# Questions tagged [metrology]

The study of measurements

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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 ...
5answers
7k views

### 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 ...
1answer
100 views

### Unit definition concerning light and metre [closed]

A stupid question. I see metre is officially defined based on the speed of light: The meter is the length of the path travelled by light in vacuum during a time interval of 1 / 299 792 458 of a ...
3answers
267 views

### Why did scientists need to invent light years?

Why did scientists need to invent light years? What's so important about having a light year? I have been learning that a light year is $9.461 \times 10^{15} \, \mathrm{m}$. My question is, why are ...
5answers
7k views

### Why is energy not an SI base unit?

According to a textbook I have begun to read, there are seven base SI units: Length Mass Time Temperature Amount of a substance Electric current Luminous intensity What I do not understand is, why ...
2answers
880 views

### Significant figure rules [duplicate]

In a simple physics experiment, we take the average a few readings to reduce the random errors. I apply significant figure rules to these. Say we round off at each step: (8.0+9.0+10.0)/3 = 27.0/3 (...
4answers
623 views

### Why was the original definition of meter abandoned?

The meter was originally intended to represent $1$ ten-millionth($10^{-7}$) of the distance from pole to equator of the Earth along a meridian of longitude. The definition was later discarded. Now, ...
5answers
528 views

### Why is the speed of light in vacuum constant? [duplicate]

Are there any proof of the speed of light in vacuum being constant? All I hear is that light in vacuum travels at a constant speed because that's an observation and that it fits in a coherent theory ...
4answers
216 views

### How confident can we be that the speed of light in a medium is constant?

I have recently found this article http://news.harvard.edu/gazette/1999/02.18/light.html it tells that physicists have been able to slow the speed of light. Is this hokum? If not how is it possible to ...
5answers
11k views

### Speed of light definition [closed]

I understand the speed of light to be an EXACT number. If permeability of free space has a factor of pi in it, how can this be?
7answers
10k views

### Where on Earth does the mass of 1 kg actually produce a 1 kg reading on a digital scale?

Gravity on Earth varies by about 0.1% between poles and equator. If someone was buying/selling something mass critical e.g. gold, where is the standard place on Earth where a 1 kg mass produces a 1 kg ...
2answers
3k views

### 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 ...
1answer
395 views

### Experimental determination of speed of light

How is the speed of light measured by the most current method i.e. using lasers and atomic clocks? Alternatively, is there a better method?
2answers
956 views

### How many psi's are in one bar?

6894.7573 bar = 100000.0 psi according to google 6894.7573 bar = 100000.0001 psi according to wolfram alpha which is it? How many psis are in one ...
1answer
695 views

### How should I calculate uncertainty of measurement calculated as average of two measurements

I am measuring force with two channel transducer. Both channels (separately) of this transducer has been calibrated and I can calculate uncertainty of measurement for each of it. However I want to ...
3answers
467 views

### Generic term comprising everything that can be represented with a number and a unit?

I am looking for the generic term comprising all of the following: $23.42\,\text{m}$ $200\,\text{K}$ $123\,\text{MeV}$ $ħ$ with other words, everything that can be reasonably represented ...
3answers
30k views

### $\rm Lux$ and $W/m^2$ relationship?

I am reading a bit about solar energy, and for my own curiosity, I would really like to know the insolation on my balcony. That could tell me how much a solar panel could produce. Now, I don't have ...
3answers
701 views

### A question about atomic clocks

I have a rather simple question about atomic clocks. I have read that: Microwave radiation with a frequency of exactly 9.192.631.770 cycles per second causes the outermost electron of cesium-133 ...
3answers
1k views

### Definition of a year

Is there an acceptable definition of a year (in number of days)? Google Calculator: https://www.google.com/search?q=seconds+in+1+year returns 3.15569e7 seconds and then https://www.google.com/...
3answers
389 views

### Does the definition of the SI unit “second” require that possible perturbation of primary frequency standards should be measured?

The definition of the SI unit "second" is stated as The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground ...
4answers
265 views

### How units were defined?

I was wondering how we humans can be sure that one meter is one meter and that one second is one second. Nowadays, except for the Kilogram, all other units are well defined using highly accurate ...
3answers
7k views

### Why is the second the SI base unit for time?

Specifically, during the moves towards Le Système international d'unités in the 18th and 19th centuries, why didn't anyone attempt to move us away from the definition of there being 24 hours in a day? ...
1answer
5k views

### Why is charge not taken as a fundamental unit? [duplicate]

According to the definition of electric current, it appears to be a derived quantity. Charge on the other hand seems more fundamental than electric current. Then why is current taken as fundamental ...
4answers
1k views

### Why isn't it $E \approx 27.642 \times mc^2$?

Sorry for the strange question, but 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 ...
1answer
302 views

### Do quartz watches actually implement the SI definition of the second?

I always wondered how much time means 1 second? Technically, the SI second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two ...
3answers
1k views

### Why wasn't the meter defined using a round-number fraction (like 1/300 000 000) of the distance travelled by light in 1 second?

We know that 1 meter is the distance travelled by light in vacuum within a time interval of 1/299,792,458 second. My question is why we didn't take a simpler number like 1/300,000.000 or why not just ...
2answers
2k views

### Why is the absolute zero a rational number in Celcius?

From the question "Why is the absolute zero -273.15ºC?" I understood that 1°C is the 100th part of the difference of melting and boiling temperature of water (this is my high school physics, ...
3answers
1k views

2answers
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### Definition of Ampere

On Wikipedia it says: This force is used in the formal definition of the ampere, which states that it is "the constant current that will produce an attractive force of $2 × 10^{-7}$ newton per ...
2answers
2k views

### Significant figures vs. absolute error

On NIST Avogadro's Number, $N_A = 6.022\;141\;29 \times 10^{23} \text{ mol}^{-1}$ has 9 significant figures and a standard uncertainty of $0.000\;000\;27 \times 10^{23} \text{ mol}^{-1}$. First, can ...
3answers
302 views

### $1.7\cdot 10^{-24}$ mole apples a day

As the title suggests I was wondering why the International Bureau of Weights and Measures decided a mole to be a standard (SI-)unit. After some research I found I was not alone with this problem. ...
2answers
3k views

### Which is the “roundest man-made object”: Gravity probe B rotor or the Avogadro project sphere?

The Avogadro project is part of the efforts to replace the artefact-based standard kilogram with a definition based on naturally reproducible objects. The main challenge is to make a very precise ...
1answer
167 views

### How is a standard unit divided into equally smaller or fractional units physically/experimentally?

Consider the standard unit of length: meter. How was it divided into decimeter, centimeter, millimeter, etc. when there were no shorter lengths than the standard? What is the physical/experimental ...
8answers
15k views

### 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 ...
2answers
5k views

### The famous drop of $c$

In this (in my opinion) intriguing speech, Rupert Sheldrake tells the story of the drop in the measured value of $c$ between 1928 and 1945. When he goes to visit the Head of Metrology of the Physics ...
5answers
764 views

### Could velocity be taken as fundamental instead of time?

In physics time and length are taken as fundamental in the SI system and, as it seems, in the thinking of physicists. Could one instead take velocity, with c as its unit, together with length as ...
3answers
640 views

### Age of the universe in years

It seems to be commonly accepted that the Big Bang occurred roughly 13.7 billion years ago. My question is what is the meaning of the year in this context? When I type year definition into google, ...
3answers
4k views

### Why is the absolute zero -273.15ºC?

I can't find an answer of why the lowest temperature is -273.15ºC. Is it deduced theoretically or is it experimental? An explanation is that when any gas volume tends to zero, the temperature will be ...
3answers
24k views

### How is a second measured? And why is it measured that way?

The earth's rotation around the sun isn't exactly 24 hours. It off by some seconds which becomes somewhere around 6 hours per year and 1 day in 4 years(leap year), which brings the question why didn't ...
2answers
19k views

### 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 ...
2answers
21k views

### Why is a degree Celsius exactly the same as a Kelvin?

How on earth is it possible that the difference between two temperatures in Celsius and Kelvin is exactly the same. Given the historical definition of Celsius, I find it hard to believe that this is ...
1answer
211 views

### How many measurements should be done? [closed]

I am measuring time of a computer operation. The operation should run roughly same time each time I measure it. How many times should I measure it to get good average and standard deviation?
3answers
20k views

### Why does the metric system use “kilogram” as a base SI unit?

SI system uses all (that I know) measurement basic units as 1 (single) instance: meter, second, ampere, etc, except the KILOgram. It already defined with 1000 multiplier (kilo). It prevents from ...
1answer
223 views

### Zero uncertainty constant and a unit change

So, we know the speed of light with zero uncertainty. We also know that values of $\epsilon_0$ (electric constant) and $\mu_0$ (magnetic constant) are known with zero uncertainty. My questions are ...
1answer
138 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 ...
5answers
749 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?
1answer
627 views

### Difference between nautical and terrestrial miles

Does someone know the historical reason behind the difference in physical units between nautical and terrestrial miles?
4answers
21k views

### Temperature in CGS (Gaussian) units

I've been struggling with conversion from Gaussian to SI units for sometime, trying to figure out how derived units in CGS (current, charge etc) relate to the SI units. But I couldn't find any ...