18 October 2010 Last updated at 21:10 ET

Quiz: Firkins, hides and metres

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How much, how long, how hot...

Wednesday marks the 50th anniversary of the SI system of units - better known as the modern metric system. Defining units, though, is no simple task - as this quiz will show....

1.) On the unity of units

Some of the SI units are familiar: the metre, kilogram, second. In October 1960, there were three others. What were they?

1. ohm, joule, watt
   
2. volt, celsius, lumen
   
3. ampere, kelvin, candela
   
4. firkin, pennyweight, jiffy
   

2.) Who counted those?

A seventh unit was later added, in 1971: the mole. What was it?

1. the number of atoms in 12 grams of carbon
   
2. the number of photons in one watt of light
   
3. the number of metres from the Earth to the Moon
   
4. the number, in cords, that a woodchuck could chuck if it could, in fact, chuck wood
   

3.) Turn up the volume

The SI system defined all the basic units, from which other units could be derived. A litre, for instance, is 10cm x 10cm x 10cm - and is how many pints?

1. 0.76
   
2. 1.76
   
3. 2.76
   

4.) A long time ago - but how long?

The metre supplanted an untold number of archaic and irreconcilable measures of length. Which of these is NOT a former length measure?

1. pace
   
2. hide
   
3. barleycorn
   
4. chain
   

5.) Forcing the issue

In 1998 Nasa lost a $125m Mars orbiter because its software was working in pounds-force, while the hardware was programmed in the SI unit for force, which is...

1. newton
   
2. pascal
   
3. einstein
   
4. hawking
   

6.) Measuring candle power

The candela was first based on a candle made using * fat.

1. pig
   
2. whale
   
3. walrus
   
4. hippo
   

7.) In league with an author

Jules Verne popularised a unit of length with "20,000 Leagues Under the Sea". But how many kilometres is that?

1. About 100,000
   
2. About 1,000,000
   
3. About 1,000,000,000
   

8.) Weighty question

Which of these weighs the most?

1. 84 pounds
   
2. 6 stone
   
3. 3 quarters (or tods)
   
4. they are all the same
   

9.) How low can you go?

Kelvin was the agreed SI temperature scale. Which most accurately describes the zero on this scale (or "absolute zero")?

1. temperature at which atoms stop moving
   
2. point at which liquid hydrogen freezes
   
3. the lowest attainable temperature of an ice/water/salt mix
   

Answers

1. In addition, the ampere (to measure electric current), kelvin (temperature) and candela (luminous intensity) were formalised. The others can be derived from these fundamental units, except the firkin (a quarter of a barrel, 40.9 litres), the pennyweight (1/20 of a troy ounce, or 1.56 grams) and the jiffy (two shakes of a lamb's tail).
2. A mole is the number of individual atoms in exactly 12 grams of the most common form of carbon. And how many is that? A shade over 602,214,179,000,000,000,000,000.
3. One pint is 568 millilitres, and a litre is 1.76 pints.
4. A hide was actually a hotly disputed unit of area of around 120 acres (ahem, 49 hectares). A pace was a Roman's full stride (about a metre and a half), a barleycorn was a third of an inch (8.5 millimetres) and a chain 22 yards (about 20 metres and incidentally the distance between the stumps in cricket).
5. The newton is the SI unit of force - it's the push needed to get a kilogram going one metre per second faster, every second.
6. It was whale fat. More formally, a candela is the intensity of a source that puts out yellow light at 1/683 watt per steradian of solid angle. Aren't you glad they worked that out?
7. It would have been 100,000 km. A league was originally the distance one could walk in an hour - the title refers to an underwater journey equivalent to about two and a half times around the Earth.
8. They're all equal, and all based on the pound, with 14 pounds in a stone and 28 pounds in a quarter (it's a quarter of a "hundredweight" which is in turn 1/20 of an Imperial ton). The tod was the same 28-pound measure, used specifically for wool. See why one agreed unit is a good thing?
9. Like the speed of light, absolute zero - where even the tiniest atomic motion freezes solid - can never really be reached, although scientists can get incredibly close. Zero on the Fahrenheit scale was defined as the ice/salt/water mix, and hydrogen freezes at a cool minus 259C, or 14 kelvin.

Your Score

0 - 3 : it hertz to say

4 - 6 : parsec for the course

7 - 9 : treat yourself to a dram

Pictures courtesy National Physical Laboratory, Science Photo Library, Thinkstock