# Questions tagged [metrology]

The study of measurements

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### 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 ...
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 ...
70 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 ...
299 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?
3k views

### 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 ...
12k 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).
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 ...
169 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 ...
159 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 ...
109 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 ...
303 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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### 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 ...
232 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 ...
215 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 ...
179 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 ...
1k 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 ...
397 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 ...
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_{\...
318 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 ...
2k 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 ...
83 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,...
491 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 ...
905 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 ...
1k views

### 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 ...
111 views

### 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 ...
106 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 ...
359 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 ...
294 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 ...
2k views

### 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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### 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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### 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 ...
264 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?
934 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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### 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 ...
496 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 ...
2k views

### 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 ...
389 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....
11k views

### 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 ...
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 ...
237 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 ...
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 ...
7k views

### 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 ...
727 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. It is about equal to ...
218 views

### 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 ...
1k views

### 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 ...
84 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?) ...
462 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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### 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 ...