Seven housings of measure

2000/10/17 Roa Zubia, Guillermo - Elhuyar Zientzia

• Industria
• Historia

The considerations on historical contributions for the definition of basic units are maintained today.

When the hunter catches wild boar many excesses are heard. It is an animal of two meters of length and a half ton of weight. Each one can ask if the meters and kilos of the hunter are smaller than the rest...

It was a concern of long ago. In the commercial field, at least, it was essential to define all this. The first method was sought. The meter and kilogram were defined by two platinum standards. But these objects were not available to everyone and were also physically very variable. It was something that had to be more concrete. After the French revolution, when modern society was organizing, the International Bureau of Measures and Weights (Bureau International des Poids et Mesures, BIPM), near Paris, was responsible for this. The objective was to define exactly the units. In fact, on June 22, 1799, the day of birth of the current metric system can be considered.

Modernity Modernity Modernity

Very advanced ideas were applied. The modern world needed modern definitions. Ten million of the distance between the north pole and the equator was the metro. They took as reference the meridian that passes through Paris. However, later errors were discovered, due to the rotation of the Earth the meridians are gradually decreasing. Therefore, this definition is not adequate. For this reason, in 1872 a new metro prototype was made.

For the definition of the unit of mass, authorized by King Louis XVI, a commission supplied the mass of a grave, that is, of a liter of water. Like other laws, the scientific norms generated by the monarchy were ruled out. The revolutionaries were looking for a smaller unit and defined the gram. But the concrete prototype of the gram was very difficult to make and measure. Therefore, a unit equivalent to that given by the actual board was needed. However, the ideologues of the revolution called him a kilogram.

It should be noted that from there comes the prefix kilo that we use to express «a thousand times». In addition to the metro, at the end of the century, new kilogram prototypes were made. However, for the definition of this unit no other method has been applied. In fact, it is currently the only basic unit defined by a prototype.

For other units, however, XX. The closeness of the twentieth century required a new methodology. The seconds and degrees of temperature had a very old definition and, however, it was not possible to renew the system. In 1893 the Amperium was launched as an electric current unit in tribute to the French physicist Andre Marie Anpere. Fifteen years later this unit was approved. The first General Assembly of Measures and Weights held in 1889 (Conference Generale des Poids et Mesures, CGPM) decided to solve the problem of prototype-based definitions.

New concepts New concepts

XIX. At the end of the twentieth century physicists investigated electromagnetic waves. It was discovered that the light of each color has an exact wavelength, a length that did not change. Thus, the American physicist Albert Abraham Michelson proposed to use waves to define lengths. In addition, in 1927 a new definition of length units was established with this base. The wavelength of the red line of the cadmium spectrum was called angstrom and ten billion angstromas form the meter.

XX. The twentieth century has brought novelties, such as the definition of the kelvin degree, candle and mol. But they have determined which are the basic units and which are not. Of these basic units you can define all the others. The definition of all this is not a simple task.