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Questions tagged [metrology]

The study of measurements

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2answers
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What experimental bounds do we have on big $G$?

I know that there has been a large amount of controversy surrounding the exact value of the gravitational constant $G$, but I know that there is not a substantial difference in the measured value. So ...
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2answers
554 views

Complete list of fundamental properties [closed]

What are all of the fundamental properties, that is all measurable quantities which are not derived from anything else? Many quantities are derived e.g. area is length squared, velocity is length per ...
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0answers
94 views

Thermometer calibration at temperatures away from the ice point and the triple point of water [closed]

Calibrations of thermometers at 'Ice point' as well as at 'triple point of water' are well known and promoted by standard issuing bureaus. However, I experience difficulty to find a simple and ...
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5answers
72 views

Is there such a thing as a work meter to measure work?

I know there are devices to measure power (e.g., dynamometers) but I can't think of any example of a device that measures work directly. Is there such a thing?
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1answer
35 views

Definitive measurement of the correct “gravity potential difference” and uncertainty of its determination by “conventional measurement”

In this article (doi) describing recent experiments on "Geodesy and metrology with a transportable clock" featuring a "transportable Sr optical lattice clock" there is an interesting remark towards ...
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2answers
950 views

What makes the thorium-229 nuclear transition special?

Thorium-229 has a famous isomer with an excitation energy of only about 7.8 eV. As I gather from the wikipedia page, this transition was discovered essentially by accident from gamma ray spectroscopy....
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3answers
690 views

Definition of Joule?

The Wikipedia definition of Joule is: the energy transferred to (or work done on) an object when a force of one newton acts on that object in the direction of its motion through a distance of ...
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1answer
107 views

Why is the kilogram defined using Earth's gravity? [closed]

Since there are variations of $g$ depending on location on Earth's surface, why not use a reproducible lab experiment using a vertical axis centrifugal balance, and say that one kg is defined by ...
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2answers
334 views

Why must the kilogram standard be based on a kilogram mass object?

Inspired by the accepted answer to a question about the Avogadro Project, why must an object used to define a new standard for the kilogram have a mass of one full kilogram? If a smaller mass were ...
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2answers
5k views

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 ...
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1answer
101 views

Why aren't SI units squared or cubed, e.g. cubic centimetres?

For example when we look at cm^3: the multiplier is then (10^-2)^3 so why don't we write (cm)^3 instead?
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2answers
630 views

Why frequency is a SI-derived unit?

Frequency is a derived SI unit but its unit is 1/s. It uses only time once and no other fundamental quantities. So why is not included in SI base unit?
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1answer
988 views

Is displacement vector fundamental or derived quantity?

We know that we have 7 fundamental quantities (all scalars) and length is one of them. I classify velocity as a derived quantity. What about a displacement vector? How do I classify displacement ...
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1answer
189 views

What's the point of normalizing air pressure in the weather report?

If I were to check the weather report on a typical day where I live I would probably see that the air pressure is something like 101kPa. I understand that this has been normalized to sea-level. In ...
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2answers
970 views

Proper reference for the International System of Units

I would like to cite a relevant document that guides usage of SI units. It seems like currently there are two nearly identical literature sources: 8th edition of SI Brochure issued by BIPM, 2006; ...
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2answers
597 views

What is the name and symbol of the derived unit for one thousand metres? [closed]

I'm still getting confused of what a derived unit is. I used the internet to find an answer but I'm still getting confused.
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3answers
7k views

Is the fact that 100 kPa equals about 1 atmosphere accidental?

Typical atmosphere near sea level, in ambient conditions is around 100,000 pascals. But the pascal, as the unit, is not defined through Earth atmospheric pressure. It's defined as one newton per ...
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1answer
160 views

Speed of light in vacuum - is it really a constant and what is the accurate value? [duplicate]

Let's suppose that the speed of light would not be a constant but a function of something. As is quite clear now, universe is expanding exponentially. If the speed of light would actually be a ...
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1answer
605 views

Why is a Coulomb not a fundamental unit? [duplicate]

Why is a coulomb not a fundamental unit but an ampere is considering that a coulomb is more ' fundamental '?
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2answers
3k views

Is 12 amu for Carbon-12 exact or rounded?

I'm having a difficult time understanding how Carbon-12 has an atomic mass of 12. Each proton and neutron has an atomic mass that is just slightly above 1 amu, so wouldn't Carbon-12 also be above 12? ...
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1answer
113 views

Means to improve precision and accuracy

I have sensor measuring a distance to a rotating disk. Later on I want to use these values to produce some results like the range of the values. What options do I have to improve accuracy and ...
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3answers
2k views

How can we define the kilogram in terms of Planck's constant?

Scientists are going to redefine the kilogram by using the value of Planck's constant. What is the physics behind this would-be definition of a kilogram? How is Planck's constant related to kilogram? ...
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2answers
369 views

Redefining the kilogram using Planck's constant instead of the density of water among other examples

The kilogram is in the process of being redefined in terms of Planck's constant so as to eliminate its dependence on a physical artefact. Since the length and temperature units are already precisely ...
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2answers
190 views

Why are the electric and magnetic constants where they are?

$ε_0$, the electric permittivity and $μ_0$, the magnetic magnetic permeability were introduced in Coulomb's Constant and Ampere's Constant in order to make units and magnitudes match, in Coulomb's ...
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2answers
574 views

Watt (Kibble) balance and the kilogram - how does the dependence on $g$ get eliminated?

The standard ${kg}$ is now in the process of being redefined by the watt balance (rather than the lump of metal in Paris) A watt balance is very simple, you measure the force needed to support a mass ...
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3answers
165 views

Base unit definition and realization under relativistic conditions

Question Section 1.5 on page 107 of the full SI brochure, Le Système international d’unités, 8ᵉ édition, comments on physically realizing unit definitions when accounting for relativistic effects (I ...
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1answer
144 views

How much variability was there in the O=16 atomic mass standard, and how does it compare to the shift to the current carbon standard?

This is a follow-up question to this excellent answer by David Hammen on the reasons and history of the choice of carbon as the element used to define the atomic mass unit. As related in that answer, ...
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3answers
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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 ...
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4answers
810 views

Why are electrical units (specifically, electrical current) considered a base unit?

Note: this is NOT a question why current is the base unit as opposed to charge—that’s because measuring $1 \ \mathrm{ A }$ through a wire is easier to measure in a lab than is $1 \ \mathrm{ C }$ in ...
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0answers
75 views

Why is ampere defined with $2 . 10^{-7}$? [duplicate]

The formal definition of the Ampere is that it is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one ...
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6answers
4k views

Why is the meter considered a basic SI unit if its definition depends on the second?

The metre is the length of the path travelled by light in vacuum during a time interval of 1  ⁄ 299792458 of a second. – 17th CGPM (1983, Resolution 1, CR, 97), source The meter (or metre) is ...
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1answer
69 views

Why do physical units have different prefixes than the ones used for counting? [closed]

For counting, we use thousand, million, billion, trillion etc. For physical quantities, thousand becomes Kilo, million is Mega and (short scale) billion is Giga. In both these systems, the consecutive ...
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2answers
256 views

Is a micrometer a type of caliper?

Wikipedia says: A caliper is a device used to measure the distance between two opposite sides of an object. Therefor under this definition doesn't a micrometer also count as a caliper?
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4answers
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Why is the mol a fundamental physical quantity? [duplicate]

I am starting to study physics in detail and as I read about physical quantities, I was puzzled why mol (amount of substance) is taken as a physical quantity. A physical quantity is any quantity ...
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1answer
10k views

How can a beam balance measure mass?

In Newtonian physics, mass is the amount of matter in an object. So, how can a beam balance measure the amount of matter in an object (which is the mass of the object).
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0answers
100 views

What is the meaning of non-integer powers of physical units in electrodynamics? [duplicate]

In the gaussian system the unit of the charge is the statcoulomb (statC), defined as $$ \mathrm{statC =dyn^{1/2} cm= cm^{3/2} g^{1/2} s^{−1}}$$ to be consistent with the fact that the Coulomb force is ...
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1answer
138 views

A unit that is not coherent?

"Derived units are defined as products of powers of the base units. When the product of powers includes no numerical factor other than one, the derived units are called coherent derived units" I know ...
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3answers
144 views

How can I calibrate a limited-range thermometer?

I've got an electronic weather thermometer with a limited range (roughly -20°C to 40°C, in 0.1° intervals). How could I go about calibrating it using household equipment? Getting a 0°C data point is ...
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2answers
106 views

How measurements for defining a second were made?

As many of you know, the second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the ...
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1answer
241 views

What's the motivation to define the unit of current as fundamental in SI system? [duplicate]

The charge in electromagnetism seemingly plays the same role like mass in the Newtonian mechanics.But why we define the current unit (Ampere) in SI system as the fundamental rather than charge unit?
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1answer
63 views

How to get rid of electric attraction between PP (polypropylene, polystyrene) and human body?

I am doing some paint mixings involving quite precise proportions of paints. I am mixing my components in PP or PS (polypropylene, polystyrene) 60ml bottles, see: Bottles datasheet Quite often, my ...
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1answer
225 views

Does a precise definition of the meter not involving light exist, so that variation of $c$ can be tested?

The second is defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. The metre ...
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1answer
213 views

Why we only need three independent unit to describe physics?

From the article below, it is said that we can describe physics with only 3 independent units: length, mass, and time in cgs system/ velocity, action, and energy in natural units system. However, why ...
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1answer
169 views

Definition of the Kilogram

I have been reading about how the SI kilogram has been defined throughout history. From what I understand, the kilogram was initially defined in terms of the mass of a specific volume of water at a ...
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2answers
817 views

Why are there seven SI base units?

I have been pondering this thought recently: why are there seven SI base units? Is there any relationship between this and any more fundamental physical fact or phenomenon? Is there a absolute ...
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1answer
341 views

Is the definition of the meter arbitrary? [duplicate]

From Wikipedia, the definition of the meter is The meter is defined as the distance traveled by light in a vacuum in 1/299792458 seconds. Why is this number of seconds chosen? Is there a ...
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1answer
2k views

Uncertainty propagation upon taking the median

If I have $N$ repeated occurrences of a measurement $x$ with uncorrelated errors and identical uncertainties $u_x$, and take the mean $\langle x\rangle$, the uncertainty on the mean becomes: $$u_{\...
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3answers
267 views

If the candela is a base SI unit, why isn't the sone an SI unit at all?

Related: Why is the candela a base unit of the SI? In the answers given in the previous question, the candela is included because lighting is important for humans. By the same argument, hearing is ...
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3answers
1k views

How do we know the precise speed of light in a vacuum? [duplicate]

From Wikipedia - "Its exact value is 299792458 metres per second" I'm wondering how we can be so precise as to the speed of light given that we cannot create a true vacuum on Earth and as I ...
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1answer
65 views

Relationship between platinun and iridium in the kilogram

Kilogram, is the basic unit of mass in the International System of Units ( SI) , and your employer is defined as the mass of the international prototype is composed of an alloy of platinum and iridium,...