Celebrating metrology: 51 years of SI units

20 June 2011

It is 51 years since one of the most important ‘administrative’ actions of the scientific community, an act that will ensure the most accurate and traceable scientific measurements for many future generations of scientists and engineers, along with innumerable benefits to the greater good.

Institute of Physics News

The year 2011 marks the 51st anniversary of the international system (SI) of units, the system of seven fundamental units of measurement that underpin modern society, and which have been some time in the making.

 One of the first ideas for a universal metric system can be accredited to John Wilkins, ex secretary of the Royal Society, who outlined a new decimal system of measurement in 1668 in his book, ‘An Essay towards a Real Character and Philosophical Language’. 

However, possibly the most highlighted character in the development of a new decimal system of measurement was King Louis XVI of France. Some 120 years after Wilkins had published; it was he who first commissioned a group of scientists, which included Antoine Laurent Lavoisier, to develop a new measurement system, with a particular bias towards the unit of mass. 

It was these scientists who laid the foundations for the development of a new system, and even though significant changes were made during the succeeding French revolution, the story of the now standard international measurement system starts here.

Of course in the 200 years since there have been any number of scientists strongly promoting the applications of a metric system in scientific measurement: from the first absolute measurement of the earth’s magnetic force in 1832 by Gauss (based on the millimetre, gram and second) to James Clark Maxwell and electromagnetism. 

Maxwell, with help from the British Association for the Advancement of Science (BAAS), extended the measurement to include derived units of prefixes which allowed the expression of decimal multiples and submultiples (turning a thousandth of a litre into a millilitre for example). 

Along the way, new units were added, such as the ampere, which were seen to complement the need for more ‘practical’ units for electromagnetic measurements, along with the existing prototypes for mass and length being updated and redefined several times to keep everything in order, and finally to the final system of units that can be used to describe the whole world around us through measurement of just seven quantities.

So what are the standard units that make up the current international definitions? Well, we have the second, metre, candela, kilogram, kelvin, ampere and mole (the latter not actually added until 1971).  

These are the so called base units, the fundamental nuts and bolts of the current standard international metric system that has been widely adopted around the world, with the USA the most notable exception. 

At present all the standard units are based on natural phenomena, with the exception the kilogram (which is still quite unbelievably traced back to an original artefact), ensuring that they have the highest degree of accuracy and traceability.  

However, the SI system has to be dynamic: as the accuracy of scientific measurements continues to improve, so too must the measurement system in which they are discussed. 

It is the job of many scientists worldwide, such as those working at the National Physical Laboratory in Middlesex, to not only maintain the primary standards used to define these units, but also to develop a new set of standards to meet any future redefinitions of these units. 

The discussion of redefinition is a continual thread among the metrological community, highlighted by a recent meeting at the Royal Society to push for the change in the way we define the kilogram in particular. 

It is difficult to say exactly when this redefinition will happen (although we can be fairly certain that it will), as with any good science experiment there are different ways of approaching the problem, and hence different solutions, which need unanimous agreement in order to be pushed through. 

After the kilogram, we may even seen redefinitions of the second or the metre in the next 20 years, which promise great advances in further understanding of fundamental physics in particular.

International agreement of these units is now essential to trade, economy, communications - everything that we might take for granted in the 21st century, but at some point would have no doubt significantly hindered cultural and economic development. 

Without an agreed definition of units such as length and mass, there existed only a vast sea of uncorroborated values, and a huge discrepancy between one man and the next. 

Therefore, agreement and standardisation of measurement units are not only good common sense, but essential to a modern society, and a modern world.