Questions tagged [metrology]

The study of measurements

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228 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
214 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
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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
852 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
353 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
283 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
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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
69 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,...
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3answers
468 views

What is precision?

I’ve learnt that accuracy is how close to the true value a measurement is (i.e. low discrepancy between the experimental and theoretical values). Precision, on the other hand, is the reproducibility ...
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1answer
838 views

What's the difference between a realization, a representation and an implementation in metrology?

In a recent answer, a metrologist casually used the terms 'realization' and 'implementation' of an SI unit as if they were different, which looks very strange to the untrained eye. Some further ...
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1answer
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What is a base unit in the new SI, and why is the ampere one of them?

One question that comes up pretty much always in introductory electromagnetism courses is Why the base unit of electrical measurements is the ampere and not the coulomb, and the usual answer is that ...
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1answer
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Why is the kilogram the last SI unit which is defined in terms of a physical prototype? [duplicate]

All elementary SI units, except the kilogram, have been redefined depending of a physical constant. However kilogram still depends on the International Prototype Kilogram. Why is this? Is there no ...
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1answer
103 views

Absence of “charge” as a fundamental unit in natural system

In natural system of units, using speed of light, Planck's constant, and eV (for energy), we express "charge" dimension as: $[charge]=[Force]^{1/2}[length] $ (from Coloumb's law equation) There is ...
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4answers
358 views

Is there a case (besides light speed in any given medium) where speed is experimentally measured rather than theoretically calculated? [closed]

I studied physics throughout college, but I cannot recall a single time where I directly measured the velocity of an object or force. Every time I measured the components of velocity (distance and ...
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292 views

SI Base Unit definition of mass - obsolete?

According to the formal definition of the SI Base unit of mass, the kilogram, it is stated that "The kilogram is the unit of mass; it is equal to the mass of the international prototype of the ...
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4answers
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Can the accuracy in Planck's constant ever be increased?

I guess I am having some confusion about the history of calculating Planck's constant. I see the mass of the electron may come into the equation here but isn't the measurement of mass based mostly ...
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0answers
111 views

How can I estimate meta-uncertainty?

A type A uncertainty estimate is derived from repeated measurements. For example, I may estimate the uncertainty on a measurement by repeating the measurement $N$ times and then calculating some ...
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1answer
75 views

Will Project Avogadro introduce a new constant? [closed]

Will the Avogadro Project introduce a new constant? It seems to me that the aim of the project is to define the kilogramme as some constant number of silicon atoms. What would this constant be ...
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2answers
245 views

Do we have a better approximation of $c$ than 299792458 m/s? [duplicate]

All sites give this value as "exact" value. I mean, what's after the comma? 299792458,000 m/s?
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1answer
816 views

How are thermometers calibrated?

I know this is quite vague, but I was just thinking about it......like obviously now we mass produce things and we don't really think about them. But how was the first thermometer calibrated/how are ...
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2answers
70 views

Where is the periodic nature in the Cs atomic clock? [closed]

In case of pendulum clock,lets say one swing ticks one second..but what is the analogy in case of CAESIUM atomic clock? Is 9,192,631,770 ticks is equivalent to one tick in pendulum clock? And how we ...
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4answers
446 views

Is there an official list of independent units of measurements?

When I say 'independent units', I mean those which cannot be broken down anymore, and simultaneously forms the basis for any more, complex measurements. For example, height, length, and width can all ...
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3answers
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Is there any truth to interpreting definition of a second as corresponding to oscillations?

As far as I understand the definition of a second, the Cs-133 atom has two hyperfine ground states (which I don't really understand what they are but it's not really important), with a specific energy ...
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2answers
379 views

How do cesium atomic clocks measure time?

I am trying to understand cesium atomic clocks better. I am not getting HOW the cesium oscillation is actually being counted. So from my understanding of an older atomic clock: Cesium gets heated....
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3answers
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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 ...
3
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1answer
219 views

Is it experimentally proven that photons travel at speed $c$ in vacuum?

Are there experiments which show that single photons (not classical em waves) travel exactly at $c$ in vacuum? What is the error bar in that case? The question is posed due to the fact that loop ...
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0answers
227 views

Software engineer vs physics engineer vs computational physics [closed]

Don't know if this question is off topic, but i think it is the best place to get an answer. I am finishing the 3rd year of a degree in physics engineering. I enjoyed the first year, but in the ...
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2answers
91 views

How are the SI units “generalised”?

How exactly are the SI units generalised from their definitions? E.g. the kilogram is a weight of an object of cylindrical form, with diameter and height of about 39 mm, and is made of an alloy of ...
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4answers
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How many fixed points does a Kelvin scale have?

I have a book that says: In the absolute Kelvin scale, the triple point of water is assigned the value of 273.16 K. The absolute zero is taken as the other fixed point. But, then another ...
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2answers
649 views

Pressure at ground level and at sea level

$1$atm $= 101325$pa $1$bar $= 100000$ pa $1$atm $= 1.013$ bar $1$bar $= 0.987$atm From wiki: The bar is a metric (but not SI) unit of pressure exactly equal to $100000$ Pa.[1] It is about equal to ...
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1answer
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AFM cantilevers driven below resonance?

Is there a physical reason why AFM cantilevers are driven below their resonance frequencies? In all of the AFMs I have used, once you measure the resonance frequency of the cantilever, it is set up ...
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2answers
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Why isn't a meter defined from a kilogram of water?

Why are there different official definitions for a kilogram and for a meter when a meter can be defined by the volume of a kilogram of water? For instance, using the triple point or some other state ...
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3answers
83 views

Why are measurements standardized the way they are?

Using meters as a base length, squaring or cubing lengths smaller than 0.67m makes the square term larger than the cubed term. This fact causes certain properties of physics (how rain needs to form?) ...
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3answers
440 views

Are the 7 base quantities in SI system really independent?

In a typical description of the 7 base quantities of the SI system we see the following two points: All other quantities can be derived from them. They are "independent". My question is about the ...
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2answers
166 views

What is time measured against? [duplicate]

Today I was observing a clock and its movement, every second is an exact second on every clock. I was making a comparison between a second and a meter. I know in France there is a metal stick one ...
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3answers
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Oscillation of Atom

What exactly does it mean when one says 'one atom of Caesium 137 oscillates 9,192,631,770 times'? I do understand the general thing about oscillation but what exactly is the oscillation of atom, what ...
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3answers
349 views

Is the speed of light in vacuum constant or does the math just happen to work out?

My apologies if my question is really idiotic, but I ask sincerely because I want to learn. Based on this question and lots of other places on the web, this topic seems to be really confusing. Let's ...
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5answers
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Is the speed of light in vacuum always the same value?

The escape velocity of different planets and stars vary. If they vary, the velocities of bodies escaping from the respective stars or planets should also vary. Like, if I want a ball to reach 10 ...
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5answers
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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?
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1answer
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What is the reason we originally and still use the non-SI unit, the Jansky?

The Jansky is the unit for spectral flux density. It is defined as $$1 {\rm \ Jy} = 10^{-26} {\rm W \ m^{-2} \ Hz^{-1}}$$ in terms of Watts per square meter per Hertz. I've never quite understood ...
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1answer
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Definition of 1 calorie

Why has the definition of 1 calorie been taken to be the energy required to heat 1 gram of water from 14.5 to 15.5 degree centigrade at standard pressure? Is there any specific reason for taking the ...
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3answers
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How can we be sure that a new frequency standard is better than the old one?

Lets assume for this question that at one point in scientific history everyone was running along well with their cesium frequency standards, and someone has a brilliant idea and builds an active ...
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1answer
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How can I measure a length of over 4 feet to an accuracy of 1/100th of an inch?

How can I measure a length of over 4 feet to an accuracy of 1/100th of an inch ? I want to make several metal standard bars of a particular lengths, some of which are over 4 feet and I want the ...
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1answer
108 views

Mass-energy-equivalence's effect on SI units?

Maybe I'm misunderstanding mass energy equivalence, but can we use it to get rid of the Joule (or the kilogram) and have a single unit for both? It seems weird that if they're equivalent they'd have ...
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Why is the candela a base unit of the SI?

The candela is defined as The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency $540\cdot10^{12}$ hertz and that has a radiant ...
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1answer
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Experimentally finding units of physical quantity?

Say you had a new physical quantity you wanted to determine the units for. How do you go about this? For the strength of an electromagnet for example, you could carry out a simple experiment like the ...
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2answers
3k views

Is the second defined arbitrarily? [duplicate]

According to the definition a 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 ...
3
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2answers
8k views

What unit system does Fahrenheit belong to?

Wikipedia's page for Imperial Units does not list Fahrenheit. The corresponding page for SI Units lists Kelvin as an SI unit, and Celsius as a derived SI unit. This leads me to believe that ...
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2answers
381 views

How are weights and scales used by the public calibrated?

My physics teacher has a set of masses: things from 1 to 10 grams. My chemistry teacher has electronic balances that measures things in grams. In France, there is a block of metal we hope no one ...