26] something they discovered when all the [5:28] original kilograms were returned to [5:29] Paris for a weigh-in including k20 how [5:32] did it get there it gets there by a [5:36] person hand carrying it each prototype [5:38] gets taken out of the bell jar and put [5:42] in its container how do we sound after [5:49] we get it in there all nicely secured [5:53] then we wrap the whole thing in bubble [5:57] wrap and put it in a camera bag and [6:00] sling it over our shoulders don't let it [6:04] out of our sight it sounds a little [6:05] casual so you actually like while you're [6:07] on the plane do you put it in like the [6:09] overhead stretch [6:11] know it stay just at all times like it's [6:14] a you know the nuclear football codes [6:16] were setting off a nuclear weapon and [6:19] you never have any scares while you're [6:20] carrying the only scare comes that [6:23] somebody wants to see it like a customs [6:24] official I've never had to open it [6:26] although I had a kind of a scary moment [6:29] at one time when they asked me what it [6:32] was made of and I told them it was [6:33] platinum iridium and somebody heard the [6:36] word iridium and connected that with [6:39] radioactivity no no and kind of you know [6:42] flew off the handle a little bit and I [6:43] had to calm them down on the shoreland [6:46] that it wasn't radioactive there was no [6:48] threats going on here [6:49] the real threat was the unreliable [6:51] weight and the kilogram mass standard [6:53] what they do is a series of comparisons [6:55] they compare every one of those with the [6:58] International prototype kilogram using [7:01] that data and plotting it it looked as [7:03] if there had been a change in the [7:05] International prototype that made it [7:07] about 50 micrograms heavier than one [7:11] kilogram now over the course of a [7:13] hundred years all right [7:14] but the interesting thing is if one [7:17] looks at the data that was recently [7:18] taken at the end of 2013 international [7:22] prototype kilogram did not show an [7:25] increasing mass telling me for several [7:28] decades it was gaining mass and also on [7:29] the stop which you know I find that hard [7:33] to explain but it seemed like all of the [7:35] 40 masses in to be somewhat diverging at [7:38] that weigh-in they went different ways [7:40] they went in different directions yes so [7:43] that some changed a lot some changed [7:46] little some hardly changed at all but [7:48] it's hard to tell I mean they could [7:49] we'll all be changing a lot but because [7:51] all you can tell is the difference [7:52] between them they've already married [7:53] all that matters is the difference [7:55] between them and the International [7:58] prototype because the International [7:59] prototype is the only thing you need the [8:01] only thing that you really know because [8:02] it's defined as one kilogram [8:06] right but I mean the outside of that [8:08] definition there's a chance it could [8:09] have gained you know five grams as long [8:12] as they all gain five grams you know I'm [8:13] having these ridiculous obviously it's [8:15] ridiculous but the point is they could [8:18] have all been gaining or they could all [8:19] been losing right it's a relative [8:21] measurement and that's the weakness of [8:23] the system [8:24] and that's why as of 2018 the kilogram [8:27] will no longer be defined as the mass of [8:29] a metal cylinder why if you want to find [8:37] out how the kilogram will be redefined [8:39] be sure to subscribe to Veritasium