Boltzmann and Planck constants remeasured

NIST has announced more accurate measurements of the Boltzmann and Planck constants. The new values will contribute to an international effort to move away from defining basic units using physical objects and instead, starting in 2019, derive them from seven fundamental constants of nature. The kelvin, for example, is currently defined by measuring the triple point of water in a sealed glass cell. Going forward, metrologists want to define the kelvin in terms of the Boltzmann constant, which NIST researchers determined to be 1.3806410 × 10^–23 J/K by measuring the degree of random motion of electrons in a resistor (Metrologia 2017, DOI: 10.1088/1681-7575/aa7b3f). Meanwhile, the kilogram is currently defined as the mass of a plum-sized cylinder of platinum-iridium alloy. Instead, metrologists want to define the kilogram in terms of the Planck constant. NIST scientists have now measured that constant’s value as 6.626069934 × 10^–34 kg·m²/second by using a Kibble balance, formerly known as a watt balance, which balances the weight of a test object by passing a current through a coil suspended in a magnetic field (Metrologia 2017, DOI: 10.1088/1681-7575/aa7bf2).