Kilogram
From Simple English Wikipedia, the free encyclopedia
The kilogram is a metric unit that describes mass. The official kilogram equals the mass of a particular piece of platinum-iridium metal kept in Paris. It is the only SI unit left that is defined such that it needs to be compared to some object. There are now attempts to define the kilogram in a different way, for examply by specifying a nuber of atoms of a certain substance (at a certain temperature).
One kilogram is a little more than 2.2 pounds. One tonne is one thousand kilograms.
[edit] History
For a long time, the kilogram was the mass of a litre of water at 3.98 degrees Celsius. This was not precise enough, because the density of water depends on the air pressure, and the air pressure cannot be described without already having a unit of mass. So in the 1880s the piece of metal was made. Platinum-iridium was chosen because it does not rust or corrode the way other metals do. Kilogram was historically named by the original unprefixed term "grave".
[edit] Problems
There are various copies of the original Kilogram. The problem is now that some of these copies have gained or lost some mass. They no longer weight the same.