how many digits is too many?

[CopperCone]

[Echo88]

Thats pretty much how it feels to finally own a 3458A and first time switching it on. 

[Mr. Scram]

The question is whether "true level" is perfectly aligned with two axes resulting in what's essentially a 2d surface, or perfectly aligned with the center of gravity of the Earth and thus subtly curved in three dimensions.

[Kleinstein]

The true level fixed to the ground would not last very long, as the moon is constantly moving and causing tidal waves.

When measuring frequency one might get into the problem that the result get too many digits to be handled with the normal 32 Bit or similar numbers. This sometimes gives odd effects with older plotting programs.

[kj7e]

Thats pretty much how it feels to finally own a 3458A and first time switching it on. 

That moment when all your DMM's read perfectly with each other.

[tggzzz]

Thats pretty much how it feels to finally own a 3458A and first time switching it on. 

That moment when all your DMM's read perfectly with each other.

That's relatively cheap and easy.

It becomes difficult and expensive when you want them to agree with other peoples' meters.

[Gyro]

It's easy. Required digits:

Normal healthy person: One more digit than you need.

Person infected by Metrology-nut disease: One more digit than you have.

[CopperCone]

digits become interesting since the charge carriers are discrete.

When you get to low enough current levels, it seems things are bumpy and averaging does not really give you a sense of whats going on. Electron counting blah, you will then begin to want to trace the path of every electron in the circuit and make 3d intensity graded plots like traderoutes in the sea.



Right now we are still basically doing what the highway department does, and setting up strips that cars drive over connected to counters..

I'm sure we are all familiar with the concept but:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0ahUKEwir3fX3uoPbAhWErFkKHbYLB5wQFgg1MAI&url=http%3A%2F%2Fdownload.tek.com%2Fdocument%2F2648%2520Counting%2520Electrons1.pdf&usg=AOvVaw0uWfMMADkG6OddS9lQ38NT

1 attoamp is only 6 electrons per second.

The whole idea of current measurement kinda gets less interesting at that point, it begins to feel like population infographics rather then cool science, in my humble opinion.

But charge transfer is interesting, being able to transfer a few electrons with no losses 

[rhb]

The question is whether "true level" is perfectly aligned with two axes resulting in what's essentially a 2d surface, or perfectly aligned with the center of gravity of the Earth and thus subtly curved in three dimensions.

If you've seen those world maps showing the seafloor, that's because gravity does NOT point to the center of the earth.  The maps are made by measuring the bumps on the surface of the ocean caused by water piling up on the bumps in the sea floor.  Every state in the US has its own coordinate system and a map projection selected for best fit to the non-flat reality of the state.

This effect was first recognized during the British survey of India and environs.  Eventually it was recognized that the Himalayas were affecting the direction of a plumb bob or spirit level .  When the gravimetric plane was adjusted for that, the errors were removed.

[Mr. Scram]

If you've seen those world maps showing the seafloor, that's because gravity does NOT point to the center of the earth.  The maps are made by measuring the bumps on the surface of the ocean caused by water piling up on the bumps in the sea floor.  Every state in the US has its own coordinate system and a map projection selected for best fit to the non-flat reality of the state.

This effect was first recognized during the British survey of India and environs.  Eventually it was recognized that the Himalayas were affecting the direction of a plumb bob or spirit level .  When the gravimetric plane was adjusted for that, the errors were removed.

That's why I made sure to mention the center of gravity of the Earth and not the center itself. There are other effects in play though, like the rotation combined with the location.

[eurofox]

Not less than 8 1/2 digits, below it is not accurate enough   

[Gyro]

Also the local composition and density of the rock. I understand that this can cause (relatively) significant local variations.

[GerryBags]

The curve doesn't even look that subtle from this:

http://bgi.omp.obs-mip.fr/activities/Projects/world_gravity_map_wgm

Then there's that big ol' South Atlantic Anomaly. Given the vaunted smoothness of the globe (smoother than a billiard ball, you'll hear) it does make you wonder if there is more to the phenomenon of gravitation than currently accepted. There are other models that seek to explain some of the anomalies, such as gravity measurements taken far below ground that yield anomalously low readings, but assessing these models on anything deeper than personal preference would obviously take years of study. Still interesting to think of the possibilities.

https://www.researchgate.net/publication/258847712_In_Gravitoelectromagnetic_Theories_HISTORY_OF_ELECTROGRAVITICS

[rhb]

Thanks for posting that.

Center of the earth or center of gravity makes no difference.  The equipotential surface which is what the figure @GerryBags posted shows is not at a constant distance from either.  The surface is determined by the vector sum of all the mass in the universe relative to a particular point on the equipotential surface.

The moon orbits the center of gravity of the earth.  But it doesn't change the center of gravity.  It *does* change the equipotential surface.  They are called tides.

What  a spirit level does is show a straight line approximation to the equipotential surface.  A plumb bob shows a perpendicular to that surface.

The center of gravity is the point at which mathematically an infinitely dense mass would have the same effect upon other bodies.

In general the distinctions don't matter unless one is involved in geodesy or doing gravity mapping.

I apologize for my poor explanation.  I am a reflection seismologist, not a potential fields geophysicist.  In industry gravity and magnetics are generally combined because of the mathematical similarities.  So while I understand the subject, I'm not practiced at explaining it.

FWIW When I first learned that there was a bump on the surface of the ocean over a seamount it really blew my mind.  I know it's true and I understand why it's true, but it is so counterintuitive I still struggle with the idea.

[Mr. Scram]

Thanks for posting that.

Center of the earth or center of gravity makes no difference.  The equipotential surface which is what the figure @GerryBags posted shows is not at a constant distance from either.  The surface is determined by the vector sum of all the mass in the universe relative to a particular point on the equipotential surface.

The moon orbits the center of gravity of the earth.  But it doesn't change the center of gravity.  It *does* change the equipotential surface.  They are called tides.

What  a spirit level does is show a straight line approximation to the equipotential surface.  A plumb bob shows a perpendicular to that surface.

The center of gravity is the point at which mathematically an infinitely dense mass would have the same effect upon other bodies.

In general the distinctions don't matter unless one is involved in geodesy or doing gravity mapping.

I apologize for my poor explanation.  I am a reflection seismologist, not a potential fields geophysicist.  In industry gravity and magnetics are generally combined because of the mathematical similarities.  So while I understand the subject, I'm not practiced at explaining it.

FWIW When I first learned that there was a bump on the surface of the ocean over a seamount it really blew my mind.  I know it's true and I understand why it's true, but it is so counterintuitive I still struggle with the idea.

The Moon doesn't orbit the center of the Earth. The Earth-Moon barycenter is slightly below the surface of Earth, but far from the center or center of mass or gravity.

[chickenHeadKnob]

FWIW When I first learned that there was a bump on the surface of the ocean over a seamount it really blew my mind.  I know it's true and I understand why it's true, but it is so counterintuitive I still struggle with the idea.

I am even more impressed that current generation radar satellites (synthetic aperture type) can see the moving surface "hump" of a submerged nuclear sub, and also see the small high speed step of a tsunami in mid ocean travel.

[GerryBags]

Mr. Scram, yes, and the same applies to the sun.

[rhb]

FWIW When I first learned that there was a bump on the surface of the ocean over a seamount it really blew my mind.  I know it's true and I understand why it's true, but it is so counterintuitive I still struggle with the idea.

I am even more impressed that current generation radar satellites (synthetic aperture type) can see the moving surface "hump" of a submerged nuclear sub, and also see the small high speed step of a tsunami in mid ocean travel.

I have long had a theory that the first Seasat SAR did not fail, but was shut down because someone realized you could see body waves on the surface.  These had been predicted theoretically for many years but no one had observed them.


But what about the other galaxies and black holes?  Won't someone, please think of the galaxies and black holes?

[TiN]

Is this really metrology section, or general chat? 

[JS]

  In a machinist forum, looking for some flat surface, equipotential surface popped out as an error while using low viscosity, self leveling rosin. It was discarded later as a source of error because of the granularity of the rosin being reasonable bigger than the curvature would be in the accounted surface.

JS