Ruler

Metric System

Ruler
This website is about Imperial units of measure, but I suppose that there should be one page about the metric system. These units should really be known as SI units. The SI stands for Système Internationale because the system was invented by the French. It can be called the MKS system (for metre/kilometre/second). This is why the British spell metre and litre in the French way, either as a compliment, or to point out that these foreign units are nothing to do with us. (I'll leave you to make up your mind!) SI units are logical, easy to understand, easily convertible, easy to learn, but unfortunately very BORING. Once you know the basic unit of a system, then you can generate all the other names. Take, for example, a metre.

1 metre = 10 decimetres
1 metre = 100 centimetres
1 metre = 1000 millimetres
1 metre = 1000,000 micrometres
1 metre = 1000,000,000 nanometres
1 metre = 1000,000,000,000 picometres
1 metre = 1000,000,000,000,000 femtometres
1 metre = 1000,000,000,000,000,000 attometres
1 metre = 1000,000,000,000,000,000,000 zeptometres
1 metre = 1000,000,000,000,000,000,000,000 yoctometres
10 metres = 1 decametre
100 metres = 1 hectometre
1000 metres = 1 kilometre
1000,000 metres = 1 megametre
1000,000,000 metres = 1 gigametre
1000,000,000,000 metres = 1 terametre
1000,000,000,000,000 metres = 1 petametre
1000,000,000,000,000,000 metres = 1 exametre
1000,000,000,000,000,000,000 metres = 1 zettametre
1000,000,000,000,000,000,000,000 metres = 1 yottametre

You will probably only have heard of millimetres, centimetres, metres and kilometres, but the rest are there if you want them. Once you have grasped the idea, then you can work out other units, for example, 1000 milliltres = 1 litre, and 1000 grams = 1 kilogram. To find the Imperial equivalents of these units, see length & area, weight and capacity.

The universe is about 100 yottametres across (but it's getting larger all the time).

You can see where Kilobytes, Megabytes and Gigabytes came from!

The metric system was first proposed by the French astronomer and mathematician Gabriel Mouton (1618-94) in 1670, and was standardised in France under the Republican government in the 1790s, its use being made compulsory there in 1801. They defined a metre as one ten-millionth of the length of a line from the North Pole to the Equator, going through Paris. It was intended that a Gradian of Latitude should be 100 Kilometres. The centigrad (Kilometre) would replace the nautical mile. They got it slightly wrong - the poles are 10,002 Km from the equator. Now SI units are strictly defined using scientific constants, or in the case of the kilogram, a specific lump of metal.

Now most (all?) of mainland Europe uses metric measures. Australia changed to metric in 1966, although informally they still use Imperial quite a lot. Britain is partly metric, partly Imperial (see index page). America seems fairly aggressively non-metric (they describe their system of measures as U.S. Customary systems) apart from their scientists, of course, although I have been informed that the American Military also use the metric system. I'd be interested if anyone can tell me about when other countries became metric or stayed with their own systems.

I must tell one anecdote about Britain becoming metric. My husband and I wanted to buy some net curtains. We needed two, one 5 foot deep by 6 foot wide, and one 3 foot square. The net came in rolls of various depth (or drop), and the shop cut off the width that you wanted. We were brought up entirely in Imperial units, and were happy with "12 inches makes a foot, 3 feet makes a yard". But we were served by a young shop girl (very young - it was a Saturday, so I think it was a school-girl earning a bit of pocket money). At the time, they only taught metric units in British schools, in a desperate attempt to metricate the country, despite the fact that things were still being sold in Imperial, like these net curtains. Anyway, we asked the girl for two net curtains. The first one was 5 foot by 6 foot. She said "What's the width and what the drop?" Fair enough - the drop was 5 foot. "What's that in inches?" Sixty inches. She got the right roll and started to unroll it. "What's the width?" Six feet. "What's that in yards?" By now, we are beginning to bite our lips. But it wasn't her fault. Two yards, we said solemnly. She cut the net, wrapped it and then asked about the second curtain. "How big did we want?" (It was 3 feet square, remember.) My husband said with a straight face, trying to cut down the amount of questions and conversion which was going on, "36 inches by one yard". The girl said "What's the drop and what's the width?" We were very tempted to give them the wrong way round, and then convert them for her, but we didn't. Poor lass, I hope she didn't meet too many people like us!

By the way, an inch is defined as being 2.54 centimetres by law, so Imperial units are really part of the metric system anyway!



This appeared in New Scientist: "Drinking a litre of beer is like emptying a bottle the length of the universe with the cross-sectional area of a medium-sized nucleus." So there!



Modern definitions of SI units.

More information about SI units including their history.

United Kingdom Metric Association

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