The phrases “what is that in old money?” or “what’s that in mph?” can often be heard up and down the country but to be honest mostly in retirement homes nowadays. It is true that some of those who grew up using Imperial measurements don’t like these newfangled meters and kilograms but the clincher in this argument is the following. It is certainly possible to find people who are used to Imperial measurements who will if nothing else accept that metric makes more sense but nobody has ever grown up with the metric system and thought the old system was better. That’s because it was insane. Whilst it’s true that you would just become accustomed to the everyday use of these old units. Trying to do scientific calculations in pounds and ounces is madness and they realised this way back in the 16th century, well on the sane side of the pond anyway.

In the 16th Century there was an issue that blighted the progress of science. Each region had it’s own complicated systems of units. The bread and butter of the scientific method is the verification, review and development of the other scientists works but this was difficult when everyone was using different units; or worse, the same unit defined differently. Granted this was perhaps not high on the list of problems for the average person on the street but for your wealthy, highborn man the problems of poverty and disease were more remote and so these first world problems seemed more important. Now you could only really be a scientist if you were wealthy and as such it was those who cared about this lack of unitary consistency that were also likely to be people with the influence to do something about it.

Before we had a clear universal set of units, people were using anything they like to take measurements. If I was to tell you that I’d just build and amazing sports car with a top speed of 1,000,000 “cosmos” you may be impressed by the size of the number and assume it’s fast but you wouldn’t know how fast. It turns out that the cosmo is defined by the number of lengths of my forearm (say 25cm in metric) travelled per lunar cycle (about 29 days) giving my car a top speed of 0.23 meters per second or just over… wait for it… half a mile an hour… tortoises are making slow jokes. You may think I’m being ridiculous but up until 1824 units used in some area of England included the Barleycorn (approximated to 1/3 inch) which is hardly a consistent or well reasoned unit. Nor is it ideal to try and make calculations using even the well-defined imperial units when there is 36 inches in a yard or 16 ounces in a pound. Luckily by the 16^{th} century people had started to discuss the idea of a metric system. A system where all units have base 10, 100 or 1000 and that comprises of only a few base units – with all other units of measurement being made up of these base units (speed for example can be measured in terms of the base units for distance and time – meters per second). James Clark Maxwell proposed the following base units; the meter for distance, the second for time and the kilogram for weight. All other units would be derived from these base units. Now when the CGPM launched the International System of Units in 1960 it actually consisted of seven base units. Additional base units were added as some physical measurements proved too complex to express in terms of just mass, time and distance.

The first unit is that of time – the second. The second was first defined a fraction of the day however it is now defined (far more accurately) as 9,192,631,770 periods (time an oscillation takes) of radiation emitted from a caesium atom. We can define the second so accurately in this way that it is used in the definition of some of the other base units.

The unit of length is the meter. It has been redefined a number of times but it was first defined as one ten-millionth of the distance between the equator and the north pole. Now it is defined as the distance travelled by light in a 299,792,458^{th} of a second.

The base unit of mass is the kilogram which was initially defined as the mass of 1 litre or 0.001 cubic meters of water at 4 degrees Celsius. It is now simply defined as the mass equal to the *International Prototype of the Kilogram.*

The Ampere is the base unit of electric current. It’s original definition is a bit more tricky so we will just focus on the current definition of the ampere: The ampere is the magnitude of constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10^{−7} Newtons per metre of length. Not the clearest of definitions but this demonstrates precisely why the Amp was added because electrical properties are not easily defined otherwise.

The base unit of temperature is the Kelvin. The Kelvin was originally defined the same as a degree Celsius but with the scale shifted so that 0 Kelvin was -273.15 degrees Celsius. This number was chosen because it is absolute zero. 0 Kelvin is the coldest it is possible for something to be. The modern definition is similar; one Kelvin is one 237.16th of the thermodynamic temperature of the triple point of water (the point at which water can exist as both solid, liquid or gas). In the official definition it goes on to define the isotopic composition of the water for which this is true.

Luminous intensity is quantified by the candela, named because originally it was based on the light emitted by a standard candle. The current definition is more accurate: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency of 540×10^{12} Hz and that has a radiant intensity in that direction of one 683rd of a Watt per steradian. To be honest the light of a standard wax candle is a good enough definition for me.

The final base unit is the mol which quantifies the amount of a substance. It is defined as the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.

From these 7 base units every other physical quantity is derived. Take for example the Newton, the unit of Force. It is defined as the amount of force required to accelerate a mass of 1 kilogram by 1 meter per second.

The CPGM wasn’t done there. It’s all very well defining a meter but that isn’t very helpful if you are measuring the size of an atom or measuring weight of the Earth. So they defined some prefixes to be used to donate a fraction or multiple of the unit. Each was a power of 10. Going up in size we have Kilo which is 1,000 (or one thousand) of then unit. So a kilometre is 1,000 meters. They then go up at every three powers of ten. So 1,000,000 (or one million) is mega, 1,000,000,000 (one billion) is Giga and 1,000,000,000,000 (one trillion) is Tera. Going the other way (getting smaller) we have milli for 0.001 (or one thousandth), micro for 0.000001 (or one millionth), nano for 0.000000001 (or one billionth) and pico for 0.000000000001 (one trillionth).

Now we have a system of units that allow measurements of all kinds to be accurately defined and agreed upon. If you’re not a fan of the metric system, then you’re not alone. France introduced and withdrew the metric system so many times that it resembled the Hokey Cokey but then again they are on their fifth Republic. They have settled on the metric system now though, the only countries who haven’t are Burma, Liberia and the US. Interesting.

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