The watt balance and atom-counting techniques now give nearly identical values of Planck's constant, with an uncertainty of less than 20 parts in a billion, says metrologist Ian Robinson of the National Physical Laboratory in Teddington, England.
Once Planck's constant has been fixed, watt balances then use the constant to calculate unknown mass.
The other, called the watt balance
, measures the constant by an indirect or "virtual" comparison of mechanical power to electromagnetic power.
The National Institute of Standards and Technology (NIST) watt balance experiment completed a determination of Planck constant in 1998 with a relative standard uncertainty of 87 X [10.
Key words: absolute watt measurement; electronic kilogram; electronic watt determination; fundamental physical constants; Planck constant; SI power measurement; SI watt unit; watt balance.
The main rival relies on a watt balance
, which measures the mass of a test cylinder by suspending it using a combination of electrical currents and magnetic fields.
the NIST Watt Balance
Experiment ) or using electrostatic forces .
The first, called the moving coil watt balance
method, relates electrical energy to mechanical power at the quantum level.
The NIST watt balance experiment is being completely rebuilt after its 1998 determination of the Planck constant.
Key words: DLC coating; hysteresis; Josephson constant; kilogram artifact; Planck constant; watt balance.
A recent moving-coil watt balance
experiment carried out at NIST, based on equating electric power and mechanical power, has determined the Josephson constant to be 483 597.
A NIST research team has reassembled the NIST watt balance
experiment inside its newly decorated home and the reborn apparatus is showing the first signs of life.