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About these units
Foot (ft)
A foot, equal to 12 inches, has been used for thousands of years and likely originated from the length of a human foot. Though early definitions varied, the modern standard foot—established internationally in 1959—provides a stable and consistent reference. The foot remains the foundation of architectural and construction measurements in the United States, influencing everything from lumber dimensions to ceiling heights and room layouts. Because the foot is intuitively relatable to human scale, people often estimate heights, furniture sizes, and walking distances in feet. In aviation, altitude is universally measured in feet worldwide, even in countries that otherwise use metric units—an unusual but deeply embedded global convention. The foot's persistence shows how cultural inertia can remain strong even when a unit predates modern scientific measurement.
Electron Radius (re)
The classical electron radius, approximately 2.818 × 10⁻¹⁵ meters, is a theoretical value derived from classical electromagnetic theory rather than an actual measured size. It represents the radius a charged sphere would need to have in order for its electrostatic self-energy to equal the electron's rest energy. Although electrons are now understood to be point-like or extremely small compared to this radius, the classical electron radius remains useful in scattering theory, especially in calculations involving Thomson scattering — the elastic scattering of electromagnetic radiation by free electrons. Thus, while not a physical dimension of the electron, the classical radius serves as a meaningful parameter in specific areas of physics and retains importance in radiation modeling and plasma physics.