The kilogram, the actual
kilogram, sits in a vault in Sèvres,
France under numerous bell jars. It is the last SI unit to be defined
based on a physical quantity—in this case one kilogram of platinum
and iridium—as opposed to fundamental constants. There is, however,
a push underway to change this, to phase out the hunk of metal and
define a kilogram in terms of a fundamental constants of the universe.

Researchers collaborating as
part of an international effort known as Project Avogadro are working
to define the kilogram in terms of Avogadro’s constant (N_A). The
chemically inclined among the readers will know what this is without
even thinking: it is the number of atoms that make up a mole of
a substance—about 6.022 x 10²³. As it stands today, our knowledge of this number is not
known to a high enough degree of precision to redefine the kilogram and rid ourselves of the artifact in France.