OVER the last 10 000 years, most societies have evolved their own set of units for measuring mass, length and time.
These are basic units in the sense that most other units, except some electrical units, can be derived from the above. At the end of the middle ages, when world trade started to expand because of a number of voyages of discovery, the wide choice of units was a serious disadvantage.
It was therefore no accident that one of the swiftest and most lasting results of the French Revolution was the reform of the system of measuring units that gave rise to the metric system. This was eventually followed by its modern equivalent, the SI system.
HISTORY
As recently as 300 years ago, the situation inside most countries was chaotic because every district or town had its own set of units and measuring standards.
In many cases the same name had different values attached to it in different parts of the country. At present there are still differences between imperial (UK) and US units because the British units were redefined in 1824 by an act of parliament, but by then America was no longer part of the British Empire.
Modern American units generally tend to correspond to the pre-1824 British units. As early as 1586, a Flemish mathematician, Simon Stevin, published a pamphlet wherein he pointed out the many advantages of a decimal system.
Over the next 200 years many other prominent people agreed, with the result that Louis XVI of France asked a group of scientists to come up with a completely new system where every unit was either ten times bigger or ten times smaller than the next.
Amazingly enough, some of these scientists (and their ideas) survived the French Revolution and on August 1, 1793 the new French National Convention adopted a rudimentary metric system.
With the passage of time most other countries adopted a similar system, but scientists could not agree on whether the base units should be the metre, kilogram and second, the centimetre, gram and second or even the ton, metre and second.
Eventually the metre, kilogram and second system, known as MKS, won the day and in 1960 it was reorganised and expanded into what is known as the SI (Système International) system of units. At present only the USA, Myanmar and Liberia have not switched to SI.
The UK has adopted it only half-heartedly and at present use a mixture of units. In the USA, many industries have standardised on the metric system, and it is legal but not compulsory.
LENGTH
Initially, the Anglo-Saxons used the length of a barley corn as a unit. Later, the inch was defined to be three barley corns, most likely to bring it close to the which is about 660 feet.
In the SI system, the metre (m) was initially defined as the distance between two marks on a platinum-iridium bar kept in Paris.
It was supposed to be one ten-millionth of the earth’s quadrant passing through Paris, but the quadrant measurement was later discovered to be incorrect.
The bar was not changed, but in 1983 the metre was redefined as the distance travelled by light in free space in 1/299 792 458 of a second.
AREA
The acre (meaning “field”) was supposed to be as much area as could be ploughed in one day. It was equal to 4 840 square yards in England, 6 150 square yards in Scotland but 7 840 square yards in Ireland.
This unit has been replaced in the SI system by the hectare (ha), which is equal to 10 000 square metres or one square hectometre (100 metres squared).
However, for legal purposes such as title deeds, some countries also accept earlier units to avoid having to resurvey large tracts of land.
VOLUME
Fluid volumes were most likely initially based on mouthfuls. Two mouthfuls is about one fluid ounce. A pint is 16 fluid ounces (USA) or 20 fluid ounces (Imperial).
A gallon is eight pints, which is 128 fl uid ounces (USA) or 160 fluid ounces (Imperial). The official SI unit of volume is the cubic metre. The litre (L or l) is not an SI unit, but it is accepted for use because its magnitude makes it more practical.
At present one litre is equal to 0,001 cubic metre or one cubic decimetre (dm³), but it used to be defined as the volume of one kilogram of pure water at 4 degrees C and 760 mm of mercury pressure, which was equal to 1,000028 dm³
MASS
In ordinary speech the words mass and weight are often interchanged, but engineers and scientists get hot under the collar when this happens. To them, these words have different meanings.
Mass is the amount of material in a body, as measured on a chemical balance. This means that one kg of butter will still have a mass of one kg on the moon. By contrast, weight is a force, measured in newtons in the SI system.
It is the force with which the earth attracts an object, as weighed on a spring balance, because of the gravitational attraction of nearly 10 m/s2. On the moon, the gravitational attraction is one-sixth its value on earth, so that one kg of butter will have a weight of almost 10 newtons on earth but only 10/6 = 1,67 newtons on the moon.
The various early systems of mass measurements are particularly confusing. In the UK there are Avoirdupois (for general use), Troy (for precious metals) and Apothecary (for medicine) systems, but we will concentrate on the Avoirdupois system.
The word just means “it has weight”. Initially one grain was the weight of a grain seed from the middle of an ear of barley. An ounce was chosen to be 437,5 grains and one pound is equal to 16 ounces.
For heavier objects the long hundredweight (used mainly in the UK) is equal to 112 pounds and the short hundredweight (used mainly in the USA and SA before we went metric) is equal to 100 pounds. One ton is equal to 20 hundredweight so that a long ton is 2 240 pounds but a short ton is 2 000 pounds.
A kilogram is defined to be the mass of the International Prototype Kilogram stored in vault at the International Bureau of Weights and Measures in Paris. The metric ton (1 000 kg) is used for larger masses.
TIME
In the 18th century, one second (s) was defined to be 1/86 400 of the average time required for the earth to complete one rotation about its axis.
However, modern astronomical calculations have shown that the earth is slowing down, so a new definition had to be found. I give it here for interest’s sake, but don’t telephone me for an explanation.
It is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium 133 atom…
OTHER UNITS
The mass, length and time units have in recent times been joined by a further four base units.
These are the ampere (A), a unit of electrical current, the kelvin (K), a unit of thermodynamic temperature, the candela (cd), a unit of luminous intensity, and the mole (mol), a unit of amount of substance often used by chemists. All other SI units can be derived from these units.
AUTOMOTIVE UNITS
The units CAR readers are familiar with can all be derived from the base units. Thus: One newton is equal to one kg.m/s² because a newton is defined to be a mass.acceleration from one of Newton’s laws of motion.
One newton.metre is equal to one kg.m²/s² obtained by multiplying one newton by one metre. Other units such as kW and kPa can be derived in a similar manner.
ODD UNITS, JUST FOR FUN
Sheep.hours per morgen is sometimes used by sheep farmers to indicate how much grazing a particular camp has endured.
A strontium unit is a measure of contamination in food. It has nothing to do with the similar Afrikaans word, but refers to strontium 90, which is one of the natural chemical elements. A force of one pound-weight will cause a mass of 32,17 pounds-mass to accelerate at one foot per second squared.
This mass is called a slug. It was in general engineering use until the change to the SI system. A metric slug is called a mug or a par.
Miner’s Inch is an amount of water, as sold in British mining towns in the middle ages. The inch refers to a graduation on the water container.
Donkey power is a facetious unit dating from 1884. It was equal to 250 watts, so that three donkey powers are close to one horse power. (One HP = 746 W)