A watt balance is an experimental electromechanical weight measuring instrument that measures the weight of a test object very precisely by the strength of an electric current and a voltage. In 2016, metrologists agreed to rename watt balances as Kibble balances, in honour of and it is being developed as a metrological instrument that may one day provide a definition of the kilogram unit of mass based on electronic units, a so-called electronic or electrical kilogram. The name watt balance comes from the fact that the weight of the test mass is proportional to the product of the current and the voltage, which is measured in units of watts. In this new application, the balance will be used in the opposite sense, the weight of the kilogram is then used to compute the mass of the kilogram by accurately determining the local gravitational acceleration. This will define the mass of a kilogram in terms of a current, the principle that is used in the watt balance was proposed by B. P. Kibble of the UK National Physical Laboratory in 1975 for measurement of the gyromagnetic ratio. The main weakness of the balance method is that the result depends on the accuracy with which the dimensions of the coils are measured. The watt balance method has an extra step in which the effect of the geometry of the coils is eliminated. This extra step involves moving the force coil through a magnetic flux at a known speed. This step was done in 1990, in 2014, NRC researchers published the most accurate measurement of the Planck constant to date, with a relative uncertainty of 1. 8×10−8. A conducting wire of length L that carries an electric current I perpendicular to a field of strength B will experience a Laplace force equal to BLI. In the watt balance, the current is varied so that this force exactly counteracts the weight w of a mass m. This is also the principle behind the ampere balance, W is given by the mass m multiplied by the local gravitational acceleration g. Kibbles watt balance avoids the problems of measuring B and L with a calibration step. The same wire is moved through the magnetic field at a known speed v. By Faradays law of induction, a potential difference U is generated across the ends of the wire. The unknown product BL can be eliminated from the equations to give U I = m g v. With U, I, g, and v accurately measured, this gives an accurate value for m. Both sides of the equation have the dimensions of power, measured in watts in the International System of Units, the current watt balance experiments are equivalent to measuring the value of the conventional watt in SI units. The importance of measurements is that they are also a direct measurement of the Planck constant h, h =4 K J2 R K. The principle of the kilogram would be to define the value of the Planck constant in the same way that the meter is defined by the speed of light
The NIST-4 Kibble balance, which began full operation in early 2015, measured Planck's constant to within 13 parts per billion in 2017, which was accurate enough to assist with the redefinition of the kilogram planned for 2019.
Precision Ampere balance at the US National Bureau of Standards (now NIST) in 1927. The current coils are visible under the balance, attached to the right balance arm. The Kibble balance is a development of the Ampere balance.