It will me measured now by the Planck constant, ending decades of definition by a small piece of metal held in a vault in Paris. The changes will come into force on 20 May next year and also affect the ampere, the kelvin and the mole. The principal pro argument was that the small cylinder of titanium alloy, which has set the standard since 1889, has lost atoms and therefore mass. This is important and must be considered to maintain accuracy. “Using the fundamental constants we observe in nature as a foundation for important concepts such as mass and time means that we have a stable foundation from which to advance our scientific understanding, develop new technologies and address some of society’s greatest challenges,” Martin Milton, director, International Bureau of Weights and Measures (BIPM) said.
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The ampere will now be defined by the elementary electrical charge, the kelvin by the Boltzmann constant and the mole by by the Avogadro constant. Other measurement units previously changed: as an example, the meter is no longer defined by a rod of metal but by the distance light travels in a set, and very small, fraction of a second. A number of authors have published however criticisms of the revised definitions.