# Tag Info

### Why was carbon-12 chosen for the atomic mass unit?

Why was Carbon-12 chosen for the atomic mass unit? As is the case elsewhere in metrology, the answer is tied up in history, measurability, practicality, repeatability, past misconceptions, and ...
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### How do you make more precise instruments while only using less precise instruments?

I work with an old toolmaker who also worked as a metrologist who goes on about this all day. It seems to boil down to exploiting symmetries since the only way you can really check something is ...
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### What reference clock is an atomic clock measured against?

This is a good and somewhat tricky question for a number of reasons. I will try to simplify things down. SI Second First, let's look at the modern definition of the SI second. The second, symbol s, ...
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### Has anyone charged an object with 1 coulomb? Why was such a ridiculously large charge chosen as the unit of charge?

Actually the ampere (SI unit for electric current) was defined first (in 1881, see Wikipedia: Ampere - History). They chose this size for $1$ ampere, probably because at this time such a current could ...
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### Why should a clock be "accurate"?

why it is necessary for a reference clock to worry about this, if the definition of the second itself is a function of the number of ticks the clock makes. The concern is that somebody else (say a ...
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### In nuclear physics, what length year in seconds is used?

A "year" without qualification may refer to a Julian year (of exactly $31\,557\,600~\rm s$), a mean Gregorian year (of exactly $31\,556\,952~\rm s$), an "ordinary" year (of exactly $31\,536\,000~\rm s$...
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### How can a day be exactly 24 hours long?

You can't calculate the length of a mean solar day just by taking the mean of the shortest & longest apparent solar days. That would work if the apparent day lengths varied in a simple linear ...
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### Why are leap seconds needed so often?

It's not the rate of change of the rotation speed that's important, it's the current rotation speed (in the rotating reference frame that stays facing the sun) not matching a 24h day. Thus leap ...
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### How do you make more precise instruments while only using less precise instruments?

The more you measure things and add or multiply those measurements, the greater your errors will become. Not necessarily. If the errors in a series of measurements are independent and there is no ...
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### Has anyone charged an object with 1 coulomb? Why was such a ridiculously large charge chosen as the unit of charge?

Has anyone charged an object with 1 coulomb? Not a problem nowadays with supercapacitors. Why was such a ridiculously large charge chosen as the unit of charge? Once the second and the ampere (both ...
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### What is a joule? I find the definition confusing

Pushing the ball bearing with 1 N for one meter and pushing a bowling ball with 1 N for 1 meter do exactly the same amount of work: 1 joule. As you say: it will take a much longer time for the ...
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### Will the volt, ampere, ohm or other electrical units change on May 20th, 2019?

Late last century electrical standards based on Josephson junctions became common. A Josephson junction together with an atomic clock can give an exquisitely precise voltage standard in terms of the ...
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### Is the fact that 100 kPa equals about 1 atmosphere accidental?

This is a coincidence. There's nothing about the atmosphere that would make it have a nice relationship with the Earth's rotation or diameter, or the fact that water is plentiful on the surface. On ...
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### How do you make more precise instruments while only using less precise instruments?

One thing I haven't seen mentioned is amplification. Amplification: Imagine you have a lever that is 10 cm on one side of the pivot and 1 m on the other. Then any change in position on the short side ...
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### Why is the Planck constant an exact number with defined value?

Planck's constant relates two different types of quantities, namely energy and frequency. That means it is a conversion factor which converts the units of quantities from one form to another. If the ...
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### What is a joule? I find the definition confusing

Maybe imagine the ball is being pulled by a weight, or a spring, back here on Earth. The weight falls down and pulls the ball along: It is easy to see that the weight goes down 1 meter when the ball ...
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### How do you know mercury changes monotonically with temperature if mercury itself is used to make the thermometer?

This has actually been a very important problem in the history of measurement and physics more broadly. The solution was an iterative process of increasing internal consistency and precision of ...

### How it was found that 12 g of carbon-12 has Avogadro's number of atoms?

This Chemistry.SE answer captures most of the argument, but I'll paraphrase here. (Oh, who am I kidding. The history of units and conventions is never short. "Paraphrase" should be taken ...
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### What's exactly the new definition of kilogram, second and meter?

The SI system is now defined entirely by physical constants. There are no more “prototype” artifacts. How it works is thus: https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9-concise-EN....
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