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.
The "actus" was a unit of length used in ancient Rome. It was primarily a measure of distance and was approximately equivalent to 120 Roman feet or about 35.5 meters (116.5 feet) in modern measurements.
The actus was commonly used for various purposes in ancient Rome, including land measurement, surveying, and property boundaries. It was also used in the construction of roads, aqueducts, and other civil engineering projects.
It's important to note that ancient Roman units of measurement, including the actus, were not standardized in the same way that modern units are today. Therefore, the exact length of an actus could vary in different regions and over time.
The use of the actus and other Roman units of measurement has largely been replaced by modern metric and imperial systems in contemporary times, but they remain of historical interest and can still be encountered in the study of ancient Roman culture and history.