Pre 2019 definitions for an Ampere, how is this useful in practice?

[Glenn G]

I've been looking into definitions for SI units (pre 2019) I'm fine with length based on a certain transition of a Krypton atom and the wavelength associated with it, a spectrometer can be used to find the wavelength of the spectral line and then multiply up...

but with the ampere, the idea of finding the force per unit length of 2 infinitely long thin metal strips 1m apart in a vacuum giving a force of 2 10-7m etc

How is this used in practice? Was an experiment ever set up in a vacuum with some long metal strips and the current turned up until the force was the required amount (also how was that force measured?) and then how is this replicated or used?

would really appreciated your help here please.

 

[CPW]

Good question.

Consider the size of one Newton of force: about equivalent of holding up an apple in your hand in Earth's gravitational field. A human-sized force.

Is your concern about the SI definition of the unit for electrical current that the forces involved are so much smaller?

Sensitive laboratory apparatus is involved, no doubt.
Just as careful measurements are involved to determine other fundamental constants of nature, such as G=6.67x10^-11 Nm^2/kg^2.

The labortory (NIST) can explain more about the details of the measurement methods:
https://www.nist.gov/si-redefinition/ampere/ampere-history

 

[Glenn G]

Thank you for your reply.

 

[Glenn G]

Thank you for your reply.

I’ve read those pages, someone has copied them to wikipedia.

I know of the Cavendish expt for G, in that case G comes out from measuring the F (from torsion in wire) the r and the two masses so it’s kind of indirect.

I still can’t find any practical details of defining an amp ie of experiments were actually carried out in a vacuum and how the f was measured.

 

[Vanadium 50]

Did they do an experiment with infinite wires? Of course not. That would be silly.

Does nature have perfect circles? No. That doesn't stop us from talking about pi. That would be silly too.

The article you were pointed to discusses realizable standards.

 

[phyzguy]

There is a fairly standard lab experiment often done in first year physics labs. The forces are small, but easily measurable. There are several techniques, but usually some sort of balance is used where the Lorentz force is balanced by a gravitational force. This site has an explanation of one such experimental set-up, but if you Google it you will find many others.

 

[vanhees71]

Well, the problem is that the (now old) definition of the Ampere cannot be realized exactly, but you can use Maxwell's equations to calculate what follows for realizable configurations of wires using this definition of the Ampere. The most accurate realizations today use quantum phenomena to establish the units. You find many details for each of the 7 base units here:

https://www.bipm.org/en/publications/mises-en-pratique/

 

[f95toli]

Summary:: SI units; Amperes; force
How is this used in practice? Was an experiment ever set up in a vacuum with some long metal strips and the current turned up until the force was the required amount (also how was that force measured?) and then how is this replicated or used?

The experiments were done but as far as I know the last time Ampere was realized this was back in the 70s (perhaps early 80s, I've seen photos). It is a very complicated experiment and you end up with a significant uncertainty budget. It has -AFAIK- never been used for practical calibration, even by NMIs.

In reality the Ampere has (and still is, at least mostly) nearly always been realized using Ohms law. That is, you use a known voltage and a calibrated resistor. These days you get the voltage from a Josephson voltage standard (back in the day they used chemical standard cells, these are still used for secondary standards) and the resistors are calibrated using the quantum Hall effect.
This is how all primary current calibration has been done for the last 30 years or so. With this you get an uncertainty of about 1 part in 10^7 or perhaps somewhat better (depends on the current range).

Once you have a calibrated current (which is typically quite small) you can then use what is known as a cryogenic current comparator (CCC) to transfer the calibration to other current ranges.

This process has worked extremely well for everything expect very small currents (which has to do with issues related to the stability and noise of large resistors), the uncertainties goes up once you go below a few hundred pA or so. The latter has become a bit of an issue in e.g. DNA sequencing and some sensor applications. The first real applications of the new charge pump realisations of the Ampere are therefore most likely to be for very small currents.

Note that there isn't really much of a market for current calibration. Most electronic instrumentation is based around precise voltages and resistances meaning there has never been much demand for "direct" current calibration except in some very specialised sectors (radiation detectors etc). Hence, whereas the somewhat weird old realisation was annoying and there were obvious "philosophical" issue it has not really been a practical problem.

 

[Glenn G]

A fascinating response. Thank you for taking the time to reply.

 

[Meir Achuz]

You can define something one way, but check it another. When the meter was first defined, who walked from the north pole to the equator to check it?

 

[vanhees71]

Well, they didn't walk from the north pole to the equator, but the measurement campaign to measure the length of the meridian through Paris was nevertheless adventurous enough (partially even life-threatening)!