The National Institute of Standards and Technology (NIST) was initially founded in 1901 to develop nationwide standards for newly emerging electrical technology. Over the years, NIST has created standards for everything from fire hydrant connectors to computer chips.
Standards, however, are of little use without a widely accepted system of measurement units. The metric system, also known as the Système International d'Unités or SI, was established in France in 1799. In the subsequent two centuries, definitions of standard SI units have improved as technology progressed. Here is a list of a few SI standards, including both their original and modern definitions.
The meter was defined in 1791 to be one ten-millionth of the distance from the North Pole to the equator, on a meridian line passing through Paris. Unfortunately, due to a miscalculation in the shape of the Earth, the original standard meter (a platinum-iridium bar kept at the freezing point of water) was 0.2 millimeters shorter than it should have been from this definition.
The modern standard meter is defined to be the distance light travels in vacuum in 1/299,792,458 of a second.
The second was initially defined as 1/86,400 of the mean solar day (the average length of the day over the course of a year). But scientists have since realized that the mean solar day may fluctuate by appreciable amounts.
The modern definition of the second is the number 9,192,631,770 divided by the frequency of a specific atomic transition in cesium atoms. An atomic transition occurs when an electron orbiting an atom jumps from one orbit to another.
The gram was first defined to be the mass of 1 cubic centimeter of water. Weighing precise volumes of water can be tricky, however, and in 1889 a piece of platinum-iridium metal was defined as the international kilogram standard.
Unlike the meter and the second, the kilogram -- defined as 1,000 grams -- has yet to be replaced with a new standard, and the original kilogram prototype from 1889 is still kept in vacuum under glass at the International Bureau of Weights and Measures in France. The kilogram is defined as "equal to the mass of the international prototype of the kilogram."
Recent research suggests that the mass of the standard kilogram may be changing over time, and researchers at NIST and other research institutions are currently working to develop better mass standards.