The kilogram (kg) is the base unit of mass in the International System of Units (SI). It is defined as the mass of the International Prototype of the Kilogram (IPK), a platinum-iridium cylinder kept at the International Bureau of Weights and Measures (BIPM) in France.
Until May 20, 2019, the kilogram was defined by the physical prototype, but as part of the redefinition of the SI units, the kilogram was redefined in terms of fundamental constants. Since that date, the kilogram is defined by fixing the numerical value of the Planck constant (h) to be exactly 6.62607015 x 10^-34 joule-seconds.
This redefinition ensures that the value of the kilogram is now based on a constant of nature rather than relying on a physical object that could potentially change over time. It allows for a more precise and stable definition of the kilogram and other SI units.
A microgram (µg or mcg) is a metric unit of mass or weight in the International System of Units (SI). It represents one millionth of a gram or 10^-6 grams. The metric system is a decimal-based system of measurement used worldwide, and it provides a convenient way to express both large and small quantities of mass.
To provide context for the size of a microgram:
Micrograms are commonly used in scientific and medical contexts, especially when dealing with substances or quantities that are very small. For example: