The metrology tag has no wiki summary.
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1answer
51 views
A sphere, a simple object?
In this video, the woman says that a sphere is a pretty simple object. What intrigues me is the use of a sphere for such a calculation. First of all, the sphere wouldn't be perfect as a perfect sphere ...
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4answers
191 views
Why a day is divided by 12/24 hours? Why the number 12?
Why a day is divided by 12/24 hours? Why the number 12? Why not using 10 or 6 or 14, 16? Who invented this? Any physical reasons behind this?
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1answer
61 views
Difference between nautical and terrestrial miles
Does someone know the historical reason behind the difference in physical units between nautical and terrestrial miles?
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1answer
92 views
Does the Kelvin have a rigorous definition?
From Wikipedia:
The kelvin is defined as the fraction 1⁄273.16 of the thermodynamic temperature of the triple point of water.
That presupposes that we can take a fraction of temperature.
Now, ...
2
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1answer
121 views
How do physicists and astronomers handle leap seconds?
I'm confused by the many contradictory descriptions I see about how UTC leap seconds are accounted for. I understand that there are various ways to handle them in common practice, and I've seen a ...
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3answers
153 views
Why are precise frequency standards important in the telecommunications industry?
I understand why having a precise definition of time can be very useful for navigation purposes. The GPS system works using the simple equation $d=ct$, where $c$ is the speed of light, so the ...
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2answers
224 views
Atomic clocks and how to synchronize them
During teaching measurement section in the class, Our teacher told us about atomic clocks. I have two questions:
What is exactly an atomic clock?
and how do we synchronize two atomic clocks far ...
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1answer
99 views
How is charge measured and mass of the electron at the same time?
There are few constants that usually come together, $e/m$ also $h/e$. How are they decoupled?
If the speed of light is "derived" as Wikipedia states how meter defined and time?
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3answers
304 views
How do Temperature Scales work?
How exactly do temperature scales work?
If my understanding is correct, the Celsius scale has two fixed points: (definitions of temperature irrespective of scale)
1. The freezing point of pure water ...
3
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1answer
113 views
There are plans to develop a better definition of a “second”. How does the current definition fall short?
The current definition of a second is stated here and I found a presentation on the BIPM site which discusses plans to change to a "better" definition of a second. You can find the presentation here. ...
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3answers
159 views
Can you use PCR to make a standard kg?
While reading this question: Why do we still not have an exact definition for a kilogram? , I had a crazy thought.
Using PCR, you make a known number of copies of a DNA strand where the length and ...
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3answers
316 views
Why do we still not have an exact definition for a kilogram?
I read that there is an effort to define a kilogram in terms that can exactly be reproduced in a lab. Why has it taken so long to get this done? It seems this would be fairly important.
Edit
Today I ...
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1answer
57 views
How to properly read a measurement result if it is a number?
If the result of a measurement is i.e. $3.2 \pm 0.7$, what is 0.7? At which confidence level we know that the real result is inside of this interval?
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4answers
214 views
Are circularly defined {velocity, distance, and time} a problem in physics?
In order to measure velocity, one needs a calibrated measuring stick and clock. But in order to calibrate a measuring stick you need a calibrated clock and velocity. And in order to calibrate a clock ...
4
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1answer
145 views
Avogadro's number
Could I get an explanation of Avogadro's number and how it relates to determining the mass of a substance? My chemistry textbook only serves to confuse me and the Wikipedia article is aimed towards a ...
2
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3answers
165 views
What widely recognized organizations set standards used by physics?
I recently answered a question about the meaning of the word "dimension" as used in physics. In that response, I provided the definition given in the International Vocabulary of Metrology (VIM) and ...
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3answers
531 views
Meaning of dimension
I was wondering what dimension can mean in physics?
I know it can mean the dimension of the space and time.
But there is dimensional analysis. How is this dimension related to and different from the ...
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5answers
862 views
Is anything actually 1 meter long (or 1kg of weight)?
I believe that no real objects are actually (exactly) 1 meter long, since for something to be 1.00000000... meters long, we would have to have the ability to measure with infinite precision. ...
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2answers
3k views
refractive index of air in dependence of temperature
What is the exact dependence of the refractive index of air and the temperature? Is there a theoretical derivation of it?
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3answers
351 views
If time standard clocks and any memories about the time standard are destroyed, can we recover the time standard again?
Assume the time standard clocks and any memories about the time standard are destroyed. Can we recover the time standard again exactly?
Recovering the time standard again means we can determine the ...
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4answers
633 views
Measuring the speed of light and defining the metre - absolute or relative?
If the metre is now defined as the distance light travels in vacuum in 1⁄299,792,458th of a second and the speed of light is accepted to be ...
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6answers
834 views
What's keeping us from simply redefining Avogadro's Number / the Mole as a definite integer?
This might be a question to ask in a Chemistry site, but because there is a lot of talk about redefining many units of measurements in terms of Avogadro's Number / the Mole, I was wondering why we ...
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5answers
420 views
Why were the SI base quantities chosen as such?
The reasons for choosing length, mass, time, temperature, and amount as base quantities look (at least to me) obvious. What I'm puzzling about is why current (as opposed to resistance, electromotive ...