The ninth Conférence Générale des Poids et Mesures (General Conference on Weights and Measures) ratified in 1948 a definition of weber based on Faraday's law for magnetic induction. Faraday's law connects electromotive force (in volt) to the rate of change of magnetic flux (in Wb/s) through one loop of conducting wire (a "turn"):
If the magnetic flux Φ is linear in time—Φ changes with uniform rate—and if Φ = 0 at t = 0, then it follows that
This equation forms the basis of the formal definition, which reads:
One weber is the magnetic flux which, linking a circuit of one turn, would produce in it an electromotive force of 1 volt if it were reduced to zero at a uniform rate in 1 second. (Resolution 2, International Committee for Weights and Measures. 1946).
- BIPM brochure about SI. PDF page 52; paper page 144.