This query has been received from a reader of Metric Views:

“If Britain were to revert to the exclusive use of Imperial measures, could it actually help trade with the USA, who (sic) uses US customary measures? I know that while I am an US customary/Imperial supporter in the USA, would British goods suffer in the USA if Britain reverted to Imperial units?”

The ‘Review of external trade statistics’ on www.statistics.gov.uk provides information on trade with EU and non-EU countries. It shows that in 2009 the EU accounted for 53.6% of UK trade in goods, both exports and imports. Non-EU countries, including the US, accounted for the balance.

HMRC statistics for general trade are found on www.uktradeinfo.com, and these provide a break down by country. The table for the ‘top 50’ countries shows that in 2009 trade with Germany alone (£64 billion) exceeded that with the US (£62 billion), which represented only 12.2% of UK trade with the ‘top 50’.

It is clear therefore that any reversion to the use of pound/inch units in UK manufacture would not be helpful for UK trade.

A reversion to imperial units for liquid volume would serve no purpose as these are unique to the UK and differ significantly from homonymic US customary units.

The questioner asks if British goods in the USA would suffer if Britain reverted to imperial units. A look at two of the more successful manufacturing companies in the UK indicates that this would be so.

Nissan’s car manufacturing plant in Sunderland is the most productive in Europe, and the second most productive in the world. It exports 85% of its output, some to the USA. High productivity is aided by the ability to source parts and components, metric of course, from around the world.

Rolls Royce in Derby produces, arguably, the best aero engines in the world, fitted to both Airbus and Boeing aircraft – the RR Trent 1000 engine powered the first flight of the Boeing 787 Dreamliner. RR aero engines include components and complete assemblies brought in from plants in the UK and the continent in order to optimise the efficiency of a predominantly metric production process.

Clearly, reversion to a measurement system that is not shared with suppliers would increase costs for both companies, and result in products becoming less competitive.

So if metrication has opened world markets to UK manufacturing and brought efficiencies in production, then why does this sector now form only 12% of the UK economy? The new government is talking of ‘rebalancing the economy’ – will manufacturing be able to make the contribution expected from it?

During the ‘Tonight’ programme on ITV1 on 15 July 2010, Lord Digby Jones, Director of the CBI 2000-06, Minister of State for Trade 2007-08 and on the board of JCB (another successful UK manufacturing company), spoke of ways to increase manufacturing output. He pointed out that the UK can not compete on something that sells on price. He said we need to look for quality, value added and innovation – areas where we can’t be undercut, and for innovative products that others can not make.

But he also said that half the kids who leave school this year will do so without a grade C in maths and English. He quipped, “Half the schools’ input to the world of work is not fit for purpose”.

Sir James Dyson was also interviewed during the programme and echoed some of the points made by Digby Jones. He pointed out that the UK is producing one twentieth of the number of engineers of China or India, and half the engineers of the Philippines or Mexico.

By the time Vince Cable, Secretary of State for Business, had made his contribution to the programme, speaking of changing the culture in education in favour of maths, physics and engineering and creating a new skills base, the message could not be clearer.

But have not readers of Metric Views heard this before? Remember our article ‘Kids don’t count’ published on 20 May 2010? This suggested that, so long as there is a difference between the measurement units used at school and those on the street and in the home, then this cultural divide will continue, and for many kids the prospect of learning a skill may be far from enticing.

The United States has avoided a divide between school, home and work but at the same time has excluded itself from many world markets by retaining pound/inch units beyond their sell by date. The UK faces the other side of the coin – opportunities for UK manufacturing industry have been created by US inaction but many of these may be lost because too few kids can count.

Both countries need to change. Which will be first?

The Système International (SI) has been carefully designed as a coherent system of units intended for use in all applications independent of language and national origin. At its heart is the principle that one and only one unit is used for each type of measurement. The same unit is used regardless of scale and the former traditional practice of distinct units for different ranges is supplanted by the use of a common set of prefixes based on powers of ten as an optional alternative to exponential notation also based on powers of ten.

Some measurement applications however do have key units of measurement that, by virtue of their definition, are very convenient for the purpose and if were to be incorporated into the SI would create anomalies, contrary to its principles.

In this article the discussion will focus on astronomy which has a significant number of non-SI units in common use. Readers knowledgeable in other subjects will no doubt think of examples in their own sphere.

For a very recent example consider the tables of data from the NASA/Ames Kepler mission:

http://kepler.nasa.gov/Mission/discoveries/

Note in particular the mass measurements: Solar (mass of Sun = 1), Jovian (mass of Jupiter = 1) and Earth masses which are in wide-spread use. The Astonomical Unit (AU where the mean distance from Sun to Earth = 1)  is also very common. Other common units of distance are the parsec (distance of an object from Earth such that two position measurements made over a 6 month interval yield a discrepency of 1 arc-second due to parallax) and the light year (the distance travelled by an object in a year if moving at the speed of light).

The kilogram is probably used in astro-physics for calculation involving fundamental physical laws (e.g. the gravitational constant is a key parameter in the equations of stellar structure) but the results are presented and discussed in terms of the mass ratios referred to above. This is not because the SI is being rivalled by dogmatic alternatives but, probably, for valid scientific reasons. It is likely the comparisons are significant in helping to assimilate the data and to formulate or refine theories about the formation and evolution of stars and planets.

The AU has its uses in helping to visualize the scale and structure of the Solar system which is probably more memorable than distances in metres. 1 AU is about 150 Gm so conversion to metres isn’t too bad e.g. the mean distance of Pluto at roughly 40 AU is about 6000 Gm or 6 Tm. However, its use should really be confined to circumstances where a comparison with Earth’s orbit is significant (its use in the Kepler observations may well be important).

Incidentally, when interplanetary distances are expressed in metres suitable prefixes should be used e.g. Tm not billions of km.

The light year has some usefulness for larger distance scales. For example a galaxy 2 million light years away is effectively being seen as it was 2 million years ago. It is also a significant parameter for very distant objects because a quasar say 13 billion light years away is effectively a window on the very early universe. As it happens the light year is within 6% of 10 Pm (approx 9.46 x 10¹⁵ m).

In the opinion of this author the parsec, however, is overused. It is of course sensible for the intermediate results of parallax measurements (which is necessarily limited to relatively nearby distance determination) but a poor choice as a general purpose unit due to its awkward relationship to the light year (3.24 ly) and metre. Retaining it is almost (but not quite) as bad as retaining the mile alongside the kilometre.

As a general conclusion, standard SI units should be used where it is feasible to do so and non-SI units, where justifiable, should be rationalised to avoid unnecessary proliferation and confined to appropriate circumstances.

Readers may disagree with the tolerance expressed here to some non-SI units and are invited to comment or give other examples in areas of their own knowledge and interest.

Eric writes:

“Fully metricated Australia seems to lose its metric bearing completely when it comes to news about the tragic oil spill in America. All media outlets except newspapers, who at least convert outdated medieval measurements, use a mixture of both. From 2000 up to 5000 feet, down to barrels and gallons, with litres thrown in, are often used without conversions. The irony is that only older people know what a gallon is and probably nobody knows how many litres a barrel holds? I did write to two TV stations pointing out that needless retro step, but have as yet received only an acknowledgement. It sure would help if a few more Australians take the time to object to this irritating anomaly.”

Are more of our readers being confused by reports which use a mix of US customary, Imperial and metric measures?

(Note: one barrel of oil is about 159 litres or 42 US gallons or 35 Imperial gallons)

Ezra wrote on 27 February:

“I just posted this comment to the USMA mailing list. My encounter with Canadian metric weather reports makes me wonder if this is one area (other than road signs) where Canada is ahead of the UK in terms of metrication. Can you enlighten me?

I just happened to check out an article in the Vancouver (Canada) Sun online and noticed the link for the current weather, so I thought I’d have a look.

You can choose many different  countries and cities within each country, so I tried where I live (Seattle area): http://www.vancouversun.com/weather/index.html?rg=us&city=seattle

I notice that everything is given in proper SI (except for wind speed, which is in km/h rather than m/s, but I’ll take it anyway, especially since they use the proper syntax instead of some monstrosity like “kph”), including the use of kPa for barometric pressure. And there is no option I could find to switch to Imperial!

I’ve seen Canadian national weather reports on the CBC and the story is the same – not a whisper of Imperial anywhere.”

Tony replied:

“As I understand it, Canada uses Celsius only with the exception of the Windsor area (where the US influence is very strong due to many working over the border) and in cooking where F lingers due to old habits/recipes and US influence. Canadian ovens generally dual labelled for sale across N America while ours are Celsius only.

In the UK, Celsius has been used for many years (see UKMA’s YouTube channel, http://www.youtube.com/user/UKMetric, for a news clip on Kennedy’s assassination; it starts with a weather report which gives temps in Celsius first, F second, although the term centigrade was used).

So Celsius is well embedded in the UK and is without doubt the principal unit used, but Fahrenheit lingers on as secondary units in certain places, such as papers with a more elderly following. Personally I almost never hear or read any Fahrenheit anywhere as they don’t use it at all in the papers I read or radio stations I listen to, but others may hear more imperial if they read other papers and listen to other radio or TV broadcasts.

Rain is given in mm, and snow generally in cm, although inches also sometimes used for snow in forecasts or news reports. Wind speeds usually quoted in mph, as that’s the only measure of speed most people know due to road signs. Main exception is in sports such as athletics where m/s is used; times only count as records if wind is under 2 m/s so that’s why that measure is used there.

Pressure in kPa rare I think, but used on the Met Office website.”

Ezra responded:

“I had heard that Celsius was regularly used in the UK for cold weather reports and Fahrenheit in the summer time because the latter numbers are larger! It struck me as a strange and jarring way of doing business, but it sounds like that report is untrue.

If all ovens sold in the UK show Celsius only, then those who have cook books showing just Fahrenheit for baking or oven cooking temperatures need to consult a conversion chart, I presume. (I hope nothing like that comes affixed to the oven, itself!)

If kilopascals are not used for barometric pressure, do you folks use millibars or inches of mercury on television, radio, and in the newspapers?

As for wind speeds in mph, we see once again the pernicious effects of having failed to convert the road signs long ago as originally planned. It is striking how a single decision can have such far-reaching effects down the road (if you’ll pardon the pun!)

Let us hope that the rising concerns for safety (drug dosages, malnutrition in patients, bridge strikes, speeding by lorry drivers from the Continent who have km/h-only speedometers in their vehicles, etc.) can be parlayed into a general recognition on the part of the government that finishing metrication (including conversion of all road signs) should be delayed no longer.”

Tony brought the exchange to a close:

“Regarding Fahrenheit in hot weather, it certainly used to be the case that the media would hype hot temperatures in Fahrenheit in summer just because it sounds more extreme. But such usage has receded significantly in recent years as they have realised that hardly anyone under 50 knew what they were talking about; see these articles on the same topic, from the same newspaper:

http://www.thisislondon.co.uk/news/article-5741008-97f-on-the-tube.do
http://www.flickr.com/photos/lmg/21280730/in/set-69593/
http://www.thisislondon.co.uk/standard/article-23714728-tube-too-hot-for-cattle-as-station-shops-cash-in.do
http://www.thisislondon.co.uk/standard/article-23710914-cooler-summer-for-commuters-as-mayor-unveils-aircon-tube.do

From 2003 to 2009 you’ll see that the articles have gone from dual unit (with hyped F headlines) to Celsius only. And there was a front page headline in the same paper which I can’t find (but is mentioned here http://www.timesonline.co.uk/tol/news/uk/article689454.ece) which quoted the recorded highs of 52C and 47C on London buses and tubes in 2006, so extreme highs are now much more likely to be in Celsius.

Regarding ovens, I have never seen an oven in the UK with Fahrenheit markings, nor a recipe book with only Fahrenheit (though older books tend to have both scales), so the change happened long ago.

On pressure, millibars are used.

Visibility (which I forgot to mention) is given in km; it’s not something used in mainstream forecasts but where it is given I have only ever seen or heard it in km. See here for example:
http://uk.weather.com/weather/today-London-UKXX0085?fromSearch=true
(Although that site does contradict something else I said in giving wind speed in km/h!)”

Consider some of the different units that are or have been used for measuring energy and power (not an exhaustive list):

Energy

• British Thermal Units (BTU), typically for domestic boilers (although they usually mean BTU/h) – see below
• kilowatt hours (kWh), for electricity (and sometimes gas) bills
• joules (J) – the SI unit – used for food energy (nutrition) on package labels
• calories (cal) (often confused with kilocalories – kcal) also for food energy
• ergs – an obsolete unit from a previous version of the modern metric system
• electronvolts (eV) – used by physicists to measure very tiny quantities of energy

Power

• horsepower (HP) – still sometimes used to describe car engine output
• brake horsepower (BHP) – now rarely used
• tax horsepower – formerly used to calculate tax on cars (e.g. Austin 7)
• Pferdestärke (PS) – a German version of HP for car engines
• chevaux fiscaux (CV) – similarly, a French version of “tax horsepower” (e.g. Citroën 2CV)
• watt (W) – the SI unit, defined as a joule per second (J/s)
• British Thermal Units per hour (BTU/h) – used for heating systems

How – if at all – do all these units relate to each other? To explain this we need to revise a little basic science.

What are energy and power – and how do they relate?

There are various forms of energy (which may be why different measurement units have evolved), but they have one thing in common: energy is what makes things move, or heat, or light up, or make a sound. So it can be thermal (i.e. heat), or electro-magnetic, gravitational, or mechanical etc. But it is all energy. Energy cannot be created or destroyed, but one form of energy can be transformed into another form of energy – e.g. when a turbine generates electricity, which in turn heats a kettle or drives a motor: or when chemical energy stored in a battery is transformed into sound waves from your radio. As it is all basically the same “stuff” – energy – it is helpful if it can be measured in the same way.

Power is the rate at which energy is transformed – e.g. a 60 watt incandescent light bulb converts 60 joules of electrical energy every second into light and heat (mainly the latter by the way). Similarly, it is the rate at which chemical energy stored in petrol is transformed into mechanical energy to drive the car and generate electrical energy to charge its battery. Again, although it can take many forms, it is useful if it can be measured in the same way.

Making comparisons

So if we were to measure energy and power using a single, common unit for each, which units should we choose, and what sort of comparisons would be possible?

For energy, the obvious choice has to be the SI unit, the joule. It has the merit that it is defined in terms of other SI units and does not have to be established experimentally. Energy is in fact “mass × acceleration × distance”, so a joule is defined as the quantity of energy needed to accelerate a mass of one kilogram at a rate of one metre per second squared over a distance of one metre.

All the other possible units listed above have disadvantages. The calorie is the amount of energy required to raise the temperature of 1 g of pure water from 14.5 °C to 15.5 °C at atmospheric pressure of 100 kPa. However, these conditions can only be reproduced in a controlled laboratory situation, and are not generally applicable. Similar objections apply to the British Thermal Unit, which is the amount of energy required to heat one pound of water by one degree Fahrenheit, which also makes it completely incompatible with other SI units. The electronvolt is appropriate for nuclear physics (albeit its value must also be determined experimentally) but it is far too small for normal use.

The “kilowatt hour” (kWh), as seen on electricity and gas bills, is an especially unsatisfactory unit.  As we have seen, a watt is a joule per second. So a kWh is a kilojoule divided by a second multiplied by an hour. There are 3600 seconds in an hour, so a “kilowatt-hour” is in fact 3600 kJ – that is 3.6 MJ.  So wouldn’t it be more sensible to measure the energy contained in electricity or gas in joules – or in this case, megajoules?

Since, as shown above, power is the rate of conversion of energy, the choice of unit for measuring power follows from the choice of unit of energy. “Horsepower”, with all its variants, is not a serious candidate since very few people could actually define it – except to say that an engine with a big number is more powerful than an engine with a small number. “British thermal units per hour” (BTU/h) are unsatisfactory for the same reason as the BTU is unsatisfactory. So it has to be the watt.

Not rocket science

The above relationships are not rocket science, and anybody within the normal intelligence range can easily understand them. So why aren’t they better known and used? I suspect a combination of reasons, such as:

• inertia – people are comfortable with what they are familiar with – so people carry on using different units to measure the same thing.
• the media dumb down to the lowest common denominator of understanding.
• ignorance of basic science (like not being good at maths) is tolerated and even considered fashionable by people who would not admit to ignorance of, say, Charles Dickens or the Battle of Trafalgar.
• mistaken beliefs that traditional “British” units (such as Fahrenheit!) have cultural value.
• fear of incomprehension or even ridicule from their peer group if units are used in an unfamiliar context – e.g. kW for measuring a car’s engine power, or metres in cricket or football.

How can these barriers to understanding be overcome? Basically, by increasing familiarity. Much depends on setting a good example – and this will depend on schoolteachers (especially outside the maths or science lesson), politicians (Tony Blair, to his discredit, did great harm by feigning ignorance of kilometres in 2006), broadcasters and journalists, role models in sport and show business. A lead from the Government would also help.

Reverting to the issue of “calories” on menus (which is where this article started from), it is quite deplorable that a Government agency – the Food Standards Agency – should have encouraged extended use of the unsatisfactory unit, the “calorie” (or did they mean “kilocalorie” or indeed “Calorie”?) instead of the proper SI unit, the joule. They even used the word “calorie” as a synonym for “energy”. In doing so, they are promoting ignorance and misunderstanding, and they are failing even to try to educate people to relate the energy they absorb in food and expend in their physical activity to the energy that is so wastefully used in transport, electricity generation and in domestic heating systems – to the detriment of our planet.

Consultation closes today on the FSA’s proposal that restaurants (including fast food bars) should state the energy value of the food on their menus.  The purpose of this proposal would be to enable customers to relate their energy intake to their daily energy requirement – an important factor in leading a healthy lifestyle.  (In principle, if your energy intake exceeds your energy use you will gain weight – and vice versa.)

The catering industry has been wary of this proposal (not least because many fast food outlets rely on people eating unhealthily!) and the FSA’s proposal is for a voluntary rather than a statutory scheme.  It would be difficult if not impossible to enforce against the thousands of individual fish and chip shops and Chinese or Indian takeaways, so it is mainly targeted at the chains of fast food restaurants that populate every High Street, shopping mall, and motorway service station.

In its submission to the FSA, UKMA has not commented in detail on the (obviously laudable) principle of including energy values on menus, but has recommended that any scheme that is agreed with the industry should use proper measurement units that are compatible with those used in nutritional science.  In particular it has advocated the use of the joule (J) as the primary (or preferably the only) measurement unit rather than the obsolete and unsatisfactory “calorie” – or “kilocalorie” – or “Calorie”.

Unfortunately, the FSA consultation paper set a very poor example by equating the physical concept of “energy” with the misused word “calorie” – for example, writing “calorie intake” rather than “energy intake”.   This is in direct contradiction to the recommendation of the Royal Society – as long ago as 1972 – that “calories” should be discontinued – including in the media.

We give below an extract from UKMA’s submission (text in blue):

“The use of the kilojoule (kJ) vs. the use of the calorie (cal), Calorie (Cal), and kilocalorie (kcal)

We applaud the principle of giving consumers the ability to make purchasing decisions based on the energy content in food. However the consultation document’s proposed continued use of obsolete measurement units presents several issues:

The “calorie” is often confused with, or used in equivalence to, the “kilocalorie”.

A convention is sometimes applied which attempts to avoid the inevitable misunderstanding that this causes. This involves the use of a capital letter ‘C’ when “calories” are to read as “kilocalories”, such that:

1000 calories = 1 kilocalorie = 1 Calorie

Indeed, the consultation document itself is a good illustration of this issue as it uses the word “calorie” erroneously in several instances where the word “kilocalorie” or “Calorie” is intended. e.g. Annex H, 7.3 (text in green):

“Note: “kcal” is used in these statements but “calories” should be substituted if        “calories” are declared as the energy information at point of choice.”

The consultation document acknowledges that …

“36. To aid consumer understanding and contribute to consistency of labelling only one        form of expression (either kcal or calories) should be used in an outlet. “

However, this stipulation will not prevent inconsistency of labelling across different establishments.

In their 1972 report on nutritional sciences, the Royal Society identified the problem of the continued use of calories to describe energy content of food. Its conclusions remain valid nearly 40 years later (text in dark red):

“We are very much aware of the problems that arise because as a result of 30 years of education the public has an awareness of the term ‘calorie’. We cannot see any easy solution to the problem of substituting the concept that man has a requirement for the energy-yielding constituents derived from food, and this is measured in joules, …”

“We recommend that editors of journals should not allow the use of the word ‘calorie’ and list below some obvious alternatives :

calorie intake                                  energy intake

calorie requirement                         energy requirement …”

The Units Of Measurements Regulations, which implements Directive 80/181/EEC, requires that energy should be measured using the SI derived unit, the joule. The fact that the calorie is not an SI unit, and is not listed in the Directive, means that calories can only be authorised for use as supplementary indications, and should not appear more prominently than the primary measurement, in joules (J) or kilojoules (kJ).

Many packaged foods are already labelled in kilojoules (kJ).

Progressive countries such as Australia, have already adopted the kilojoule as the primary unit of energy to indicate energy content of food.

Conclusion

A single unit, the joule, used for all purposes regarding energy (not just food), will both benefit the consumer, and increase the general public’s understanding of the concept of energy in general.

It is for these reasons that we strongly recommend that the opportunity that this consultation presents should be taken to begin the phasing out of the obsolete unit “calorie” in favour of the “joule” (which incidentally is named after the British scientist, James Prescott Joule).

References:

REPORT OF THE ROYAL SOCIETY’S BRITISH NATIONAL COMMITTEE FOR NUTRITIONAL SCIENCES

METRIC UNITS, CONVERSION FACTORS AND NOMENCLATURE IN NUTRITIONAL AND FOOD SCIENCES

Report of the Subcommittee on Metrication of the British National Committee for Nutritional Science

Proc Nutr Soc. 1972 Sep;31(2):239-47.

http://journals.cambridge.org/action/displayFulltext?type=1&fid=980848&jid=PNS&volumeId=31&issueId=02&aid=655296 “

[UKMA submission ends]

Further comment

Some have argued that the general public is familiar with “calories”, and to replace them with joules would be confusing and would reduce the effectiveness of the proposal to include energy values on menus.  This is to patronise the general public and underestimate their intelligence.  It is not difficult, for example,  to remember that the average daily energy requirement of an adult male is approximately 10 megajoules (10 MJ) and hence to relate that to a meal of, say, 4 MJ, or a bottle of wine at 2 MJ.  Moreover, to continue the dumbing down of energy information by using non-scientific units helps to maintain the gulf between the educated scientific community and people who have to rely on the popular media for their information.

The question of how best to measure energy (and also power) is a theme to which we shall return in a forthcoming article.

Technical footnote:

What the above Royal Society quotation does not explain is the reason why the joule is a better unit than the “calorie” (in all its variations).  This is because, whereas the value of the “calorie” is determined experimentally (by heating water), the joule is defined in terms of other SI units.  Thus, since energy = force x distance, a joule is a newton times a metre, or in other words the quantity of energy needed to accelerate a mass of one kilogram at a rate of one metre per second squared over a distance of one metre.  Similarly, a joule can be directly related to the watt (1 W = 1 J/s).  By contrast  the “calorie” is simply an unrelated anomaly that – unfortunately – has gained some currency in the popular media and some parts of the weight-watching industry.  It should be phased out as soon as possible, and the FSA should be helping in this – rather than prolonging its life.

For those who have not come across his books, a few words about Bill Bryson may be helpful. Mr Bryson was born in the USA in 1951, and lived both in England and the USA before settling in England in 2003. He worked as a journalist until 1987, and then became a freelance writer.

For his travel books, Mr Bryson uses the units of measurement he finds in common use in the country he is describing. Easy for him, and it should seem logical to the reader. What a shame that the BBC does not adopt this policy for its news reports from around the world.

But when Mr Bryson embarked on ‘A Short History of Nearly Everything‘, or, as John Waller of the Guardian called it, ‘a rough guide to science’, his decision on units was not so simple.

Should he:

• Use metric, and put off many US readers?
• Use US customary units (USC), antagonise many readers outside the US, reduce the credibility of the science, and defeat one of the purposes of the book?
• Use a mixture of units, and risk antagonising everyone?
• Abandon the project?

Fortunately for his readers, he persevered, trying to use USC for ‘conversational’ English and metric for the science. Thus, the Introduction of the book uses USC entirely; Chapter 1, about the universe, is 5:1 in favour of USC; Chapter 2, about the solar system, is 3:1 in favour of metric; and so on.

In my view, as a UK reader, this is not entirely successful, but I have some sympathy for Mr Bryson who had to reach a decision on which system to use each time he needed a measurement unit. There must have been many occasions when he was writing the book that he wished the English-speaking world used a single system of measurement.

What would you have done in his shoes?

In 2004, the book won the Aventis prize for general science, and in 2005 the EU Descartes prize for science communication.

Finally, here is a quote from the popular video “Globalisation and the Information Age” by Karl Fisch (http://www.youtube.com/watch?v=ljbI-363A2Q ) for would-be Brysons to consider:

“China will soon become the number one English-speaking country in the world.”

“Now navigation.  There are still lots of aircraft that are flown around the world that do not have sophisticated navigation aids and pilots need simple ways of mentally calculating navigational requirements.  One of the most common is based on the fact that 1 radian (the angle at the centre of a circle that is subtended by an arc equal to the radius) is approximately 60 degrees.  The navigational trick is known as the “one in sixty rule.”

Very simply put, if a pilot is 1 mile off track after 60 miles then the  error is 1 degree which gives a simple way of calculating the change needed to take out the error..  As an example, if the distance to be flown is 120 miles and after 60 miles the pilot identifies that he is 1 mile off track then he needs to turn 2 degrees to make good his destination (one to fly parallel to his track and one to close the destination).   Now this will work for any unit of measurement.  One banana off after 60 bananas is still an error of 1 degree.   The crunch it seems to me is that, as I understand it, the internationally agreed global positioning system is still based on latitude and longitude (all the GPS systems I have dealt with start with a very sophisticated lat/long model of the earth) and the angle subtended by one minute of arc at the earth’s surface on a latitudinal meridian is a nautical mile.  Now any maritime chart or aviation chart/map is overprinted with the lat/long grid so it is very easy to see 1 minute of arc and therefore see what 1 nm [sic] looks like irrespective of the scale of the map.   It makes using the one in sixty easier.  One could do it in kms but I think you would need to know the scale and use a ruler to measure kms so why make life difficult?.  As the Merecats would say – Simple.   Incidentally this also explains the dominance of using knots as a measurement of speed.”

This was my reply:

“Thanks for the explanation, which I think I understand. [Actually, I didn’t fully]

In your example, the deviation from course (1 nautical mile or banana) divided by the distance travelled (60) is in fact the sine of the angle subtended, and inverting this gives 0.955 degrees – i.e. roughly one degree off course. Presumably, this is good enough for travelling short distances. ( I couldn’t see the relevance of radians, but I note that one radian is 57.296 degrees, and this divided by 60 degrees gives 0.955). So far, so good. However, as you say, this relationship is independent of measurement units.

Turning to the latitude and longitude grid, surely this only works in a due north-south direction, as the parallels of latitude are shorter as you approach the poles. Measuring from my Philips world atlas, I calculate that 5 degrees along the equator (i.e. 300 nautical miles) in Brazil represents 554 km, which gives 1846 m per nautical mile (the SI definition of a nautical mile is 1852 m), whereas, measuring horizontally from British OS maps, one degree at latitude 50 degrees north (The Lizard) represents 1190 m. At 60 degrees north (Shetland) it is 940 m. So even at the scale of the UK, there is a considerable difference – and this ignores the related problem of portraying a curved surface on a flat map! (I also thought of introducing the Heisenberg Uncertainty Principle, but that would be a digression).

I suppose that if you have a pair of dividers to hand you could use the vertical scale of minutes of latitude to set the dividers to measure in nautical miles, but assuming that the map or chart has a scale in km, it would be just as easy to set it to measure in kilometres. Or use a standard scale rule calibrated in metres at the appropriate scale (such as architects and planners have used for 40 years). (The 1939 OS national grid, which I believe is used by the army, is based on kilometre squares).

(Incidentally, I gather from browsing internet sites that GPS can use decimal degrees as an alternative to degrees, minutes, seconds – so it is not necessarily dependent on minutes – hence, nautical miles are not essential to GPS systems. As I understand it).

So my conclusion is that the claimed advantage of using nautical miles is fairly weak. It must be primarily a question of resistance to change and the historical domination of the Americans in the aviation industry. Of course, changing the habits of a lifetime is always inconvenient at first, but I would have thought the long term advantages of a world-wide system, used and understood by all for all purposes, far outweighs the temporary inconvenience of a small minority having to adjust to change.

Incidentally, as NATO armies work in km, whereas air forces work in nautical miles (and feet for height?), what units do they use when they need to talk to each other about ranges, distances, heights etc – e.g. an OS map shows that a mountain is 782 m high, so what altitude do I need to fly at to clear it? (I seem to remember a Chinook helicopter flying into a hillside on the Mull of Kintyre – attributed to pilot error – could confusion over measurement units have had anything to do with it?).

Anyway, I have gone on too long. Hope this makes some sense.”

There was no reply from my correspondent – so I did a little more research.

It seems that the fundamental problem is that maps are flat, whereas the Earth is (roughly) spherical. So a co-ordinate map grid based on kilometre squares that is suitable for a relatively small land area – say, the UK – does not work when extended to a continent. (I am advised that navigators on ships crossing the Irish Sea have to make minor adjustments when they sail from the British National Grid area into the Irish grid area, as the latter has a different origin). For longer distances, navigators use latitude and longitude as a co-ordinate system in order to determine their position and their course.

However, what I still do not understand is why this should affect the units of distance used. There is no particular logic in dividing the Earth’s circumference into 360 degrees of longitude and then into    21 600 minutes (i.e. 360 x 60), and then using the distance that one minute represents at the equator (and only at the equator) as the basis for a unit of measurement.  Wouldn’t it perhaps be more useful to divide the distance from the equator to the poles by a convenient number – say, 10 000 – and then base measurements on that?  But, oh, I forgot: that’s exactly what the founders of the original metric system did.

In fact, until the Second World War, most aviation outside America and the British Empire actually did use the kilometre for distances (and, consistently, metres for height), and indeed, for domestic aviation, Russia still does.  It was only the post-war dominance of the USA in IATA and ICAO (supported of course by the British) that imposed nautical miles on an otherwise metric world.

Or have I missed or misunderstood something?  Can anybody help?  Above all, is there any hope of getting the situation changed?

Go into your local DIY superstore, and head for the lawn mowers. Now try to compare the power of different models. For small petrol mowers it is likely to be given in cc, for large petrol mowers in hp, or PS for German models; electric mowers will be rated in W or kW.

Of course, it is rarely necessary to compare the power output of different types of mowers, because the type required is usually determined by the lawn to be mown. However, for cars the choice is not so simple, and anyone considering the purchase of an electric car will surely wish to evaluate its performance against that of comparable petrol or diesel models.

Tim Bentley, a frequent contributor to MetricViews, writes:

“With a huge growth in the number of electric cars about to be launched on the British market, it is now time to adopt the kW as the standard unit of power for all cars. Whilst kW is generally used for electric cars, hp, bhp and PS are used for petrol and diesel cars. It is important that customers are able to compare the different cars on offer and the use of a standard unit (kW) is not only sensible but essential.”

A webpage from ‘Which’ highlights this problem: www.which.co.uk/advice/power-converter/index.jsp

The arrival of electric cars also provides scope for confusion in the matter of fuel consumption. A useful article about this appears on the US site Metrication.US:

www.metrication.us/content/demise-mpg

David Brown, well known to readers of MetricViews, provides a helpful comment on the US web site.

The US article understandably omits to mention the confusion arising from the difference between the US gallon and the imperial gallon, which is still used alongside L/100 km for fuel consumption in the UK.

However, if both the British Government and the shadow transport secretary don’t want road users to get their heads round km for distance and km/h for speed, as appears to be the case, then what chance is there for MJ/km for consumption of fuel (or energy) and p/MJ for price?

The United Kingdom (of Britain and Ireland at that time) came late to the party and was the eighteenth signatory of the Convention in 1884.

The importance of the Convention is illustrated by the recent history of the yard and the pound.

Until 1893, the United States had attempted to maintain its standards of length and mass to be identical with those of the UK. However the fire at the Palace of Westminster in 1834, the resulting damage to Imperial fundamental standards, the lack of stability of the replacements, and the requirements for greater accuracy in measurement made the continuing alignment of the standards of the two countries increasingly difficult. Then in 1890, the US, as a signatory of the Metre Convention, received national prototype metric standards of length and mass. In 1893, it abandoned its national standards of the yard and the pound and adopted the following definitions:

One yard equals 3600/3937 of the metre
2.204 622 34 pounds avoirdupois equals one kilogram

The differences between US and Imperial measures of length and mass were a continuing problem for scientists and industry, and agreement was reached in 1959 between Australia, Canada, New Zealand, South Africa, the UK and the USA to establish uniformity in technical and scientific fields. These new definitions were agreed:

One yard equals 0.9144 metres
One pound avoirdupois equals 0.453 592 37 kilogram.

With the Weights and Measures Act 1963, the UK finally adopted these definitions for all purposes. Corresponding Imperial fundamental standards passed into history.

Of course, the Convention is not concerned only with length and mass, but has facilitated international agreement on standards for time, electricity, temperature, illumination and, for the atomic physicists, “substance”.

One question remains. When should we celebrate the benefits of a universal, simple, coherent system of measurement: on 20 May, the anniversary of the initial signing of the Convention, or on 10 October, a reminder of the link between the metric system and the number ”10″?