Tackle Kilogram Problem S cientists love the International System of Units, which is the basis of practically every measurement in research and commerce. More precisely, they really love six of the system's seven basic units of measure, which define how we quantify such things as time, length and temperature. However, the unit for mass, the kilogram, poses a problem.
Physical standards are hard to reproduce accurately and can change over time.
Scientists at the National Institute of Standards and Technology are working to remedy this situation. They've just completed the first phase of a watt-balance experiment that can determine mass by comparing mechanical and electrical power through calculations based on other quantum standards. Their system can measure mass accurately to seven decimal places and with refinements could reach eight or more decimal places. That would rival the accuracy with which scientists currently can measure the physical kilogram standard and would eliminate the disadvantages of basing mass on a physical standard. NIST scientists are presenting their electronically derived kilogram results at the Conference on Precision Electromagnetic Measurements in Washington, D.C., July 10.