Counting down to the new ampere

August 29, 2016 by Michael Stewart
The SET chip. Credit: National Institute of Standards and Technology

After it's all over, your lights will be just as bright, and your refrigerator just as cold. But very soon the ampere—the SI base unit of electrical current—will 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 10–7 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 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 108—in the uncertainty range of other electrical units—at 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.

Explore further: Helmholtz Prize for the 'new' ampere

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21 comments

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jalmy
1 / 5 (1) Aug 29, 2016
I don't understand.

Voltage is a property of electricity and resistance is a property of material. If you derive amps from volts and ohms. Why would you need to give it its own quantity at all? It varies with the material its in. Also if you did want to set a standard. Why would you just not use a measure of the magnetic field it generates as a constant? That's how ammeters measure it after all.
24volts
1.4 / 5 (9) Aug 29, 2016
Jalmy,
They can't stand it because it's not a metric system of measurement. The fact that resistance and voltage works just fine seems to be beyond their understanding. If they can't complicate it to the point that no one but a scientist can understand a measurement value they aren't happy.
That's the problem with the entire metric system. It has no real world values that people can relate to. I'm Sure I'm going to get a bunch of flack for that statement by the diehard metric users on here but truth is truth.
Shootist
1.4 / 5 (9) Aug 29, 2016
They can't stand it because it's not a metric system of measurement.


There are two types of nations in the world: Those that landed men on the moon and those that use the metric system.
retrosurf
3 / 5 (9) Aug 29, 2016
I'm Sure I'm going to get a bunch of flack for that statement by the diehard metric users on here but truth is truth.


How do you feel about fluoridation of drinking water, 24volts?
24volts
3 / 5 (4) Aug 29, 2016
I'm Sure I'm going to get a bunch of flack for that statement by the diehard metric users on here but truth is truth.


How do you feel about fluoridation of drinking water, 24volts?


It's slightly helpful in preventing tooth decay but officials tend to put too much in most water supplies. They also tend to use too much chlorination in drinking water too but I guess officials feel it's better to be safe than sorry. No, I don't believe the stuff is put into water to make people dumb.

Exactly what did your question have to do with amps and scientists trying to figure out some way of using metric system values to define them? If your question was some form of advance attack in your mind trying to make me look like some kind of quack, don't waste your time. I don't care at all what anyone's opinion on here is of me. I do get very tired of all the personal attacks that go on here though. I normally just block those people.
retrosurf
2.8 / 5 (8) Aug 29, 2016
.. tend to put too much in most water supplies. They also tend to use too much chlorination in drinking water ..


That's what I thought. Bye.
FineStructureConstant
2.3 / 5 (6) Aug 30, 2016
@24volts - do you realize that fluoridation is the most monstrously conceived and dangerous communist plot we have ever had to face?
Auntiegrav
not rated yet Aug 30, 2016
@FineStructureConstant : technically, it was an anticommunist plot. Not so much a plot as protectionism for the nuclear processing program, but there is also the fertilizer, dentistry and foundry lobbies...in other words, "profitable industries that don't want to be seen as poisoning people".
Back to the amp:
It's important to standardize the amp because the meters used to measure volts and ohms generally rely on the flow of electrons in one way or another. Using force (the weight of gold foil leaves, for example) translates voltage to force, and then can be used as an indirect measurement of electrons present, but there are added levels of measurement uncertainties compared to directly counting the electrons. Counting electrons can also be used for future development of precise chemical reaction processes and some interesting detecting devices when combined with Wheatstone bridges.
FineStructureConstant
5 / 5 (2) Aug 30, 2016
So, the count so far is:

- whoosh!
- whoosh!

Anybody up for a hat-trick?
FineStructureConstant
1 / 5 (1) Aug 30, 2016
Any road up, back to the plot. I always found the bit about the 2 x 10E-7 newtons per meter length of perfect parallel conductors etc. troublesome - it must have been a pig to measure. Counting electric sheep - oops, electrons - seems a safer bet. Just need a fine pair of tweezers and one of those counting gadget thingummies...
jalmy
not rated yet Aug 30, 2016
@Auntiegrav

Also completely impractical as a unit of measure. As abstract and hard to use as the original. Again I propose using magnetism generated as a baseline. I/E what is the magnetic field generated by the current if you run a 24V system through a superconductor attached to a repeatable load such as a specific length of graphene or carbon nanotube. Repeatable, accurate, easily measurable. It is electromagnetism after all, would it not make sense to use one to measure the other? Again indeed we do such as clamp on ammeter is used to measure amp draw.
lengould100
5 / 5 (3) Aug 31, 2016
What the heck is all the anti-metric nonsense about? EVERY scientist in every country uses the metric system. Only a few obsolete engineers and tradespeople use the old armfuls and leg-lengths and half-a-chicken-weight measuring system to produce bridges that fall down and autos which need double the usual engine displacement to keep up.
scordiopetsu1980
1 / 5 (2) Sep 01, 2016
yes
maks06
1 / 5 (1) Sep 01, 2016
The ampere [A], is the unit of electric current; its magnitude is set by
fixing the numerical value of the elementary charge to be equal to
exactly 1.602 .10(-19) when it is expressed in the unit [As], which is
equal to C.

Thus we have the exact relation e = 1.602 .10(-19) [C]. The effect of
this definition is that the ampere is the electric current corresponding
to the flow of 6.242 10(18) elementary charges per second.

The present basic unit of electric current Ampere can't be basic unit
because is defined with Coulomb and second. Ampere is not unique unit, because depends on other units. From this is obviously that the Coulomb has no relation whit any other units and because of that it's most convenient this unit to be proposed as basic units.
agref
3 / 5 (2) Sep 01, 2016
There are two types of nations in the world: Those that landed men on the moon and those that use the metric system.


The people who landed men on the Moon used the metric system.
del2
not rated yet Sep 02, 2016
The present basic unit of electric current Ampere can't be basic unit
because is defined with Coulomb and second.

Not in SI, it's not.
carbon_unit
3 / 5 (2) Sep 04, 2016
There are two types of nations in the world: Those that landed men on the moon and those that use the metric system.

The people who landed men on the Moon used the metric system.
However, those who used english units in a system that was supposed to be metric didn't get to Mars orbit.
Phys1
not rated yet Sep 08, 2016
by the diehard metric users

It is a conspiracy against the furlong, the chain and the cowdung mass unit.
OdinsAcolyte
not rated yet Sep 16, 2016
Sounds a lot harder than I=V/R.
OdinsAcolyte
not rated yet Sep 16, 2016
Sounds a lot harder than I=V/R.
barakn
1 / 5 (1) Sep 21, 2016
They can't stand it because it's not a metric system of measurement.


There are two types of nations in the world: Those that landed men on the moon and those that use the metric system. -shootist

So Liberia and Myanmar have landed men on the moon? News to me.

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