To speak of length, mass and time is to be referring to three fundamental concepts that help us to dimension very important physical properties, such as the distance between two points, the amount of matter in a body, and the duration of some phenomenon, respectively. These three notions are handled by all of us with more or less precision, but they are certainly part of our lives, as well as those of our ancestors.

Physical magnitudes are said to be those properties of bodies that can be measured; all ancient peoples have had their ways of measuring these characteristics. Knowing the duration of some important event within the activities of the community, or the perception of natural phenomena only took on a true dimension when it was compared with a similar event or phenomenon that had been assimilated beforehand, that is, that had helped ancient human beings to locate themselves in time and space. Thus, the process of comparison became what we know as measurement.

When we say that something is 75 meters, we are referring to three fundamental things. A mathematical quantity is equivalent to 75 units. A standard unit that is taken as a reference, in this case, the meter. A physical quantity or property that is being measured, i.e., length.

The first two characteristics must be expressed for the length to take its true dimension. For a measurement to be considered valid, it must always give the same result, regardless of who makes it. If there are small differences between several measurements, it is convenient to establish criteria or ranges of precision within which the measurement is considered to be consistent. For example, to measure the distance between two cities it is sufficient to give a precision of kilometers, while to measure the diameter of a bacterium, it is necessary to give a precision of a few millionths of a meter, i.e. microns.

The first measurements

However, it is important to remember that the units of measurement of length, like those of any other physical magnitude, have been arbitrarily defined. Until the end of the 18th century, there was no unit of measurement for each country. The expansion of trade and communications made it necessary to adopt general measures that were equivalent in all countries. Transactions between countries were made difficult because many of the measures were parts of the body of some important personage. The foot, inch, and yard were used, which corresponded to the measurements of the foot, thumb, and arm, respectively, of the king in turn. Another inconvenience was that the relationship between them was not decimal, that is, the multiples were not decimal. For example, one yard = 3 feet = 36 inches, which generated arithmetical complications to make conversions.

The first measuring instruments were the senses and the parts of the body. The Egyptians used the arm stroke (which seems to be the oldest known length measurement) equivalent to the measurements of a man with arms outstretched. The English used the royal foot as their standard. The Romans used the step and the mile (mille passum, meaning "a thousand steps"). For them, one step was equivalent to two steps, since they considered the stride with both feet, that is, the complete cycle. The Roman Empire (which lasted from the 2nd century BC to the 4th century AD) imposed the pound as a unit of mass and the foot as a unit of length, the latter being a bronze standard, replicas of which were used throughout the Empire.

As nations grew, they became more complex in their political and economic organization and their relationship with other peoples. This situation was reflected in their ways of measuring. For example, the historical events that led to the decline of the Roman Empire had a definite influence on measurements, to the extent that a kind of metric anarchy arose for the established measurements, as a form of protest against the Roman yoke.

Without going too far away, let us remember that among the caudillo presidents of the Mexican Revolution, or at the time of the Reform, there was a somewhat similar situation, but in the field of the national economy. The competition for power was such that when any of them took office, they issued new currency to physically and economically destabilize their predecessor.

The meter: revolutionary unit

Returning to the units of measurement, it was not until 1790 when the French Constituent Assembly decided to invite the most famous scientists of the time to try to unify the measurement systems. Some scientists proposed during the Assembly in 1795 that a system of measurement be adopted in which the measurements would have decimal relations between them, and that these measurements would have a natural standard much more stable than the parts of the body. It was then established that the distance between the north pole and the equator of a meridian passing through Paris, divided into ten million equal parts, would be the standard meter.

So, a group of French scientists set out to measure part of that distance using the triangulation method, from Duquerque to Barcelona, between 1792 and 1799. They considered the Earth to be a sphere and took the arc corresponding to the longitude measured during these seven years, thus obtaining the distance from the pole to the equator. Later, when it was discovered that the Earth was not a perfect sphere, the meter was established as the length found between two very fine lines drawn on a bar made of an alloy of platinum and iridium.

Napoleon imbued with a spirit that made him look like the true leader of the French Revolution, decreed the introduction of the Decimal Metric System in schools so that this would be France's contribution to the world. The difficulty and resistance of the population to the new measures was very significant, but, in the name of the Revolution, the people accepted the change.

Metric unification

Almost a century later, the "Metre Convention" was held in Paris, where 18 of the most important European nations of the world at that time decided to adopt the metric system as the universal system. England did not attend this meeting and continued to apply its foot-yard-inch systems.

In 1960 the meter was redefined in terms of the wavelength of the red light coming from the krypton 86 source because the platinum and iridium rod was not as stable as it was thought to be because its length was affected by temperature changes. Finally, due to the need for greater precision in modern instruments, it was established in 1983 that the meter is the distance that light travels in a vacuum in a time of 1/2 999 972 458 seconds.

It was not until 1960 that England and the English-speaking countries adopted the International System of Units (SI). The United States is the only country that has not made the use of the SI mandatory and exclusive. Canada, Australia, New Zealand, and almost all of Great Britain are already metrically standardized countries or are about to become so. In the United States, there is a group called the US Metric Association (USMA), which works intensively for the metrication of the country.

In Mexico, the Sl has been the official system for many years and is called the General System of Units of Measurement, whose mandatory use is under the jurisdiction of the General Directorate of Standards of the Ministry of Commerce and Industrial Development.

By Alejandra González Dávila