The kilogram is defined to be the mass of a particular
piece of platinum-iridium alloy that was made around 1880 and
is at the Bureau of Weights and Measures in Paris, France.
This is an awkward definition; indeed, there is reason to believe that the mass of the standard
kilogram has decreased slightly since it was made.
It would be preferable to have the kilogram defined in terms of a
process that can be performed without having to go to Paris.
For example, the electron has a definite mass, as does a carbon atom; so
the kilogram could be defined as being the mass of a certain (very large)
number of electrons or carbon atoms:
1 Kilogram = 1,097,769,238,499,084,016,780,676,223 electrons(the mass of an electron is not known to this great accuracy; after the first 6 digits you can put any numbers you like). However, for this kind of definition to be useful, we need to be able to count out and weigh electrons reproducibly. For the moment, the mass of the platinum-iridium cylinder can be compared more reproducibly with other objects than we can count electrons, so we are still using a lump of metal for the definition.
More about this:
http://en.wikipedia.org/wiki/Kilogram
Masses of various objects
| object | mass |
|---|---|
| A hydrogen atom | 0.000,000,000,000,000,000,000,000,001,6 Kg |
| a snowflake | 0.000,001 Kg |
| paper clip | 0.0004 Kg |
| penny | 0.0025 Kg |
| nickel | 0.005 Kg |
| dime | 0.0023 Kg |
| a sheet of paper | 0.0045 Kg |
| quarter | 0.0056 Kg |
| pencil | 0.005 Kg |
| middle school student | 45 Kg |
| small car (or small elephant) | 450 Kg |
| metric ton | 1000 Kg |
| a locomotive | 500,000 Kg |
| the earth | 6,000,000,000,000,000,000,000,000 Kg |