In arguments concerning weight, the final arbiter is a cylindrical piece of a platinum-iridium alloy kept in an airtight chamber in Sevres, France, which is deemed to weigh exactly I kg. But a Massachusetts Institute of Technology physicist says that he has a more precise alternative -the mass of an atom (Science, Vol 265, No 5179).

David Pritchard and his colleagues recently weighed ranging from hydrogen to argon to an accuracy of about 10 decimal places. The scientists exploited a device called the Penning trap, which used electric and magnetic fields to trap ions that oscillate around the lines of magnetic force. A comparison of frequencies (which have a direct relationship with energy) of oscillation of differentials using Einstein's equation, E=MC², yielded their masses.