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Questions tagged [metrology]

The study of measurements

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SI redefinition of the kilogram - what is one measuring? [duplicate]

I have been reading about the new SI units and specifically, want to get a better understanding of the definition of a kilogram. It was written that the kilogram will be defined in terms of Planck's ...
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3answers
12k views

Will the volt, ampere, ohm or other electrical units change on May 20th, 2019? [duplicate]

When watching a video by Veritasium about the SI units redefinition (5:29), a claim that the volt and unit of resistance (presumably the ohm) will change by about 1 part in 10 million caught my ...
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MKS, why we need 7 base units? [duplicate]

What were the units of temperature in MKS system? Why do we need 7 fundamental units? I think we need only 2: kg, s. All other candela, mol, etc. Should be called derived ones.
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184 views

Redefinition of everything on May 20th, 2019 [closed]

A couple of issues: So after May 20th, 2019, what exactly will be the defined value of $\hbar$? What will be the defined number of elementary charges in a Coulomb? Then $\mu_0$ and $\epsilon_0$ will ...
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Effect of variation in day length on timekeeping

I was reading about atomic clocks when I came to know that precise measurements of time have shown that the time for earth's rotation is slightly more that 24 hrs (on the order of 10^-3). I was ...
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2answers
38 views

When was Coulomb's constant made/established?

Charles-Augustin de Coulomb lived from 14 June 1736 – 23 August 1806. Coulomb's constant is $$k_{\text e}=\alpha\frac{\hbar c}{e^2},$$ a form of Planck's constant is included, but Max Planck lived ...
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49 views

Confusion on Centigrade/Celsius Scale

There came the centigrade scale. The issue was that many solid/liquid thermometric substances didn't respond to temperature linearly so different thermometers produced different results. This was ...
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4answers
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How can a day be exactly 24 hours long? [closed]

The longest solar day of year is approximately 24 hours 0 min 30 seconds (occurs at mid to late December) while the shortest solar day of year is approximately 23 hour 59min 38 seconds. If I average ...
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1answer
39 views

Significant figures in measurements, different orders of magnitude

Let's say I a have a scale that can, for example, measure a mass of some object to precision of 0.1 g. If I do some measurements of different objects I can get results like this: 9.8 g, 9.9 g, 10.1 g,...
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1answer
60 views

Are constants derived or calculated?

I am currently writing up a lab report on the determination of Planck's constant using x-ray diffraction and atomic spectra. In my introduction, I am talking about the history of Planck's constant, ...
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2answers
147 views

Why is it necessary to keep UTC within 0.9 seconds from UT1?

Why is it necessary to keep UTC within 0.9 seconds from UT1? I thought that UTC in fact more accurate than UT1, so why should it be based on UT1 within that range? Another question would be, why isn'...
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1answer
88 views

Is 1 liter always equal to 1 cubic decimeter, independently of temperature, pressure, etc?

I recently found this conversion table for the unit conversion $\rm mmol/m^3 \ \leftrightarrow\ \rm mmol/L$ (millimoles per cubic meter to millimoles per liter) My physics is very rusty, but just to ...
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42 views

Faradays constant determination? [closed]

I am a bit confused on how faraday determined his constant. So I’ve been taught that he realized that if you sent a certain amount of charge (96485 C), then one mole (or some fraction of that based on ...
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2answers
56 views

Has the kilogram changed?

In late 2018 the kilogram's definition switched from a physical object (the International Prototype Kilogram) to Planck's Constant; a grand move in my opinion. However, I haven't heard of the ...
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2answers
61 views

About the dimension of the SI units vector space

We know that the set of fundamental and derived physical units can be structured as a vector space over the rational numbers. In the International System of Units the dimension of this space is $7$ ( ...
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1answer
69 views

Is the triple point of water exactly 0.01 $\deg$C?

Is the triple point of water exactly $0.01 \deg$C, or is this an empirical value/is there some uncertainty to it? If so, how do we know it is exact, and why?
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2answers
137 views

Is there (or was there) a unit of electric current based on Avogadros number or Coulombs constant?

This has to do with the SI definition of the Ampere. Why the quantity $2*10^{-7} $ Newtons in particular? It would make more sense to define 1 Ampere = 1 mole of electron charge per second. Which ...
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54 views

How did Henry Kater measure distances down one part in $10^5$?

Wikipedia says that in 1817, Henry Kater was able to measure distances accurately enough to get at least five significant figures in a measurement of $g,$ suggesting that he could measure a distances ...
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What is the theoretical resolution limit of magnetometers / magnetoencephelography?

I recently read this article about a group of physicists and neurobiologists working on developing advancements to magnetoencephelography, presumably based on new quantum metrology and quantum ...
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2answers
157 views

Why is the MKS unit of time the same as the CGS unit? [closed]

There are many system of units used in physics. In the CGS, the units are, length : centimetre mass : gram time : second And in the MKS system the units are, length : metre mass : kilogram time : ...
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1answer
57 views

Profile of a platinum-iridium meter bar

The former meter standard, platinum-iridium meter bar, had a specific cross section somewhat resembling mixed variant of letters "X" and "H" with serifs (Image source): What was the reasoning behind ...
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3answers
104 views

What was wrong with the old definition of temperature scale in kelvin?

Wikipedia's article on the recent change to the definition of the SI base units states, as the reason for changing the definition of the kelvin: A report published in 2007 by the Consultative ...
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1answer
42 views

Is there a “standard” Newton?

Basic SI units have definitions through experiments that seems to imply a pretty obvious setup. Is there a standard experiment for calibrating Newtons? The definition is the force needed to cause a ...
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1answer
110 views

How did Coulomb arrive at value of electron charge?

Charge of one electron is known to be as $1.6$ x $10^{-19}$ C or alternative 1 Coulomb contains charge of $6.24$ x $10^{18}$ electrons. I am just wondering if these numbers are arbitrarily chosen or ...
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2answers
139 views

Does the death of Kilogram ($kg$) affect us in any means in our day to day life? [closed]

Recently, the sleek cylinder of platinum-iridium metal has been discarded and the kilogram is set to be redefined along with ampere for electricity and Kelvin for temperature. Hereafter the Kilogram ...
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2answers
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How to get the UT1 directly? or How to get the UT1-UTC

I have a question to consult you. Even though I know the relationship between time scales like UT1, UTC, TAI, and TT, I do not know which one is the key point for the computation. That is, from which ...
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1answer
59 views

Definition of a meter and Newtonian law of Gravity

Newtonian law of Gravity: $$F_g = \frac{m_1 m_2}{l^2} G$$ $$G = 6.7 * 10^{-11} \frac{m^3}{kg * s^2}$$ A meter is defined as: the length of the path traveled by light in a vacuum in $1/...
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2answers
176 views

Will the SI units need redefining ever again?

Up until recently, there were obvious problems with the SI definitions of fundamental units, like bits rubbing off the kilogram prototypes (or mercury vapour absorption), and the water used for the ...
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1answer
73 views

What is the mass of $N_A$ atoms of carbon-12?

With the recent redefinition of the kilogram, what is the mass of $N_A$ (Avogadro's constant) of carbon-12 atoms? $N_A$ was defined as exactly 6.02214076×$10^{23}$ atoms. Then how close would the ...
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1answer
154 views

Is absolute zero still 0 Kelvin?

Following the recent decision to change the definition of SI units, I understand that Kelvin is no longer defined in terms of the number 1/273.16. Does that mean that absolute zero is no longer ...
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1answer
93 views

What will happen to the International Prototype Kilogram? [closed]

The kilogram has been redefined in a way that does not refer to the International Prototype Kilogram. That kind of makes the International Prototype Kilogram useless. So what will happen to it? Can I ...
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233 views

A new definition of mass using Planck constant

Tonight in a italian television news channel well known internationally I have heard that almost certainly the definition of mass that we currently know will be obtained by means of the constant ...
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132 views

What will be the uncertainty in $\mu_0$ under the new SI scheme?

As you may be aware, a new SI system is likely to be adopted in November 2018 (see https://www.nist.gov/si-redefinition/kilogram-introduction). Whilst the speed of light remains a fixed quantity and ...
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1answer
114 views

How is the division of physical quantities into base quantities and derived quantities a matter of convention?

Several physics textbooks and even the SI-The International System of Units Brochure(8th-edition) says that, ''The division of quantities into base quantities and derived quantities is a matter of ...
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1answer
62 views

Is quantum physics a reason for the measurement differences of the International Prototype Kilogram

I've recently learned that there is an official Kilogram (IPK) kept in Paris which is used to calibrate, directly or indirectly, all weight measurement tools. I've also learned that each time the IPK ...
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1answer
134 views

What is the value of absolute zero: $-273.15\ \rm °C$ or $-273.16\ \rm °C$?

What is the value of absolute zero? 0K= -273.15 °C Or -273.16 °C It has been confused in different scientific scriptures. The first definition on Dictionary.com, for example.
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Is the International System of Units complete?

Are there any known (measurable continuous) physical quantities, which are neither base quantities of SI, nor are derivable from the base quantities? In other words, are there any quantities which ...
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1answer
66 views

When you see the atomic mass number for an element, does it take into account the atomic mass defect?

Sometimes I read that the official atomic mass number for an element on the periodic table only includes natural isotope ratios, other times I read that atomic mass defect, number of electrons, etc. ...
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Since astronomers are adding 'leap seconds' to our years nowadays, does that mean Earth's orbital period is getting longer?

How long was an Earth year several billion years ago? (I'm assuming constant days, even though I know days were much shorter back then).
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What is the difference between least count sensitivity and resolution of an instrument

The least count is the smallest measurement that can be observed in an instrument. Sensitivity is the minimum output signal for given input in a sensor. But I think the minimum output should be the ...
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2answers
778 views

Why can't we define fundamental unit of mass? [duplicate]

In my physics textbook of class $11^{th}$ The kilogram was defined as :- mass of the platinum-iridium standard cylinder kept at Sevre's France But this isn't a proper and scientific definition ...
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1answer
62 views

How is the Length of a Meter Physically Measured

I have two parts to this question. First, I understand that the meter is defined as the distance light travels in 1/299,792,458 seconds. But how is this distance actually measured? The second is ...
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1answer
191 views

Why was the fraction 1/31,556,925.9747, in the 1956-1968 definition of the second in terms of ephemeris time, chosen?

The recent question Why are leap seconds needed so often? pulled up some interesting details about the definition of the second, and I'd like to have some of them confirmed explicitly. I'm ...
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4answers
8k views

In nuclear physics, what length year in seconds is used?

So I'm working on a nuclear physics problem and am looking at radioactive decay. The common unit used for very long decays is years within the literature. Is this the sidereal or tropical year? I want ...
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7answers
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Why are leap seconds needed so often?

In Coordinated Universal Time (UTC), leap seconds are added to account for the slowing down of Earth's rotation. But the slowing down is said to be of the order of milliseconds in a century. Then why ...
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1answer
135 views

Why are the international prototype kilogram and its copies kept under nested bell jars?

I'm watching a YouTube video about the American standard kilogram by the Veritasium channel and noticed something that I'd seen before but never questioned. The standards are kept under two, nested ...
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3answers
125 views

How to define approximate meter using primitive to no tools? [closed]

Imagine you've lost somewhere in the wild due to some catastrophic event, and don't have any measurement tools with you. How do you find approximate meter, millimeter, etc. with materials like sticks ...
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2answers
79 views

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
520 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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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 ...