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Questions tagged [metrology]

For questions about the science of measurement, such as how units are defined or how practical measurements are connected to those definitions.

The International Bureau of Weight and Measures defines Metrology as " the science of measurement and its application. Metrology includes all theoretical and practical aspects of measurement, whatever the measurement uncertainty and field of application." This includes the definition of measurement units and how practical measurements can be connected to those definitions. Metrology provides the basis for the measurements that are essential in science, technology, engineering, medicine, commerce, industry, and everyday life.

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How do you make more precise instruments while only using less precise instruments?

I'm not sure where this question should go, but I think this site is as good as any. When humankind started out, all we had was sticks and stones. Today we have electron microscopes, gigapixel cameras and atomic clocks. These instruments are many…
Vilx-
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Why was carbon-12 chosen for the atomic mass unit?

The atomic mass unit is defined as 1/12th the mass of a carbon-12 atom. Was there any physical reason for such a definition? Were they trying to include electrons in the atomic mass unit? Why not define the amu as the mass of one proton or neutron…
Dieblitzen
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What reference clock is an atomic clock measured against?

I looked at a few of the other posts regarding the accuracy of atomic clocks, but I was not able to derive the answer to my question myself. I've seen it stated that atomic clocks are accurate on the order of $10^{-16}$ seconds per second. However,…
zrbecker
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Why is the mole/"amount of substance" a dimensional quantity?

According to the BIPM and Wikipedia, "amount of substance" (as measured in moles) is one of the base quantities in our system of weights and measures. Why? I get why the mole is useful as a unit. In fact, my question isn't really about the mole at…
Tim Pederick
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Why is the ampere a base unit and not the coulomb?

I always thought of current as the time derivative of charge, $\frac{dq}{dt}$. However, I found out recently that it is the ampere that is the base unit and not the coulomb. Why is this? It seems to me that charge can exist without current, but…
Greg
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Why do atomic clocks only use caesium?

Modern atomic clocks only use caesium atoms as oscillators. Why don't we use other atoms for this role?
Pinki
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What are the proposed realizations in the New SI for the kilogram, ampere, kelvin and mole?

The metrology world is currently in the middle of overhauling the definitions of the SI units to reflect the recent technological advances that enable us to get much more precise values for the fundamental constants of nature than were possible when…
Emilio Pisanty
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Why is a second equal to the duration of exactly 9,192,631,770 periods of radiations?

Why is a second equal to the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom? Why is the number of periods so complicated? It could be any…
A. Vats
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Why does the speed of light in vacuum have no uncertainty?

I could understand that the definition of a second wouldn't have an uncertainty when related to the transition of the Cs atom, so it doesn't have an error because it's an absolute reference and we measure other stuff using the physical definition of…
The Quantum Physicist
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Has anyone charged an object with 1 coulomb? Why was such a ridiculously large charge chosen as the unit of charge?

The fact that two balls charged with 1 coulomb each would repel/attract each other from a distance of 1 metre with a force sufficient to lift the Seawise Giant would suggest me otherwise, but has anyone ever charged an object with 1 coulomb of net…
moonblink
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In nuclear physics, what length year in seconds is used?

So I'm working on a nuclear physics problem and am looking at radioactive decay. The common unit used for very long decays is years within the literature. Is this the sidereal or tropical year? I want to use units of seconds but seeing as how these…
RocketTwitch
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Why is the prospective new kilogram standard a sphere?

I can understand the choice of material, silicon 28, but why is it a sphere rather than (say) a cube? Article here I would have thought that a sphere would have been the hardest shape to machine accurately.
user56903
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Will the volt, ampere, ohm or other electrical units change on May 20th, 2019?

When watching a video by Veritasium about the SI units redefinition (5:29), a claim that the volt and unit of resistance (presumably the ohm) will change by about 1 part in 10 million caught my attention: [...] I should point out that a volt will…
user231851
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Why are scientists involved in the Avogadro Project using silicon-28 atoms instead of carbon-12?

My question is, why use silicon-28 atoms to calculate the kilogram when you already have carbon-12 atoms defining the constant? Does the Avogadro Project intend to define the constant by replacing the idea of carbon-12 and putting silicon-28 in its…
Captaine Code
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Why isn't it $E \approx 27.642 \times mc^2$?

Why is it that many of the most important physical equations don't have ugly numbers (i.e., "arbitrary" irrational factors) to line up both sides? Why can so many equations be expressed so neatly with small natural numbers while recycling a…
badroit
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