Kilogram - The Nonstandard Standard

The seven basic SI units are the meter, kilogram, ampere, mol, second, kelvin, and candela. Of these, only the kilogram is still defined by a physical standard. What does this mean? Basically this means the mass defined by a kilogram isn't a constant. However, scientists at the National Institute of Standards and Technology are looking at replacing the platinum-iridium primary standard with an electronic kilogram, called the watt balance.

The watt balance measures a kilogram against the amount of force required to balance a 1-kg mass against the force of the Earth's gravity. The kilogram mass rests on the balance surrounded by a coil of copper wire itself surrounded by a coil of superconducting wire. The magnetic field resulting from electricity passing through the coils pushes on the balance. The amount of voltage and current needed to offset the weight would be the new basis for defining the kilogram. The new standard would greatly reduce the uncertainty in the definition of the kilogram. However, electrical power is related to Planck's constant, so prior to accepting the standard, metrologists would need to agree on a fixed value for the constant. Presently, Planck's constant is determined experimentally using the physical kilogram standard.

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