Kibble Balance: Weighing Mass with Planck's Constant

This simulator walks through the two-phase measurement at the heart of the modern kilogram: a coil sits in a fixed magnetic field on one arm of a balance, opposite a test mass. In the weighing phase you tune a current through the coil until its magnetic force exactly cancels the mass's weight on the beam. In the velocity phase the same coil is swept through the same field at a known, constant velocity, inducing a voltage you can read off directly. Because both phases share one coil in one field, the unknown geometric factor B·L cancels when the two measurements are combined — leaving a mass expressed purely in terms of a current, a voltage, a velocity and g. Real Kibble balances realize that current and voltage from the Josephson and quantum Hall effects, tying every kilogram measured anywhere to a fixed, exact value of Planck's constant.