Kilograms or pounds – that is the question

The historical origin of a pound lies in the ancient roman libra, the sicilian litra and the greek litron – three pound units that varied between 300 and 450 grams. The ancient roman libra is to this day referred to in the shortcut for pound – lb. In medieval times, the pound asserted itself as the main measuring unit and was used widely throughout Europe. It however became a problem, that every city had its own understanding of how heavy exactly one pound was. Fluctuations of 100 grams were not rare. In the year 793, Charlemagne introduced the so-called carl-pound, which weighed about 408 grams. To this day, the ‘pound’ unit remains an important measuring system for mass in the Anglo American regions, where it is equivalent to exactly 16 ounces, therefore 0.453592 kilograms.

Outside of the Anglo American regions, and especially in Europe, the kilogram is used as the main unit, also defining a certain mass. The idea of developing a standardized international measuring system arose in France around 1790, yet the actual instatement of the kilogram, the way we know and use it today, didn’t happen until much later. As goods and merchandise had been measured according to their volume or count for the longest time, scientists in France began working on a uniform system that would work cross-nationally. To spread measuring standards internationally, an organization was founded, that was in charge of standardizing the measuring and weighing system. Between 1870 and 1875, this organization evolved into the international meter commission in Paris.

The international meter commission determined the so-called primary kilo, which to this day remains the basis for the kilogram. The primary kilogram was presented in 1889 as an international reference tool for the first time. The weight refers to a cylinder, 39 mm in height and 39 mm in diameter, made of 90% platinum and 10% iridium with a very high density. The actual primary kilogram is kept in the town of Sèvre near Paris, stored in a safe in the international bureau for measures and weight (BIPM). Since the actual primary kilogram is kept stowed away safely, the BIPM uses a large amount of copies, which are called kilogram prototypes and only ever differ in weight by no more than one gram from the original. Every country in the world that has entered into the meter convention owns such kilogram prototypes.

The primary kilogram offers the basis for standardizing measuring systems. By gauging devices, it is ensured, that the data delivered from the measuring device ranges within the calibration error limits – in other words, the device measures according to the primary kilogram. The first handwritten law for standardizing was developed in 1871. As a certified producer of gauged medical appliances, seca owns the official licence from the German bureau of standards and possesses own kilogram prototypes as a reference. All weights that are used for producing standardized scales are checked and, if necessary, adjusted on a yearly basis. Furthermore, seca was the first foreign scale producer to acquire the rights of producer gauging for the Japanese market. To receive this certificate for a very superior market, copies of the Japanese primary kilogram had to be acquired and the entire process had to be checked and certified by the Japanese bureau of standards. The objects used for standardizing Japanese products are sent to Japan every three years for comparing to the Japanese primary kilogram.

Every region therefore has its own system, either the pound or the kilogram. The primary kilogram in the French safe is currently baffling scientists, as it is becoming lighter every year. The loss in weight has so far not been accounted for. The difference is namely only 50 micrograms, which is a reduction of fifty millionth grams in the past one hundred years. Yet, as  we are becoming more and more dependent on precise measurements in the fields of science and economy, scientists are currently trying to redefine the kilogram into a system which can be deduced from a fundamental constant of physics.