Re: What will happen if the SI prototype kilogram is lost?

First, I need to make a correction in what you were reading; your source 
was either misinformed or out of date.

For a while, the liter *was* defined in terms of a mass of water.  Which 
made a difference in about the 6th significant digit between a liter and a 
cubic decimeter (equivalently, between a milliliter and a cm3).  But this 
ceased to be true in 1964.  Since 1964, the liter has been defined as 
exactly one cubic decimeter.  (One still has to watch out for the old 
definition when looking at precise data from before 1964).

With regard to your main question, mass is the black sheep of the 
measurement standard family, because it is the only one still defined as 
an "artifact" rather than as something one can get directly from 
fundamental physics.  As you suggest, the mass of the prototype kilogram 
can drift a tiny bit depending on dust, cleaning and polishing, etc.  That 
is why it is kept in a special vault and only accessed rarely, with well-
defined cleaning procedures.  But the uncertainty in the artifact due to 
these factors is around the 8th significant digit, and metrologists would 
like to do better, both for its own sake and because this uncertainty 
carries over into other units like electrical units.

If something happened to the prototype kilogram, I don't know if there is 
a specific contingency plan.  My guess is that the kilogram would get 
defined in terms of one of the "lieutenant" kilograms which are compared 
on occasion with the primary standard and given similar special care.

There is work going on to get around this messy dealing with an artifact; 
people are trying to define a reproducible standard mass that can in 
principle be realized anywhere rather than sitting in a vault in Paris.  
One effort involves growing ultrapure silicon crystals and measuring the 
interatomic distance precisely with X-rays.  If they can accurately count 
how many silicon atoms are there, they will know what the mass is.  The 
second method is called the "Watt balance" where electrical measurements 
(which can usually be done quite precisely) are used to generate a known 
force.  Neither of these methods has yet been able to match the precision 
of the prototype kilogram, but a next generation of the Watt balance is 
coming together which may give the guy in Paris a run for his money.

The Watt balance experiment is described at: http://www.eeel.nist.gov/811/elec-kilo.html

For more on the definition of fundamental units, see: http://physics.nist.gov/cuu/index.html

Allan Harvey, Physical and Chemical Properties Division, NIST
"Don't blame the government for what I say, or vice versa."