Equation 4 · From Origins to Frontier: A History of Scientific Instruments and Metrology
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the coil’s velocity in the moving phase. Read the equation part by part below; each part has a contextual explanation and a link to its mathematical background.
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where m is the test mass, g the local gravitational acceleration (independently and precisely measured at the balance’s location), v the coil’s velocity in the moving phase, and U and I the voltage and current measured electrically. Combined with the quantum electrical measurements of U and I in terms of h , this equation is what let NIST’s NIST-4 Kibble balance measure the Planck constant to within about 13 parts per billion in 2017 — precise enough to underwrite the redefinition that followed [ 5 ] . The BIPM operated its own Kibble balance program in parallel, one of several national and international efforts converging on compatible values of h ahead of the vote [ 5 ] .
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