Principle
A conducting wire of length L which carries an electric current I perpendicular to a magnetic field of strength B will experience a force equal to BLI. In the watt balance, the current is varied so that this force exactly counteracts the weight of a standard mass m, which is given by the mass multiplied by the local gravitational acceleration g. This is also the principle behind the ampere balance.
Kibble's watt balance avoids the problems of measuring B and L with a second calibration step. The same wire (in practice a coil of wire) is moved through the same magnetic field at a known speed v. By Faraday's law of induction, a potential difference U is generated across the ends of the wire, which is equal to BLv. Hence the unknown quantities B and L can be eliminated from the equations to give
Both sides of the equation have the dimensions of power, measured in watts in the International System of Units, hence the name "watt balance".
Read more about this topic: Watt Balance
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