# 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.

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### The relationship betwen Equivalent Megatons and Yield

This article defines a concept called Equivalent Megatons (EMT) and gives a formula of calculating it in terms of Yield (measured in megatons). In evaluating the destructive power of a weapons system,...
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1 vote
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### Melting point of water [duplicate]

Wikipedia says about melting point that: The melting point of ice at 1 atmosphere of pressure is very close to 0 °C (32 °F; 273 K); this is also known as the ice point. But why it is very close and ...
1 vote
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### Is Coulomb written/found on any object/product as its unit?

Today I had physics class and the unit Coulomb made me wonder as to why it was even invented when I don't see it being used anywhere. When I asked where it may be shown just like bulbs have x Watts ...
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### I am confused about the apparent fact that the numerical values of the Planck units are unchanged by exchanging $\hbar$ for $h$ [closed]

Can you confirm that each of Max Planck's original equations for Planck units included $h/2π$. If yes, does this explain how $ħ$ can be used In Planck's formulas without changing the numerical results ...
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### Why isn't the charge of an electron taken to be "$-1.6 \times 10^{-19}$ coulombs" [closed]

Even though we refer to electrons being negatively charged, why is it that we don't write the charge as "-1.6 × 10^-19 coulombs"?
1 vote
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### Connection between quantum non demolition measurement and the Heisenberg limit in metrology

Quantum non demolition measurements are those that do not feed back action noise into the measured observable. For discrete systems, this leads to the somewhat trivial case of where the observable (...
53 views

### Why can the Ampere not be defined as the flow of $n$ Coulomb in $n$ seconds? [closed]

1 Ampere is defined as the flow of 1 Coulomb of charge in one second. However, I do not understand why it cannot be defined as the flow of n Coulomb of charge in n seconds. This definition is ...
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### How do you know mercury changes monotonically with temperature if mercury itself is used to make the thermometer?

In the book I am reading recently "Concept of physics" volume 2 by professor H.C. Verma it says that (I am just summarizing the main points chronologically) Energy is transferred from hot ...
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### What exactly is kg? [closed]

Kilogram literally makes no sense. We say that we are 10 kilogram by using the weighing scale but according to its definition, it is the amount of matter contained in an object. So how much matter is ...
53 views

### Correct way to reference to a percentage value's category/label

When a percentage is written as, for example: 20% of sold products were of product A and 80% product B. What is the correct concept / word to use to describe "product A"? Is it units? As ...
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### How it was found that 12 g of carbon-12 has Avogadro's number of atoms? [duplicate]

I just want to know the history of finding Avogadro's number of atoms in 12 gm of C-12 and why C-12 only? Like I was asking how scientists came to conclusion that there is exactly 6.023*10^23 atoms in ...
1 vote
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### How much does the International Prototype Kilogramme weigh?

From [some date in] 1889 to 20 May 2019 the International Prototype Kilogramme in Paris (called the ur-kilogram by German-influenced writers) weighed $1.000\,000\,00kg$. It seems very hard to find out ...
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### Why the specific factor of $2 * 10^{-7}$ is used while defining 1 ampere?

while defining an ampere through force between two current-carrying wires, I noticed that they considered the force between two wires as $2 \times 10^{-7}$ forming a definition as follows:- "One ...
35 views

### Base units in CGS and technic systems

What are the base units in the CGS and the technical system of units? Is there not seven? How could it be that what is fundamental in any system is not in other? By the way, how many minimal "...
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### Illogical choice for the new definition of the charge of the electron by the 28th General Conference on Weights and Measures

The present definition of the Coulomb, since May 20th 2019, is that one electron has a charge of exactly $1.602176634 \times 10^{-19}$C Previously, the Coulomb was defined (through the Lorentz force ...
133 views

### What limits mechanical watch precision? [closed]

It seems like good mechanical watches gain or lose +/- a few seconds per day. 5 s/day corresponds to a fractional frequency stability of $5\times 10^{-5}$. What limits this stability? Obvious ...
210 views

### What determines the existence of a physical dimension, as in dimensional analysis?

Dimensional analysis in physics is based on the idea that only commensurable quantities can be added, subtracted, equated or otherwise compared. The five dimensions which appear in dimensional ...
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### Metrology: How are the most accurate measurements made? [closed]

How is a most accurate measurement made when there is no other equipment to verify it? Consider you base your apparatus on a set of theory and assumptions, and the result does not match prediction. ...
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### "Earth spinning faster will break GPS and atomic clocks".. Umm, how?

Since it's just come out that Earth is apparently spinning about a milliscecond and a half faster.. I see a bunch of "articles" claiming this will.. somehow.. break atomic clocks and make ...
166 views

### Calling angles dimensionless and simultaneously distinguishing between numbers and angles is inconsistent, isn't it?

The SI Brochure says that each angle is the ratio of two lengths - i.e. a number - and therefore a derived and dimensionless quantity. On the other hand, the papers  and  suggest that angles are ...
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### In what sense is this a feedback control?

I am reading a couple of papers on control enhanced parameter estimation. One is titled "Optimal Feedback Scheme and Universal Time Scaling for Hamiltonian Parameter Estimation" (arXiv ...
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### Is the definition of a meter tautological?

The speed of light is defined as $c=299{,}792{,}458\,\mathrm{m/s}$, and a meter is defined as the distance that light travels in a $1/299{,}792{,}458=1/c$ of a second, but then we would have defined a ...
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### What is a "cycle time" in Allan deviation formula for atomic clock instability? And why does having more independent atoms reduce $\sigma_y(\tau)$?

The usual formula for clock instability is given as $\sigma_y(\tau)\approx\frac{\Delta f}{f_0\sqrt{N}}\sqrt{\frac{T_c}{\tau}}$ First off what do each of these symbols really mean? What is $T_c$? The ...
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### Why there is no unit of energy? [duplicate]

In the SI System (https://en.wikipedia.org/wiki/International_System_of_Units) there are 7 units, but no unit of energy, though this is surely the fundamental unit of physics. Yet the Joule is usually ...
1 vote
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### The definition of Time and second

From the book "University of Physics 15th edition", in chapter 1, they talk about the fundamental units. They stated that the definition of unit of time is based on an atomic clock, where 1s ...
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### How is the Heisenberg Uncertainty calculated value added to a measurement error bar?

It is not clear to me if the calculated HU for a specific experiment is added to the measured value Gaussian curve together with the statistical errors (i.e. added to the total error bar) of this ...
This question is motivated by me trying to finish my answer to this question. In the 2019 SI redefinition the kg was redefined in terms of Planck's constant (and the second and meter):  1 \text{ kg}=...