Equation 5 · Part 1 · From Origins to Frontier: A History of Scientific Instruments and Metrology
Symbol U
What this part means
U is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.
Its job in the formula
U is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.
Full expression→Symbol U→Article meaning
The passage around this formula
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 ] .
Learn the underlying idea
A variable is a named place for a value. Its letter is a local label: x can mean position in one formula and a data point in another.
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Sources cited in the surrounding passage
These citations provide research context; check each source for the exact claim it supports.