A Second Mass Standard
The masses of atoms can be compared with each other more precisely than they can be compared with the standard kilogram. For this reason, we have a second mass standard. It is the carbon-12 atom, which, by international agreement, has been assigned a mass of 12 atomic mass units (u). The relation between the two units is
with an uncertainty of 10 in the last two decimal places. Scientists can, with
reasonable precision, experimentally determine the masses of other atoms relative to the mass of carbon-I 2. What we presently lack is a reliable means of extending that precision to more common units of mass such as a kilogram.