Stoney units

In physics the Stoney units form a system of units named after the Irish physicist George Johnstone Stoney, who first proposed them in 1881. They are the earliest example of natural units, i.e., a coherent set of units of measurement designed so that chosen physical constants fully define and are included in the set. The constants that Stoney used to define his set of units is the following:[ Astrophysics, clocks and fundamental constants, by Karshenboim and Peik, p. 79] c, the speed of light in vacuum, G, the gravitational constant, ε_0, the vacuum permittivity, e, the elementary charge. This means that the numerical values of all these constants, when expressed in coherent Stoney units, is equal one: In Stoney units, the numerical value of the reduced Planck constant is where α is the fine-structure constant. George Stoney was one of the first scientists to understand that electric charge was quantized; from this quantization and three other constants that he perceived as being universal (a speed from electromagnetism, and the coefficients in the electrostatic and gravitational force equations) he derived the units that are now named after him. Stoney's derived estimate of the unit of charge, 10−20 ampere-second, was of the modern value of the charge of the electron due to Stoney using the approximated value of 1018 for the number of molecules presented in one cubic millimetre of gas at standard temperature and pressure. Using the modern values for the Avogadro constant and for the volume of a gram-molecule under these conditions of 22.4146e6mm3, the modern value is 2.687e16, instead of Stoney's 1018. Stoney's set of base units is similar to the one used in Planck units, proposed independently by Planck thirty years later, in which Planck normalized the Planck constant in place of the elementary charge. Planck units are more commonly used than Stoney units in modern physics, especially for quantum gravity (including string theory). Rarely, Planck units are referred to as Planck–Stoney units.