SI base unit referenced to the values of the fundamental constants, namely the Planck constant ("h"), the electron charge ("e"), the Boltzmann constant ("k"), and the Avogadro constant ("N"). This approach was approved in 2018, only after measurements of these constants were achieved with sufficient accuracy. SI base unit The International System of Units (SI) defines seven units of measure as a basic set from which all other SI units can be derived. The SI base units and their physical quantities are the meter for measurement of length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin
Auxiliary power unit Auxiliary power unit An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft and naval ships as well as some large land vehicles. Aircraft APUs generally produce 115 V alternating current (AC) at 400 Hz (rather than 50/60 Hz in mains supply), to run the electrical systems of the aircraft; others can produce 28 V direct current (DC). APUs can provide power through single- or three-phase systems. During World War I, the British Coastal class blimps, one of several types of airship operated by