The Humble Centimetre

25.4 mm

DarloRich said:

How many inches of rain are we talking about?

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Half an ankle.

DaleCooper said:

Half an ankle.

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Not bad then. Thanks

DerekC said:

My children went to school in the 1980s/90s and learned only metric units, only to find that the world outside still loves its feet and inches, pounds and stones, pints and gallons … and of course miles! I think the unending use of two parallel sets of units is the source of much more confusion than centimetres and millimetres ever create. And of course with increasing Americanisation post-Brexit, the problem is likely to get worse.

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Ashley Hill said:

If I'm making something I never measure in mm. It's always 1m 34 and a half cm,I only add the mm to be accurate,134cm and 6mm. It may be daft but it works for me. I can do it in imperial too. Like DerekC mentioned,I went to school in the 70s/80s learning metric whilst all around me used Imperial.

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Mostly the same here, I went to school in the 70's and 80's and was taught exclusively in metric.
As Derek says above though, the outside world also used miles and feet and inches, stones.

To be honest, I can relate to imperial and for long distances, weight and height, understand it more.
If somebody says to me that somebody is 6ft, I can picture 6ft, but if they tell me somebody is 1.82 metres tall (even though its the same height), without any reference to feet and inches, its a shrug of the shoulders from me.
Somebody is 20 stone, I can imagine that, but 127Kg, nope.
180 miles (Chester le Street to Leicester Forest East motorway services, when I was a kid), yep, but 289.69Km, not a chance

I have a lovely set of four SMP Advanced-level mathematics textbooks (plus two answer books) that I saved from a university library's withdrawals shelf. They were published in 1970, and make special reference that they've been re-written for the new metric curriculum. The books roam far and wide over pure mathematics, statisitics, physics, electrical engineering, and even computer programming - the introduction acknowledges that few schools will have access to a computer (this is 1970, a decade before the BBC's computer literacy project, after all!), but that learning the theory of their operation will be beneficial.

I dove into the introduction thinking I'd read something there about the merits of cgs vs kms, but it turns out I must have seen it someplace else.

As my Higher Physics teacher said “Look at the order of magnitude and you’ll see how out of place a centimetre is”

DerekC said:

And of course with increasing Americanisation post-Brexit, the problem is likely to get worse.

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Some interesting extracts from Wikipedia which some might find surprising.
https://en.wikipedia.org/wiki/United_States_customary_units

The U.S. government passed the Metric Conversion Act of 1975, which made the metric system "the preferred system of weights and measures for U.S. trade and commerce". The legislation states that the federal government has a responsibility to assist industry as it voluntarily converts to the metric system, i.e., metrification. This is most evident in U.S. labeling requirements on food products, where SI units are almost always presented alongside customary units.

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However:

According to the CIA Factbook, the United States is one of three nations (along with Liberia and Myanmar) that have not adopted the metric system as their official system of weights and measures.

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Although:

Metric units are standard in science, medicine, as well as many sectors of industry and government, including the military

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SteveP29 said:

To be honest, I can relate to imperial and for long distances, weight and height, understand it more.
If somebody says to me that somebody is 6ft, I can picture 6ft, but if they tell me somebody is 1.82 metres tall (even though its the same height), without any reference to feet and inches, its a shrug of the shoulders from me.
Somebody is 20 stone, I can imagine that, but 127Kg, nope.

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That's because the whole point of imperial was that it came from an era without everyone having accurate methods of measurement, hence we had measures loosely linked to body measurements where a yard was very roughly a stride, and measurements had a symmetry (i.e. often based on the 12 times table, divisible by 3 and 4, etc). Like cooking, you could halve and halve again to get a quarter of a pound if needed for cooking and you have a bag of a pound of flour. 360 degrees, can be halved to 180, halved again to 90, etc. (In fact that's an anomaly in that we don't have a metric equivalent of 360 degrees, nor a metric equivalent of 12/24 hour clock or 7 days in the week or 28 day "month", etc).

Try extracting 150g of flour from a 1kg bag without a kitchen scale - virtually impossible to do it accurately as neither weights relate to anything tangible - you can't halve and halve again, etc.

It's only readily available scales and other measuring devices that facilitated the metric measurements which are, of course, based on the ten times table, which makes manual calculations very easy and avoid logarithms etc that were needed for complex calculations a century ago. Ironically, now we all carry a high powered computer in our pocket, there's no longer a need for metric as we could all do calculations based on imperial measures at the click of an app!

underbank said:

(In fact that's an anomaly in that we don't have a metric equivalent of 360 degrees, nor a metric equivalent of 12/24 hour clock or 7 days in the week or 28 day "month", etc).

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See gradians, and decimal time.
The latter has long since gone the way of the dodo, but gradians are still commonly supported on scientific calculators (along with degrees and radians).

underbank said:

That's because the whole point of imperial was that it came from an era without everyone having accurate methods of measurement, hence we had measures loosely linked to body measurements where a yard was very roughly a stride, and measurements had a symmetry (i.e. often based on the 12 times table, divisible by 3 and 4, etc). Like cooking, you could halve and halve again to get a quarter of a pound if needed for cooking and you have a bag of a pound of flour. 360 degrees, can be halved to 180, halved again to 90, etc. (In fact that's an anomaly in that we don't have a metric equivalent of 360 degrees, nor a metric equivalent of 12/24 hour clock or 7 days in the week or 28 day "month", etc).

Try extracting 150g of flour from a 1kg bag without a kitchen scale - virtually impossible to do it accurately as neither weights relate to anything tangible - you can't halve and halve again, etc.

It's only readily available scales and other measuring devices that facilitated the metric measurements which are, of course, based on the ten times table, which makes manual calculations very easy and avoid logarithms etc that were needed for complex calculations a century ago. Ironically, now we all carry a high powered computer in our pocket, there's no longer a need for metric as we could all do calculations based on imperial measures at the click of an app!

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Extracting 2oz of flour from a 1lb bag isn't any easier, but I largely agree with the rest of the post.

However I do wonder why we ended up with 360 degrees in a circle - 240 would mean that 10 degrees of longitude was an hour of time difference. And if there are 16 ounces in a pound why are there only 14 pounds in a stone? 12 in each case would be much more sensible. And why is a gallon a different size in the US? So although some bits of imperial measurements are more relateable, the whole system is essentially a mish-mash arising from various largely forgotten historic accidents.

edwin_m said:

Extracting 2oz of flour from a 1lb bag isn't any easier, but I largely agree with the rest of the post.

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I was raised with Canadian (baking) recipes, where most ingredients are measured by volume. Scooping 1/2 cup of flour out of a bag is dead simple to do (if you own a set of measuring cups & spoons). The unit that catches me out is the "stick" of butter: it isn't packed in sticks over here, so I have to remember to translate it to 1/2 cup.

edwin_m said:

However I do wonder why we ended up with 360 degrees in a circle - 240 would mean that 10 degrees of longitude was an hour of time difference.

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The degree is such an ancient unit of measurement its origins are probably lost in time. But 360 degrees in a full rotation does tie in nicely with 360 (or so) days in a year, so it may be from that.

edwin_m said:

And why is a gallon a different size in the US?

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That's a curious one, when US and Imperial fluid ounces are nearly identical (~29ml). The fault is probably with the pint. Both systems have 8 pints to a gallon, but there are 20 fl.oz in a US pint, while the UK pint only has 16.

(The US seems to stick to fl.oz for measureing beverage sizes, and largely avoids the pint.)

najaB said:

And that's the beauty of metric - switch between units by just moving the decimal point.

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They aren't different units!

The p, n, μ, m, , k, M, G, T things are prefixes that are shorthand ways of writing *10^x.

Where you do get different units are when you have stuff where the scale of metric is wrong and you are allowed to put other units in - the litre (a cubic decimetre*) or the hectare** (100 'ares', whatever they are). Although these are also just 'move the decimal point' - a litre is a thousandth of a cubic metre (or a million cubic millimetres) and a hectare a hundredth of a square kilometre or 10,000m^2. There's the ångström (a ten-billionth of a metre or a tenth of a nanometre). The bar (100kPa) is another, normally seen as millibars (100Pa). The tonne is an odd case given that it is a kilo-kilogram, ie a Megagram.

Electronvolts (1.602176634*10^-19 Joules) and daltons (1.66053906660*10^-27 kg - the weight of a proton or neutron) are accepted for use with SI but don't have the base 10 multiple conversion. Ditto minutes (of time), hours, days, astronomical units, degrees, minutes (of angle), seconds (of angle).

*one of the few times in the English-speaking world where you see deci- used as a prefix at all (decibel being the other one). And surprisingly in science - purely because a litre isn't SI (though is officially accepted for use with SI despite not being an SI unit) but it used to get used all the time as the numbers are better than other units (eg 24 litres of a gas at STP is a mole of molecules) before standardisation and they didn't want to break that. They seem to be OK with using deci- and centi- in scientific purposes when they are cubic -metres. It's the only time you get centimetres in serious scientific stuff: to say stuff like "10ccs stat". 1cc = 1ml.

**basically the only time hecto- ever gets used as a prefix unless someone is deliberately being weird and it loses its 'o' and is attached to an archaic French psuedo-imperial measure that doesn't get used. Not (outside of India) having a good word for *10^4 means we use it.

edwin_m said:

And why is a gallon a different size in the US?.

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https://en.wikipedia.org/wiki/United_States_customary_units

The United States customary system (USCS or USC) developed from English units which were in use in the British Empire before the U.S. became an independent country. However, the United Kingdom's system of measures was overhauled in 1824 to create the imperial system, changing the definitions of some units. Therefore, while many U.S. units are essentially similar to their imperial counterparts, there are significant differences between the systems.

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edwin_m said:

However I do wonder why we ended up with 360 degrees in a circle - 240 would mean that 10 degrees of longitude was an hour of time difference.

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If there were only 240 degrees it would leave a big gap.

DaleCooper said:

Because of abominations like "10 and a half cm" would be one good reason.

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10 and a half cm I understand. It's 105mm that confuses me .....

I cut a lot of upholstry foam for private customers and and it varies between inches, cm and mm. Not unusual to be asked for 1 meter by 60cm by 2" thick .....

As already noted, standard meteorological value for measuring rainfall is the mm. So that's why that gets used. When there's snow on the ground though you may still catch meteorologists referring to inches .....

SteveP29 said:

...180 miles (Chester le Street to Leicester Forest East motorway services, when I was a kid), yep, but 289.69Km, not a chance

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One of the issues is unnecessary precision. If you round the mileage to 180, and ignore fractions, or furlongs or yards, then you shouldn’t be converting using 2 decimal places.

So “about 180 miles” becomes “about 290 km”...

swt_passenger said:

One of the issues is unnecessary precision. If you round the mileage to 180, and ignore fractions, or furlongs or yards, then you shouldn’t be converting using 2 decimal places.

So “about 180 miles” becomes “about 290 km”...

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That's an important principle that a lot of people don't appreciate, it always annoys me to see instances of unnecessary precision. In the example given of the distance from Chester le Street to Leicester Forest East motorway services it is treated as though it was 180 miles exactly. It's probably somewhere between 175 and 185 miles, depending on how it is measured.

PeterC said:

Kids today, getting all their arithmetic in base10 don't know they are born.

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But today's kids are terrible at handling fractions because they think in 10s - they get taught them several years after working in decimals and so struggle to conceptualise them.

I got taught fractions just before decimals, and can use both well because of that. My brother learnt decimals at the same age, but didn't learn fractions until the next year and so much preferred decimals and struggled a little with fractions despite being good at maths. I think it now might be a three year difference between learning decimals and then learning fractions and so the latter are really alien. Non-Brits I knew at uni had similar issues - a lot of the time fractions need to be converted into decimals/percentages (same thing really) to make sense to them.

We did unit conversion at school about aged 8. Fahrenheit was still a thing in summers back then so we did that and 2.5 cm-in-an-inch (I got told off for using 2.54 rather than 2.5 as it was apparently too hard for the rest of the group I was working with - all of which were able to do it as I taught them!) and nothing else. We did it a couple more times - but we lost Fahrenheit and went to 2.54 for inches. A couple of years above or below and we wouldn't have done that. But it was enough - we learnt the concept and could apply it and so deal with the stuff we used at home but hadn't been taught about. I certainly had to convert stuff for friends that weren't the same age as they had no clue how to do it thanks to metric idealism rather than unit pragmatism - "if we don't teach the bad units, the kids won't use them and victory for us as they'd disappear" vs "if we don't teach the bad units, the kids can't use them and so defeat for us as they haven't disappeared".

Aged 10 or 11, as a bit of extra-curricular, having learnt the curriculum and been tested on it but still needing to be taught some maths, our teacher taught us pre-decimalisation money and adding that up. Also pounds and ounces, feet and inches, etc. It was easy once you got the knack of it, though we weren't doing it in our heads. In many ways the teacher did it as metric propaganda - and it worked because we made mistakes as to which base to use and it was annoying - but at the same time we learnt that it wasn't that difficult once you got the hang of it and old people weren't totally nuts to still be using the stuff they grew up with.

AFAICS, with Old Money, until the interwar period, you were either poor and worked in shillings and pence - stuff cost 39s rather than £1 19s, you were rich and worked in pounds and shillings (treating sixpence functionally like a half-shilling and thruppence as a quarter-shilling), or you were an accountant and trained and paid to work in all three. Decimalisation happened about 20 years before inflation rendered it near useless as everyone would be on the rich scale, and about 50 years after it would have first been really useful. Now we're at the point where copper coins are near useless and we wouldn't even have to bother with half-shillings.

si404 said:

They aren't different units!

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Now that's splitting hairs! Yes, the meter is the only actual unit of measure, but for all practical purposes they are different units. After all, cartographers rarely work in μm and carpenters rarely work in km.

Oh dear!

The meter is a device for measuring things.
The metre is the internationally accepted unit of length.
Only the arrogant Americans think they can misspell the name of one of the fundamental units of science.
What about the jewel as a unit of energy or indeed the coolom as a unit of charge anyone?

edit: and yes, the metre is always written in lower case. I used capitals to emphasise the word as I can’t find bold on my phone. Now I can

LWB said:

edit: and yes, the metre is always written in lower case. I used capitals to emphasise the word as I can’t find bold on my phone. Now I can

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In fact all SI units are written in lower case when written in full, even those that are named after people or where the abbreviation is upper case, and very many people get that wrong. My physics teacher maintained it was so the unit could be distinguished from the person.

LWB said:

The meter is a device for measuring things.
The metre is the internationally accepted unit of length.

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And both exist in auto-correct.

krus_aragon said:

The fault is probably with the pint. Both systems have 8 pints to a gallon, but there are 20 fl.oz in a US pint, while the UK pint only has 16.

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It's the other way round, a UK pint is the larger one, therefore the US gallon is smaller. This is especially important when refueling an aeroplane and you need to be really sure how much volume you are really taking on.

So it seems it is an easy mistake to make mixing up gallons/pints and litres and and metric in general is superior.

Doppelganger said:

It's the other way round, a UK pint is the larger one, therefore the US gallon is smaller. This is especially important when refueling an aeroplane and you need to be really sure how much volume you are really taking on.

So it seems it is an easy mistake to make mixing up gallons/pints and litres and and metric in general is superior.

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I recall watching an episode of Air Crash Investigation where this was the main cause of the crash.

Born in 84 and growing up amongst older people still stuck in the English ways i use alsorts of units of measurement for things, often mixing them. If someone wants to built some set in feet and inches i'm happy to do that unless they start wanting to go smaller than 1/8 inch with accuracy then we have to use common sense. The entertainment industry is rather terrible for mixing and matching when describing the same thing. Standard stage decks are 8 feet by 4 feet, and we will often have them on 1m legs! It makes little sense but it works for us.
Generally as an engineer i'm quite strict on stuff being in mm so this concept of rainfall makes a lot of sense.

GusB said:

I recall watching an episode of Air Crash Investigation where this was the main cause of the crash.

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I seem to recall it was a metric / imperial conversion rather than a UK vs US gallon issue?

robbeech said:

I seem to recall it was a metric / imperial conversion rather than a UK vs US gallon issue?

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If you're thinking of the Gimili Glider, then yes it was. The FMS gave them a figure in kgs, and the ground staff loaded that number of pounds.

I think there was also a probe that was sent all the way to Mars then failed to do whatever it was planned to do because the people who set it up were working in the wrong units.

edwin_m said:

I think there was also a probe that was sent all the way to Mars then failed to do whatever it was planned to do because the people who set it up were working in the wrong units.

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Yup. That would be Mars Climate Orbiter. Lockheed Martin engineers were working in imperial and NASA JPL were working in metric. This resulted in the orbit injection burn being too long, and the probe went too deep into the atmosphere and either burned up or skipped back out into a heliocentric orbit.

While that was the proximate cause, the failure highlighted creeping complacency and bureaucracy within NASA as the navigation engineers who spotted the problem beforehand were ignored because they "didn't fill out the right form".

Doppelganger said:

It's the other way round, a UK pint is the larger one, therefore the US gallon is smaller. This is especially important when refueling an aeroplane and you need to be really sure how much volume you are really taking on.

So it seems it is an easy mistake to make mixing up gallons/pints and litres and and metric in general is superior.

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The one that often throws me with that particular conversion is seeing US car reviews reporting MPGs about 20% lower than equivalent UK ones, and consequently thinking that US cars are worse on fuel than an equivalent UK model!

edwin_m said:

In fact all SI units are written in lower case when written in full, even those that are named after people or where the abbreviation is upper case, and very many people get that wrong. My physics teacher maintained it was so the unit could be distinguished from the person.

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I think the "degree Celsius" is the exception to that rule. (It is an SI unit although not the SI base unit for temperature).