Everything is getting heavier
wired.com
wired.com
It would be interesting to know what is the source of that error (I didn't find the speculation of out gassing to be particularly compelling, but I didn't recall them mentioning how the companion standards were built).
1. Why do we need both kilogram and mole as fundamental units when they measure essentially the same thing (at least in case when the thing being measured is pure)?
2. Why not reverse the definition: postulate that 1 mole is Avogadro's number of elementary particles; 1 mole of C-12 has 12 grams and derive (kilo)gram from that.
1. They are not both fundamental, a mole is defined as the amount of substance that contains an equal number of elementary entities as there are atoms in 0.012kg of the isotope carbon-12. So the mole is defined in terms of the kg.
2. This is essentially Arnold Nicolaus' project (described in the article). He wants to define the kg as the mass of a certain number of atoms. But we don't yet have an accurate way of counting atoms, so if we pick a number (i.e. define Avogadro's number by fiat) today, then later when we count more carefully the mass of everything (denoted in kg) will fluctuate.
2) We don't exactly know Avogadro's constant. We've measured it fairly accurately, but, for your proposal to work, we'd be forced to define all 23 digits. We can then declare that 12g of carbon 12 is exactly 12g, but how do you know that you have exactly 12g of carbon twelve? Just from basic counting statistics, you'll probably be off by a few hundred billion atoms, and that's assuming that you have someone is and count all 6.02 * 10^23 atoms individually. The result is that the mass of the kilogram is now a derived and empirical value, adding uncertainty to every measurement of mass in every experiment. It's far preferable to leave the kilogram as an exactly defined unit and put the uncertainty into Avogadro's number, where it will only crop up when someone needs to make the conversion.
Dr. Emmett Brown: There's that word again; "heavy". Why are things so heavy in the future? Is there a problem with the earth's gravitational pull?
From, of course, Back To The Future (1985)
"In fact, of the seven fundamental metric units — the kilogram, meter, second, ampere, kelvin, mole, and candela — only the kilogram is still dependent on a physical artifact."
Followed by:
"And the luminosity of light, or candela, is measured in terms of power, designated in watts, or joules per second."
I'm not an SI expert, but Wikipedia suggests the candela is in fact an SI base unit with a definition based on physical phenomenon of the universe, so the second sentence seems erroneous.
[edit: clarity]
It isn't quite as fixed as some measurements since the average human's vision could continue to evolve and change the curve, or because we could find out that we measured it wrong, but it's not dependent on a single physical sample like the kilogram.
http://en.wikipedia.org/wiki/File:SI_base_unit.svg
Now, in the abstract, yours is the more natural definition. But experimentally, it turns out to not give the best accuracy.
I believe one starts with (1) the meter (which is based on a second defined using a atomic transition, plus light) and (2) the Newton (which is the force required to accelerate the all-important kilogram at 1 meter per second squared). Then an Ampere is the amount of current flowing through two parallel wires required to produce a force of one Newton per meter between the wires.
http://en.wikipedia.org/wiki/Ampere#Definition
Now, the technology is apparently improving enough that it may be possible to actually count elementary charges like you suggest. See here:
http://en.wikipedia.org/wiki/Ampere#Proposed_future_definiti...
First up is the question of defining that 1 KG itself and the configuration; how will we define the parameters of the copper wire? (edit: to clarify after reemrevnivek's excellent comment the configuration of the solenoid is important because the strength of the magnetic field generated is directly related to the configuration of the coil) We can't use the radius, length of the copper wire accurately because as it gets heated up it will expand not only longitudinally but radially as well.
Second, how will the magnetic field be measured to the precision needed? If you use induction of current in another coil then according to lenz law that will create a downward force on our coil leading to inaccurate measurements (I'm assuming that it will be constantly measured) , this means that the coil needs to be strictly constrained, by how do you constrain it without adding/rubbing material on to the surface of the copper wire?
Read up on the Biot-Savart law if you want to know more about calculating magnetic fields.
It would be nice for HP to start measuring their ink in grams instead of ml, that way they could increase their prices based on the grand K.
Other standards are related to fundamental physical constants. Time for weight to do the same.
[edit: typo]
"Aside from a yearly ceremonial peek inside its vault, which can be unlocked only with three keys held by three different officials, the prototype goes unmolested for decades. Yet every 40 years or so, protocol requires that it be washed with alcohol, dried with a chamois cloth, given a steam bath, allowed to air dry, and then weighed against the freshly scrubbed national standards, all transported to France."
Sounds sensible and reliable. No issue with corrosion, infiltration of gasses or lint I guess. And what about the other standards? Who knows which is changing?
"Steiner argues, the watt balance, with its Planck constant, is “a better realization,” because his system is self-contained and replicable, whereas the Avogadro project spans several continents and relies on a single artifact. "
A non-reproducible result is not science. It doesn't matter how many zeroes are involved; its not actually a verifiable fact if there's only one experiment, right?