A femtometer (fm), also known as a fermi, is an extremely tiny unit of measurement used to express even smaller distances than a picometer. It is equal to one quadrillionth of a meter, which is 0.000000000000001 meters or 1 × 10^-15 meters. The prefix "femto" denotes a factor of 10^-15 in the International System of Units (SI).
Femtometers are typically used in the field of nuclear physics and particle physics to describe the sizes and distances involved in atomic nuclei and subatomic particles. The nucleus of an atom, for example, is on the order of a few femtometers in diameter. In high-energy physics experiments and discussions about fundamental particles like protons, neutrons, and quarks, femtometers are a relevant unit of measurement due to the incredibly small scales involved.
A "finger" is a traditional unit of length used for measuring cloth or fabric. Similar to the span, the finger is based on the width of a human finger. It was historically used in various countries, including England.
In England, a finger was typically equal to about 2.25 inches or approximately 5.715 centimeters. However, like other traditional units of measurement, the precise length of a finger could vary depending on the region and historical context.