An "ell" is an antiquated unit of length that has been historically used in various countries, including England, Scotland, and Germany. The length of an ell could vary depending on the region and the specific purpose, but it was typically used to measure cloth or textiles. The term "ell" is derived from the Latin word "ulna," which means "forearm," as the length of an ell was often based on the length of a person's forearm.
Different regions had their own variations of the ell, but some common measurements include:
English Ell: In England, the English ell was approximately 45 inches or 1.143 meters.
Scottish Ell: In Scotland, the Scottish ell was longer, measuring around 37 inches or 0.94 meters.
French Ell: In France, the French ell, also known as the aune, was used and had its own variations, but it was typically around 1.188 meters.
Flemish Ell: In parts of Europe, including Belgium and the Netherlands, the Flemish ell was used, which was approximately 27 inches or 0.686 meters.
In the field of atomic and molecular physics, an "atomic unit of length" is a unit of measurement that is used to express distances at the atomic and molecular scale in a dimensionless way. It is part of a system of atomic units (a.u.) that simplifies calculations involving fundamental physical constants and properties of atoms and molecules.
The atomic unit of length (a.u. of length) is defined in terms of the Bohr radius (a₀), which is a fundamental constant in atomic physics. The Bohr radius is approximately 0.52917721067 angstroms (Å) or 5.2917721067 x 10^-11 meters (m).
In atomic units, the Bohr radius is set to exactly 1 a.u. of length. Therefore, when using atomic units, distances are expressed relative to the Bohr radius, and the value of 1 a.u. of length corresponds to the typical size scale of atomic and molecular structures.
The use of atomic units simplifies many quantum mechanical calculations and allows physicists and chemists to work with dimensionless quantities, making it easier to compare and analyze atomic and molecular properties.