From Rulers of Countries to Rulers of Length – Chloe McCole

From the watch around your wrist, to the speedometer in your car, our lives are filled with measuring devices. Many believe we can trace our love of measuring to the ancient obsession over measuring the length of a certain body part – No, I don’t mean that one.

In Ancient Egypt the ‘cubit’ was an important measurement and was the length of the arm, from elbow to outstretched fingertips. This simple measurement was used to realise the design and construction of arguably their greatest achievement, the great pyramids.

However, like us the Ancient Egyptians weren’t all the same size or shape so how did they cut all their stones to an accurate and consistent size? This they managed by standardising the length in the form of The Royal Cubit, a piece of black granite cut to a fixed length. This was used as a guide for the production of the wooden cubits that were then used on building sites throughout Egypt.

Jump forward 4000 years and head 3000 kilometres to the west, to the people of France who weren’t so much building pyramids as storming palaces. It’s the 18th century and the French Revolution is in full swing and while a certain monarch purportedly mocked the starving peasants with taunts of “let them eat cake”, the Academy of Sciences decided the time had come for a total overhaul of the measurement system, they wanted to be able to measure the exact amount of said brioche, in standardised units.

Whilst the timing of this may seem to many to be a little strange, in this time of great confusion the development of the metric system provided much needed order. France, like many other countries, had already defined units of measure; the problem being that even though some units shared a name across many countries, their magnitudes varied, sometimes they even varied from town to town, imagine Rotherham measuring things differently to Sheffield. So the Academy wanted to define a set of base units of measurement that they could then use to derive all other measurements. First though they had to agree on how to determine the unit for distance.

There was initially two front runners in solving this problem, the first, using pendulums was dismissed due to subtle differences in the force of gravity across the world affecting the pendulum, the second involved a much trickier proposition. Pierre Méchain and Jean Baptiste Joseph Delambre were assigned the task of working out the distance from the Equator to the North Pole, using the invisible meridian through Paris. The Academy then decided that the base unit of length would be set as one ten-millionth of this calculated distance. This unit was to be known as a metre.

As you can imagine this assignment took a while, and the Academy quickly grew impatient, so whilst these calculations were going ahead they had a number of platinum rods commissioned, and just like the Royal Cubit, these bars were used as the calibration standard of all measurements.

It actually took six long years for Méchain and Delambre to finally report their findings and the platinum rod that most closely corresponded to their resulting value gained a spot in the National Archives. The Academy didn’t stop there, with a total of seven base Système International d’Unités (SI units) being established since the French Revolution. Up next was the kilogram or, as it was known at the time, le grave. Using the newly defined metre, a base unit of mass was determined, set at one decimetre cubed of water at a temperature of 4˚C.  Engineers then created a platinum cube that corresponded to this mass and sent it to sit next to the metre rod in the National Archives.

Three of the remaining five units define the everyday quantities of electrical current (ampere), temperature (kelvin) and time (second), while the other two are more specialist and refer to amount of substance (mole) and luminous intensity (candela).

Interestingly the kilogram is the only base unit still defined by the mass of a physical object, although even this is set to change with a push to define units in terms of measurable natural constants rather than the properties of manufactured objects. For example, the metre is now defined as the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second. That second no longer defined as the archaic value set by a fraction of a day, but rather the time taken for 9,192,631,770 cycles of the radiation required for a caesium atom to vibrate between two defined states of energy! – simple right?