The metric system of measurement was created by French scientists in 1795 . It is convenient to use because units of different sizes are related by powers of ten. An international committee determines the standards of of the metric system. this committee has set up the international System of Units (SI). The SI is used throughout the world. SI units are emphasized throughout this text. The National Institute of Standards and Technology (NIST)  keeps the official standards for the units of length, mass, and time for the United States. Because other quantities can be described using these three units, they are called  fundamental units.
  The standard units of time is the second (s). the second was the first defined as 1/86 400 of the mean solar day. A mean solar day is the average length of the day over a period of one year. In 1967, the second was redefined in terms of the frequency of one type of radiation emitted by cesium - 133 atom.
  The standard SI units of length is the meter (M). The meter was first defined as one ten-millionth (10^-7) of the distance from the north pole to the equator, measured along a line passing through Lyons, France.
  In the 20th century, physicists found that light could be used to make very precise measurements of distances. In 1960, the meter was redefined as a multiple of a wavelength of light emitted by krypton-86. By 1982, an even more precise length measurement defined the meter as the distance light travels in 1/299 792 458 seconds in a vacuum.
  The third standard unit measures the mass of an object. The kilogram (kg) is the only unit not defined in terms of the properties of atoms. It is the mass of platinum-iridium metal cylinder kept near Paris. A copy is kept at the NIST.
  Two other fundamental units will be introduced as needed. A wide variety of other units, called derived units, are combinations of the fundamental units. A common derived unit is the meter per second, or m/s, used to measure speed.