A "handbreadth" is a unit of length that is approximately equal to the width of an adult human hand, excluding the fingers. It is often used in various historical and cultural contexts as a convenient way to estimate small lengths or distances. The actual measurement of a handbreadth can vary somewhat depending on the size of an individual's hand, but it is typically around 3 to 4 inches (about 7.6 to 10.2 centimeters).
In some historical systems of measurement, particularly in the ancient Near East and the Bible, the handbreadth was used as a standardized unit for measuring length. It was one of the smaller units within a larger system of measurement, often used for small-scale applications like measuring cloth, the dimensions of objects, or architectural details.
While the handbreadth is not a commonly used unit of measurement in modern times, it remains a useful and intuitive way to approximate small lengths informally, especially when a quick estimate is needed.
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.