The Metric System
In the late 18th century, Louis XVI of France charged a group of scientists to reform the French system of weights and measures. It was widely recognized at the time that it was an inconsistent and disorganized collection of measurements that varied with location and often on obscure bases. Providing a scientifically observable system with decimally based divisions was the charge assigned to a group from the French Academy of Sciences, which included Pierre Simon LaPlace and J.J. Lagrange. They sought to create bases of measurement linked to the scientifically verifiable values such as the Earth’s circumference.
The unit of length, defined as a meter, was introduced in 1791 after careful measurement of the Earth’s radius and the recognition that the planet was not perfectly spherical but instead possessed an oblate spheroid shape. The meter was designated as one ten-millionth of the length of the Earth’s meridian through the city of Paris from the North Pole to the Equator.
For the measurement of volume, the SI unit devised in 1795 was the cubic meter, which was based on the volume of a cube with sides of one meter each. The large size of this unit has largely resulted in the more common use of the smaller metric unit of the liter, defined as 0.001 cubic meters.
The kilogram was settled upon in 1799 as the mass standard, based on the value of a platinum bar. Now the contemporary standard for the kilogram is stored at the Bureau International des Poids et Mesures (BIPM) in Sevres, France as a Platinum-iridium alloy.
The original definition of the principal time unit, the second was considered to be 186,400th of the mean solar day. Due to inconsistencies in the rate of the Earth’s rotation, the modern definition is linked to the radiation correlating to the orbital transitions of the cesium -133 isotope.
Since the 1960s, the International System of Units has been internationally agreed upon as the standard metric system.
What is the Kelvin Temperature Scale?
There are three different temperature scales in use in the world today. Mainly the United States utilizes the Fahrenheit scale, which was introduced by Daniel Gabriel Fahrenheit in 1724. The non-intuitive reference points on the Fahrenheit system (212∘F and 32∘F) for the boiling and freezing points of water, respectively) are replaced in the more universally accepted Celsius, or Centigrade system, devised by Anders Celsius in 1742, by 100∘C and 0∘C for scientific applications, however, both scales are inconveniently constructed in that a substantial portion of the scale consists of negative values for temperature. For many physical considerations, the use of a Celsius or Fahrenheit temperature that is a negative number produces an impossible result, such as in the Ideal Gas Law, (pV=nRT).
In 1848, William Thomson Kelvin, a British physicist proposed the scale that is now named in his honor. In the design of this system, there are no negative values for temperature with the lowest value on the scale known as absolute zero. Substances at this theoretical point would display a complete absence of kinetic energy, thus atoms at absolute zero would cease all motion.
The Kelvin and Celsius scales are routinely used in chemical measurements and are conveniently constructed in that temperature change between any two points are exactly the same. Most laboratory thermometers available today are graduated in the Celsius system yet transition to the accepted SI Kelvin units is straightforward; since 0K=−273.15∘C, adding 273.15 degrees to the Celsius temperature will yield the correct Kelvin value. Note that because the Kelvin system is an absolute scale, the degree symbol (∘) is omitted in reporting the Kelvin temperature.