A square chain is a unit of area measurement that is now considered obsolete and not commonly used. It was historically used in some English-speaking countries, including the United States and the United Kingdom, as part of the older system of land measurement.
One square chain is equal to an area of 10 square rods, or 484 square yards. In terms of square meters, one square chain is approximately equal to 404.68564224 square meters (m²).
The square chain was often used in land surveys and property descriptions in the past, but it has been largely replaced by the more standardized metric and imperial systems of measurement. In modern times, square meters and square feet are the more commonly used units for measuring land area.
In physics, particularly in the field of nuclear and particle physics, a "barn" is a unit of area used to describe the likelihood of nuclear reactions or scattering events. The name "barn" comes from the idea that it's a very large target area, as if hitting the broad side of a barn were an easy task.
The barn is not part of the International System of Units (SI), but it's a widely recognized unit in nuclear physics. One barn is equal to 1 square femtometer (fm²), which is an incredibly small area. Specifically:
1 barn (b) = 1 square femtometer (fm²) = 10^(-28) square meters (m²)
To put it in perspective, a barn is used to describe the effective target area for nuclear reactions. Nuclear cross-sections, which describe the likelihood of nuclear interactions occurring, are often measured in barns. It's a unit that allows scientists to quantify the probability of certain atomic and subatomic processes taking place.