In order to avoid converting from one system of measurement to another, the international scene has long been committed to defining a universally recognised and accepted convention. That is to say, one that, at the least, could be accepted by a large number of countries, since that would avoid the additional effort of performing unnecessary calculations as well as the possible occurrence of mistakes, entirely avoidable.

It should be mentioned, however, that converting from one system of measurement to another still makes sense, though; if anything, we must bear in mind that there are some professional groups and countries that still prefer to use their own, different systems. Nevertheless, it is important to remember that there are also several historical reasons for the creation of this universally accepted convention. The converter/transducer available on this web site will grant a useful support to all those who are interested in improving their knowledge on the history of energy, mainly since it will help them to understand – within different contexts – the meaning of several numerical values related to different energy products and sources.

The International System of Units (IS), which became in force in the Portuguese Law in 1983, was the result of this international standardisation effort.

The origins of IS

The quest for an internationally accepted standard measure, as far as units of length – metre – are concerned, goes back to the 18th Century, a time when the scientific community was somehow grouped into two sections, each of them with a different judgment of that matter: on the one hand, there were those who believed the best solution would be to define that same unit through the length of a pendulum with an oscillation semi-period of one second; on the other hand, some supported the idea that it would be desirable to use a part in ten millions of a meridian quarter – that is a quarter of the Earth’s perimeter – instead. After the French Revolution, in 1791, the members of the Paris Academy of Science decided to adopt the meridian-based definition, setting out their arguments against the first proposal based on the evidence that the Earth’s gravitational force varies along the Earth’s surface, thus provoking several differences in the oscillation period of the pendulums. The first standard measure (wholly made of metal) was built some time later, in the 19th Century; however, experts did not take into account the flatness of the Earth’s Poles, so this first prototype lacked 0.2 millimetres.

In 1875, at the so-called Metro Convention (to which Portugal was one of the signatories), the board unanimously approved the creation of the International Bureau of Weights and Measures, ruled and sponsored by the General Conference of Weights and Measures under which it still works – nowadays in the outskirts of Paris – providing all the necessary requirements to assure that the world-wide standardisation of units of measurement in the Physics field is done properly. The members of the Conference, on the other hand, still gather every four years in order to update the IS, introducing all the changes resulting from the development of both science and technology. In addition, it is also responsible for spreading this units system. The Conference gathers a total of almost 50 countries.

The creation of the IS resulted from a decision taken at the 9th General Conference of Weights and Measures, which took place in 1948, following the analysis of a standardisation proposal made by the International Union of Pure and Applied Physics (IUPAP). At the time, IUPAP recommended the use of the MKS system as a work-base. This system, which had been first suggested by the Italian physicist Giovanni Giorgi (1871–1950), used the metre, the kilogram and the second as its fundamental units – hence the acronym MKS, from the French words mètre, kilogramme and seconde.

During the 1954 and 1971 General Conferences of Weights and Measures (the 10th and 14th, respectively) the delegates decided to adopt as base units those corresponding to the following magnitude values used in Physics: length, mass, time, electric current intensity, thermodynamic temperature, amount of matter and luminous intensity. The designation International System of Units was adopted at the 1960 Conference, bearing the acronym IS. Nowadays, this system is an integral part of the legislation of almost every country in the world.

The IS unit of energy and work is called joule and the unit of measurement of electrical power is called watt. By definition, 1 joule is the work done by a 1 newton force when the latter displaces its application point 1 metre in the force direction. Speaking in purely energy terms, we can say that 1 joule is the energy which produces the work of 1 joule through integral transformation. Watt, on the other hand, is the power that leads to the production of energy of 1 joule per second.

Introduction of IS in Portugal

Comparing to what happened in other countries, the introduction of IS in Portugal was somehow tardy (in 1983), enacted as law through the Law by Decree nr 427/83 issued on the 7th November. However, the Ministry of Education had already recommended its use in 1972. After the introduction of several amendments to the 1983 Law by Decree, legislators decided to gather all the existing laws concerning IS in a single document (Law by Decree nr 238/94, dated September 19th). This last Decree issues, among others, the following regulations:

a) The use of the International System of Units is to be enforced within the whole of the national territory (Article 1); legal units of measurement are to be applied to “all economy-related activities, to both public health and public security sectors and to all administrative operations” (Article 4).
b) The simultaneous use of other units and legal units of measurement to express a given set of values whatsoever will be allowed until the 31st December 1999 (Article 2).
c) The use of legal units of measurement is compulsory in all measuring devices built-in displays.
d) The use of non-legal units of measurement will only be allowed in some isolated cases, such as those related to spare parts of pieces of equipment already on the market (or in use) before the enactment of the law.

Joule and Watt

IS units of energy and power were named, respectively, after two famous British physicists: James Prescott Joule (1818-1889) and James Watt (1736-1819). These two scientists and technologists wrote several important essays on these subjects – hence, the designation given to these two units of measurement, which is supposed to pay homage to Joule and Watt’s scientific legacy.
James Joule was born in the bosom of a rich family from Manchester with a significant background in the brewing industry. As a child, Joule was a shy, delicate boy. He was given access to a privileged education since his early years; Dalton was chosen to be his maths and science tutor. Soon, he developed a special interest in all heat-related subjects, which led him to perform a number of experiments at his family’s brewery. Aged only 18, Joule studied the heat produced by an electric current and a few years later (sometime around the 1840s), he formulated the law that bears his name (which relates both the current and resistance of a conductor to the heat it produces). Between 1837 and 1847 he presented the so-called conservation of energy principle and found out that heat is a form of energy amongst many others. Unfortunately, following some serious health problems, Joule was forced to interrupt all his investigation activities at the age of 55, thus depriving science of his extraordinary talent in the experiments field. His most relevant work (heat’s mechanical equivalent) was carried out before he was 30.
James Watt, on the other hand, was the son of a shipbuilding worker. Unlike Joule, Watt never made it to the top as far as education was concerned, mostly due to his feeble constitution. He had, however, a number of talents, which allowed him to start working on a regular basis as an instrument manufacturer at Glasgow University. While he was repairing a steam machine from Newcomen, he realised that it was possible to make a few changes, which, he believed, would greatly improve its effectiveness. After having introduced several other changes, he presented the world with the so-called Watt machine, which played a very important role at the Industrial Revolution. In 1779, Watt invented a revolutionary copy process (for office papers), which used a kind of gelatinous ink (the photocopy’s remote ancestor). Unfortunately, in order to read these copies, it was necessary to use a mirror. When he retired, in 1800, the once feeble boy was a very rich man.