A "link" is a unit of length that is equivalent to 1/100th of a chain. In the United States customary system of measurement, one link is equal to 7.92 inches or 0.201168 meters. It is primarily used in surveying and land measurement.
Links are often used in conjunction with other surveying units like chains and rods to measure distances on land, especially for large plots or parcels of land. The term "link" is rarely used in everyday conversation or modern applications outside of surveying and related fields. Instead, more common units like feet, yards, or meters are used for measuring length in most contexts.
A "fermi" (symbol: fm) is a unit of length used in physics to describe extremely small distances at the atomic and subatomic scale. One fermi is equal to 10^-15 meters or 0.000000000000001 meters. It is named after the Italian physicist Enrico Fermi, who made significant contributions to nuclear physics and particle physics.
The fermi is particularly useful for describing the sizes of atomic nuclei and the distances between particles within atomic nuclei. For example, the typical diameter of an atomic nucleus is on the order of a few femtometers (1 femtometer = 1 fm). It is also used in high-energy particle physics to describe the cross-sectional area of particle interactions.
In summary, the fermi is an essential unit of length for understanding the microscopic world of atoms, nuclei, and subatomic particles, where distances are incredibly small.