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An italic lower-case h, denoting the quantum of action.
Also known as: Quantum of action, Italic h
h relates a photon's energy to its frequency and sets the scale at which quantum effects matter. Its value has been exact since 2019, when the kilogram was redefined in terms of it — the last SI unit to depend on a physical object.
Denotes the Planck constant, relating a photon's energy to its frequency and setting the scale at which quantum behaviour becomes significant.
Contexts: Mathematics, Laboratory
Has had an exact value since the 2019 SI redefinition, which used it to define the kilogram and so retired the last physical artefact any unit depended on.
Contexts: Mathematics, Laboratory
Appears far more often in its reduced form ℏ, the constant divided by two pi, which is a different character and a different value.
Contexts: Mathematics, Laboratory
A lower-case Latin letter h set in italic type: a vertical ascender with an arch springing from its middle to a second descending stroke.
SI defining constants.
The kilogram is defined in terms of this constant.