## Metric and Wilkins compared – not quite deja vu

A feature of the metric system, which distinguishes it from customary systems, is the use prefixes for decimal multiples and submultiples as well as the use of symbols for units. These were not part of Wilkins’ proposals of 1668. Roddy Urquhart compares these with the modern metric system (SI).

## Using metric – accuracy v precision

A fellow metric supporter who admits to being a little weak on mathematics owned up to not understanding the difference between accuracy and precision when it comes to measurement.

He is probably right in saying that he is not alone and that many people fail to see advantages with metric in this respect.

I offer here two examples in an attempt to clarify the issue, one purely numeric, the other practical involving an everyday example of measurement.

## Barrels of oil

The media always report statistics of oil production, reserves etc in “barrels”. But how many people know how big a “barrel” is?* Indeed is it an appropriate unit of measurement to use in the context of world energy policy?

The oil industry used to be dominated by American companies, and as with aviation and computers, American units have largely been adopted as the industry standard.

This might not matter since the only people who trade in oil are industry insiders, and arguably the general public do not need to know how big a “barrel” is. They will understand that if OPEC reduces output by x million barrels per day, that’s a lot of oil, and price rises can therefore be expected. In any case, outside the USA, they will still buy the end product in litres.

Yet although it is not a consumer protection issue, there is a problem. Any departure from the International System of Units should be discouraged, as it results in dual labelling, conversion errors, the need to know two systems when one will suffice – and of course general incomprehension.
More particularly, the “barrel of oil equivalent” is used as a
measure of global energy production and consumption. For this, all energy sources (m3 of gas, tonnes of coal, etc.) are converted by energy content into the equivalent energy available in a barrel of crude oil.

BP produces an annual report of world energy consumption and has just produced this year’s. See www.bp.com/statisticalreview
I notice that they are now using the tonne of oil equivalent alongside, or sometimes in place of the barrel. Whilst this is clearly an improvement, it doesn’t really give you what you’re looking for. If you’re talking about world energy consumption, rather than just oil, then surely joules (or gigajoules – GJ) are the unit they should be working with? I can see some logic in using a quantity of oil to measure reserves, and refinery throughput, but if you’re comparing nuclear and gas, for example, what has oil got to do with it? The joule only gets a mention in the conversion tables – 1 tonne oil equivalent ~= 42 GJ.

*A “barrel” is 42 US gallons (equivalent to approximately 159 litres)

## A new definition of the kilogram?

A new method of defining the kilogram is being sought by various teams of scientists around the world. However, it may be some years before a decision emerges. (NB – this will obviously not alter the actual size of the kilogram). This article, contributed by Martin Vlietstra, will be of interest to the more technically minded.

The kilogram is an anomaly in the world of physical constants – its current definition relies on a particular artefact or object – the prototype kilogram that is held by the BIPM on behalf of its “shareholders”, its subscriber governments. Every other physical constant is defined in terms of one or other physical phenomena that can, in principle, be measured in any laboratory in the world. Ever since the retirement of the prototype metre in 1960, scientists have been looking for a means of defining the kilogram by means of a scientific experiment and yet maintaining the accuracy that can be obtained using the prototype kilogram

One of the projects to redefine the kilogram is to define it in terms of a sphere of silicon. Such spheres are currently being produced in the laboratories of the Australian Council for Scientific and Industrial Research (ACSIR) – See http://www.theage.com.au/news/national/making-an-exact-difference/2007/06/14/1181414466901.html.

Once the sphere has been manufactured, there are a number of problems associated with defining the kilogram. Firstly, the diameter of the sphere must known to an accuracy of better that one part in 10^8. If the sphere has a mass of exactly one kilogram, its radius will be approximately 93.58 mm, so its diameter needs to be known to better than 1 nm (which is approximately two wavelengths of light). Details of some of the scientific techniques used and the participating laboratories (Australian, Belgian, British and German [in alphabetic order]) can be found at http://www.npl.co.uk/mass/avogadro.html.

In addition to measuring the diameter, the scientists concerned will need to identify which is the more practical – to define the kilogram in terms a specific number of silicon atoms or to define it in terms of the mass of a sphere of specified radius. Part of the experiments currently under way is to decide which of the two techniques give the better results.

This is not the only experiment that is being developed to redefine the kilogram; another is the Watt Balance which is being carried out by the BIPM. (See http://www.bipm.org/en/scientific/elec/watt_balance/ ).

Who will decide which experiment is the better? This will ultimately be decided by the CGPM on the advice of the CIPM and is likely to be some years off.

CGPM = Conférence Générale des Poids et Mesures / General Conference on Weights and Measures, a body consisting of representative of the governments that have subscribed to the Convention of the Metre.

CIPM = Comité International des Poids et Mesures /International Committee of Weights and Measures, a body of 18 eminent scientists elected by the CGPM.