## Ampere Could Be Redefined After Experiments Track Single Electrons Crossing Chip 299 299

ananyo writes

*"Physicists have tracked electrons crossing a semiconductor chip one at a time — an experiment that should at last enable a rational definition of the ampere, the unit of electrical current. At present, an ampere is defined as the amount of charge flowing per second through two infinitely long wires one meter apart, such that the wires attract each other with a force of 2×10^-7 newtons per meter of length. That definition, adopted in 1948 and based on a thought experiment that can at best be approximated in the laboratory, is clumsy — almost as much of an embarrassment as the definition of the kilogram, which relies on the fluctuating mass of a 125-year-old platinum-and-iridium cylinder stored at the International Bureau of Weights and Measures in Paris. The new approach, described in a paper posted onto the arXiv server on 19 December, would redefine the amp on the basis of**e*, a physical constant representing the charge of an electron."
## Re:fluctuating weight of KG? (Score:5, Informative)

Because a few atoms of the slug can sublimate into the surrounding atmosphere, even at room temperature. And because a few atoms of the surrounding atmosphere can adhere to the slug. And yes, at the precision we're talking about here, it makes a difference.

## Gravity is not constant... (Score:3, Informative)

It's hard to consistantly and accurately measure weight when the force of gravity constantly changes, add to the fact that there may be radioactive decay of trace elements, oxidation of metals, Dust/erosion, sublimation of trace components), it's easy to understand how using a physical object to consistantly measure a weight, would fluxuate. when your "constants" are actually "variables" it's really hard to nail down constants...

## Re:Bah, I say (Score:4, Informative)

Well, yes. But the point here isn't shuffling around the units. The point here is to increase the accuracy at which the elementary charge is known, which would be necessary whether you're defining the Ampere in terms of the charge or the Coulomb in terms in the charge. Currently, we know the elementary charge to ten decimal places. That's not good enough, so that's what this is about--finding out that figure to greater accuracy so it can be used as a universal measurement standard. For comparison, the definition of the second is accurate to 15 decimal places.

## Re:How an Ampere is defined will NOT change! (Score:5, Informative)

## Re:Gravity is not constant... (Score:3, Informative)

Placing two masses on a balance is the usual method . . . . and it is gravity-independent. Gravity is necessary, of course, but it only needs to be constant across the two platters of the balance.

## Re:How an Ampere is defined will NOT change! (Score:5, Informative)

This is not entirely correct. Ampere is an SI base unit while Coulomb is a SI derived unit (defined as 1 C = 1 A s) - not the other way round.

## Re:Gravity is not constant... (Score:5, Informative)

That approach is in fact one of the proposals [wikipedia.org] for a replacement to the kilogram. The problem is counting 10^23 atoms of a material (and getting pure material to work with).

## Re:fluctuating weight of KG? (Score:5, Informative)

http://en.wikipedia.org/wiki/Kilogram#Stability_of_the_international_prototype_kilogram [wikipedia.org]