Femtometer
A femtometer (fm) is a unit of length in the metric system, equal to one-quadrillionth of a meter (1 fm = 10⁻¹⁵ meters). It is also known as a fermi, named after the physicist Enrico Fermi. Femtometers are used to measure extremely small distances at the subatomic level, such as the size of protons, neutrons, and atomic nuclei. For example, the radius of a proton is about 0.84 femtometers. This unit is commonly used in nuclear physics and particle physics, but it's far too small for everyday measurements.
Russian Archin
The archin (also spelled arshin) is a traditional Russian unit of length that was widely used before the adoption of the metric system. It is approximately equal to 28 inches or 0.7112 meters.
The archin was commonly used in measuring cloth, land, and building dimensions in Russia and neighboring regions. It roughly corresponds to the length of a forearm from the elbow to the tip of the middle finger, similar to the cubit.
Though obsolete today, the archin remains an important part of Russian historical measurements and appears in older legal and trade documents.
Summary:
1 archin ≈ 28 inches (0.7112 meters)
Used historically in Russia for textiles and land measurement
Based on the length of the forearm
Mostly of historical and cultural interest today
No conversions available for length.