

Everything is getting heavier - mjbellantoni
http://www.wired.com/magazine/2011/09/ff_kilogram/all/1

======
nopassrecover
Editorialised title - actual title is "The Search for a More Perfect
Kilogram". Wired should use your title though ("Everything is Getting
Heavier") - it's a catchy title for an interesting article. Also the picture
half-way down is totally a TARDIS.

~~~
ChuckMcM
The article did state that the 'secondary' versions of the kilogram were
getting lighter with respect to the Grand K and it was up to 500 mG which is
quite significant.

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).

~~~
caf
It's not "five hundred milligrams" (500mg), it's "five-hundredths of a
milligram" (0.05mg, or 50μg).

------
zvrba
Can somebody (preferrably, a physicist or a chemist) explain to me two things.

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.

~~~
vilhelm_s
(I'm not a chemist, but):

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.

------
kristopolous
Marty McFly: Whoa, this is heavy.

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)

------
kevinh
Interesting article. I was under the impression that the kilogram was defined
as the mass of one liter of water, but that appears to just be an
approximation now.

~~~
Zaak
The problem with using water as a standard is that "water" isn't well defined
when you're measuring to eight decimal places.

------
aprescott
Huh?

"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]

~~~
quanticle
What I don't get is why ampere is a fundamental unit while coulomb isn't. I
mean, it seems more natural to think of amperes and coulombs per second rather
than to define coulombs as the amount of charged carried by a 1 amp current in
1 second.

~~~
jessriedel
Techinically, neither the Coulomb nor the Ampere are fundmental. Both are
derived from the meter, second, and 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...](http://en.wikipedia.org/wiki/Ampere#Proposed_future_definition)

------
MadGouki
Would it be possible to define the KG as the amount of copper wire, when
arranged in a configuration (a coil?), that causes a magnetic field of a
certain size when a specified current is applied?

~~~
GeneTraylor
The problem lies in how will you define the other parameters over here.

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?

------
suivix
I'm sure the Wall Street protesters would break into there and steal the
kilogram.

------
hmottestad
I wonder if this affects the price of very expensive per weight materials.

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.

------
tybris
In general, everything is getting X. By some definition of X.

------
JoeAltmaier
A fabulously overwrought drama of no value. Similar to Tibetan rituals.

Other standards are related to fundamental physical constants. Time for weight
to do the same.

~~~
mtts
The article makes it quite clear why it isn't possible to relate weight to
fundamental physical constants yet: the constants that can be used for this
haven't been measured precisely enough yet. Until they are, using a block of
metal in a vault that has lost about half a speck of dust worth of matter in
two hundred years is more exact.

[edit: typo]

~~~
JoeAltmaier
You are right of course.

"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?

