Counting down to the new ampere

[DocM]

NIST = National Institute of Standards and Technology, US Dept. of Commerce http://www.nist.gov

 

http://www.phys.org/news/2016-08-ampere.html

 

Quote

After

 it's all over, your lights will be just as bright, and your refrigerator just as cold. But very soon the amperethe SI base unit of electrical currentwill take on an entirely new identity, and NIST scientists are at work on an innovative, quantum-based measurement system that will be consistent with the impending change.

It won't be a minute too soon. The ampere (A) has long been a sort of metrological embarrassment. For one thing, its 70-year-old formal definition, phrased as a hypothetical, cannot be physically realized as written:

"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 107 newton per meter of length."

For another, the amp's status as a base unit is problematic. It is the only electrical unit among the seven SI base units. So you might logically expect that all other electrical units, including the volt and the ohm, will be derived from it. But that's not the case. In fact, the only practical way to realize the ampere to a suitable accuracy now is by measuring the nominally "derived" volt and ohm using quantum electrical standards and then calculating the ampere from those values.

In 2018, however, the ampere is slated to be re-defined in terms of a fundamental invariant of nature: the elementary electrical charge (e). Direct ampere metrology will thus become a matter of counting the transit of individual electrons over time.

One promising way to do so is with a nanoscale technique called single-electron transport (SET) pumping. Specially adapted at NIST for this application, it involves applying a gate voltage that prompts one electron from a source to tunnel across a high-resistance junction barrier and onto an "island" made from a microscopic quantum dot.

The presence of this single extra electron on the dot electrically blocks any other electron from tunneling across until a gate voltage induces the first electron to move off the island, through another barrier, and into a drain. When the voltage returns to its initial value, another electron is allowed to tunnel onto the island; repeating this cycle generates a steady, measurable current of single electrons.

There can be multiple islands in a very small space. The distance from source to drain is a few micrometers, and the electron channels are a few tens of nanometers wide and 200 nm to 300 nm long. And the energies involved are so tiny that that device has to be cooled to about 10 millikelvin in order to control and detect them reliably.

Conventional, metallic SET devices, says NIST quantum-ampere project member Michael Stewart, can move and count single electrons with an uncertainty of a few parts in 108in the uncertainty range of other electrical unitsat a rate of tens of millions of cycles per second. "But the current in a single SET pump is on the order of picoamperes [10-12 A]," he says, "and that's many orders of magnitude too low to serve as a practical standard."

So Stewart, colleague Neil Zimmerman, and co-workers are experimenting with ways to produce a current 10,000 times larger. By using all-silicon components instead of conventional metal/oxide materials, they believe that they will be able to increase the frequency at which the pump can be switched into the gigahertz range. And by running 100 pumps in parallel and combining their output, the researchers anticipate getting to a current of about 10 nanoamperes (10-9 A). Another innovation under development may allow them to reach a microampere (10-6 A), in the range that is needed to develop a working current standard.

"At present, we are testing three device configurations of different complexity," Stewart says, "and we're trying to balance the fabrication difficulties with how accurate they can be."

In addition to its use as an electrical current standard, a low-uncertainty, high-throughput SET pump would have two other significant benefits. The first is that it might be combined with ultra-miniature quantum standards for voltage or resistance into a single, quantum-based measurement suite that could be delivered to factory floors and laboratories. The overall effort to provide such standards for all the SI base units is known as "NIST-on-a-Chip," and is an ongoing priority of NIST's Physical Measurement Laboratory.

The other advantage is that an SET pump could be used in conjunction with voltage and resistance standards to test Ohm's Law. Dating from the 1820s, it states that the amount of current (I) in a conductor is equal to the voltage (V) divided by the resistance (R): I=V/R. This relationship has been the basis for countless millions of electrical devices over the past two centuries. But metrologists are interested in testing Ohm's law with components which rely on fundamental constants. An SET pump could provide an all-quantum mechanical environment for doing so.

In a separate effort, scientists at NIST's Boulder location are experimenting with an alternative technology that determines current by measuring the quantum "phase-slips" they engender while traveling through a very narrow superconducting wire. That work will be the subject of a later report.

Researchers surround an open dilution refrigerator that cools the SET unit to near absolute zero. Clockwise from left: Michael Stewart, Bahman Sarabi, Neil Zimmerman. Click on image for view of the SET chip. Credit: National Institute of Standards and Technology

 

Edited August 29, 2016 by DocM

[+Gary7 Subscriber²]

What is next to go Ohm's Law?

[Astra.Xtreme]

A little too much rambling in this article instead of providing details of what they're actually changing.

In simple terms, the entire SI system is being revised such that all the units will now be measured based on the constants of nature, which makes sense.  Measurements before will still be the same after, so not really a big deal.  More-so a clarification for consistency and reliability.

[DocM]

2 hours ago, Gary7 said:

What is next to go Ohm's Law?

That depends on the new ampere. It could be up for revision.

[Draggendrop Veteran]

The following is my opinion, based on my experience as an electrical engineer, to put this "article" into perspective.

 

To begin with, the post by @Astra.Xtreme   is correct, concise, and to the point.

 

"A little too much rambling in this article instead of providing details of what they're actually changing.

In simple terms, the entire SI system is being revised such that all the units will now be measured based on the constants of nature, which makes sense.  Measurements before will still be the same after, so not really a big deal.  More-so a clarification for consistency and reliability."
 

-----------------

 

I will break this event into further detail before it becomes misunderstood.

 

This "article" was probably put together by a tech/science writer with limited background and distorted (due to lack of comprehension) what trained professionals gave as the explanation for the further refinement of this particular S.I. base unit.

 

To begin...

 

"The International System of Units (SI) defines seven units of measure as a basic set from which all other SI units are derived. The SI base units and their physical quantities are:

 

metre for length
kilogram for mass
second for time
ampere for electric current
kelvin for temperature
candela for luminous intensity
mole for amount of substance

 

The SI base units form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology. However, in a given realization in these units they may well be interdependent, i.e. defined in terms of each other."

 

https://en.wikipedia.org/wiki/SI_base_unit
http://physics.nist.gov/cuu/Units/units.html

 

From the above base units, we have derived units. These derived units are in the hundreds, many are field specific, others are not.

 

The actual S.I. definition of an ampere is ...

 

""The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10−7 newton per metre of length."
9th CGPM (1948)"

https://en.wikipedia.org/wiki/SI_base_unit

and prior to this, was...

 

"The original "International Ampere" was defined electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of silver nitrate. Compared to the SI ampere, the difference is 0.015%."

https://en.wikipedia.org/wiki/SI_base_unit

 

But in reality, those just entering the field lack the knowledge to comprehend the mechanisms involved in deriving this standard. What is taught, as an easier concept to follow is...

 

"The ampere (SI unit symbol: A), often shortened to "amp", is the SI unit of electric current (dimension symbol: I) and is one of the seven SI base units. It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.

The ampere is equivalent to one coulomb (roughly 6.241×10^18 times the elementary charge) per second. Amperes are used to express flow rate of electric charge. For any point experiencing a current, if the number of charged particles passing through it — or the charge on the particles passing through it — is increased, the amperes of current at that point will proportionately increase."

 

https://en.wikipedia.org/wiki/Ampere

 

Or in more simplistic terms....

 

" The number of "majority carriers" passing a defined point, in one second."

 

Why, because there are more types of "carriers" than just electrons.

 

One will immediately notice, the use of "time". As the standard of time is improved, so does the accuracy of our base units and derived units. As the accuracy of any variable increases, the accuracy of a dependent term also increases.

 

The beauty of the S.I system is the use of base 10 for easier conversions and a defined set of descriptive units, an example being meters per second.

 

The base units must use the proper descriptor units for all derived units and accuracy is dependent on this, an area where many rookie mistakes take place during derivations of equations.

In a nutshell, all that NIST is doing, is making the unit more accurate. This is in the same context as making an improved atomic "clock" for better accuracy.
 

What will this change...for all intents and purposes, nothing. It will just make the base unit more accurate.
 

-----------

 

On the topic of Ohm's law.

 

The familiar Ohm's Law "triangle" as well as other variations such as the "power" triangle, are just a layman's guide to understanding the basic physical properties with basic electricity.

 

https://en.wikipedia.org/wiki/Ohm's_law

 

What really is Ohm's Law? It is a direct current formula, in it's simplest terms, to describe relational characteristics in a simple circuit. As frequency is introduced, the variables take on new forms, an example being resistance which now is defined as an impedance, requiring reactance and phase angles.

 

Maxwell's equations, are at the other extreme, the precision approach for describing the state of a complex activity, used in various forms, are very complex and are a tedious mathematical exercise. A better way to understand Ohm's Law, it is what remains after chopping down Maxwell's equations, to the bare minimum, in order to make sense of a DC circuit, without the mathematical onslaught. Maxwell's equations (4) in vector form were themselves "chopped down" in 1884 from the original 20 equations of 1861 which used Hamiltons rotational quaternions of 1847. 

 

The bare bones...Ohm's Law pertains to relationships of select variables in a direct current system, zero hertz ( zero frequency) which is the start of the frequency spectrum. As frequency is introduced, everything begins to break down because we need to use increasingly more complex equations to accurately describe the environment and the system variables.

 

In summation. 

 

This article did a poor job of expressing a different approach for increasing the accuracy of a base unit, the ampere.

 

@Gary7

 

The "ampere" and "Ohm's Law" are going to be around for a very long time, unless the laws of physics change, where upon, we have bigger issues to deal with.

 

Hope this helps...

 

[+Gary7 Subscriber²]

I am aware of what an Ampere is as I have an Associates Degree in Electronic Engineering.  My post on this matter was sort of sarcasm .

 

 

[Hum]

I'll take all the amperes I can get.

[Shiranui]

I call BS.

 

Everyone knows that Americans use slugs, bushels and pecks to measure electricity.

[Draggendrop Veteran]

I'm kind of partial to the battery lick test....gets your taste buds moving.....

[Draggendrop Veteran]

Just as a side note...

 

The S.I. standard and the accuracy of the base units has really made a big difference in many fields, particularly the ability to stay in a base 10 system. 

 

This is really put in context, for me, by Isaac Asimov. In 1965, he penned a book of short stories, titled... Of Time, Space and Other Things.

 

Story #11, called "Forget it" deals with the early antiquated units and tables, in an American text book called "Pikes Arithmetic"  a very important book of its time (1785). One of several examples he gives is this one pertaining to the measure of fabric...

 

Such as 2 and 1/4 inches makes a "nail", 16 "nails" makes a "yard" while 12 "nails" makes an "ell". 12 "nails" makes a "Flemish ell", 20 "nails" to make an "English ell", 24 "nails" to make a "French ell" and 16 "nails" plus 1 and 1/5 inches makes a "Scotch ell".

 

This is then carried onto other similar archaic units, to the point of "Forget it" 

 

Yes... I am happy with S.I. 

 

 

 

[compl3x]

13 hours ago, Gary7 said:

What is next to go Ohm's Law?

 

I expect resistance.