Length

 Many early measurements for length were based on parts of the human body. In Ancient Egypt they used Cubits, Palms and Digits, which were all based on parts of the arm and hand. This could obviously cause problems, as not everyone’s hands or arms are the same size.

Sometimes measurements of length were based on how far you could travel in a certain number of steps. The Roman mile used to be defined as the distance the Roman army could cover in 1000 paces. That meant that on a horrible day if they were marching up hill a mile might be shorter than on a nice day marching on flat ground.

Over time rods or bars of a standard length were created to replace the use of body parts. In the UK this started to be formalised by Richard the Lionheart in 1196, who set out the official length of a yard.

 

 

Rogers Fund, 1955. Fragment of a Ceremonial Cubit Rod circa 1295–1070 B.C.

 

Weight

To weigh things early civilisations used to compare things to the weight of a set amount of grains or stones. They would use a simple balance to compare the weight of two objects. A “standard weight” could be cast from metal or other long-lasting object and kept somewhere as the officially recognized weight. This could then be used to make and test copies and settle disputes.

This method of creating weights would eventually replace the use of grains and stones. It wasn’t always easy to make sure these weighed the same.  The ancient Greeks would employ officials to check the weights being used in the marketplaces to make sure people were not trying to trick their customers.

 

Bronze balance pans and lead weights, Vapheio tholos tomb, Laconia. Late Helladic (LH) II (15th c. BCE) National Museum, Athens. Dan Diffendale

 

Time

Early humans would measure time by simply observing the passage of the sun and the stars.

The first methods used for measuring the passage of time more accurately were sundials, hourglasses and water clocks.

Early sundials could be as simple as a stick stuck in the ground. The position of the stick’s shadow could then be marked throughout the day as it moved. They became much more sophisticated as time went on.

Hourglasses and water clocks had sand or water that would flow from one vessel to another at a steady speed. You could then add extra marks to the device to get smaller measurements of time.

The first mechanical clocks were invented in 13th Century Europe. The pendulum clock was created in 1656. This remained the most accurate type of clock until the 1930s when quartz oscillators were introduced. After World War Two atomic clocks arrived and these remain the most accurate form of time measurement.

The Prague Astronomical Clock

 

 

Temperature

The first tools for measuring temperature were thermoscopes. These were glass tubes filled with liquids that would expand or contract based on how hot they were. They were very similar to today’s thermometers, but they didn’t have the extra markings to show you how the temperature was changing. They could only tell you if something was getting hotter or colder, but not by how much.

The first modern thermometer was invented in 1714 by Daniel Gabriel Fahrenheit. His scale was based on the difference between the freezing point and boiling point of water. The Celsius scale was introduced in 1742. In 1848 Lord Kelvin introduced the Kelvin scale with use the coldest possible temperature as its lowest point instead of the freezing point of water.

 

Mid-17th century Fifty-degree thermometers. Museo Galileo

 

Standardisation

As Humans began to travel and trade with other civilisations and cultures the need to agree on measurements became more and more important. However, it still took a long time for people from different countries to agree on a standard set of measurements.

Today the most widely used system of measurement is called The International System of units or SI. This system was not introduced until 1960! It is made up of 7 base units:

  • Kilogram –Mass
  • Meter – Distance
  • Mole – Amount of substance
  • Second - Time
  • Ampere – Electric current
  • Candela -Luminous intensity
  • Kelvin – Temperature

Apart from the Kilogram, these are based on universal constants. That means everyone is using exactly the same measurements, and they wont change.

 

 

Why is the Kilogram different?

At the moment a kilogram is still based on a single kilogram weight made of platinum and iridium and kept in a vault in France. This is known as the International Prototype of the Kilogram (IPK). There are lots of official copies of the IPK that countries use as their own national standards. There are also 6 copies held in France that can be used to check the accuracy of the IPK.

Over time some of these official copies seem to have lost or gained small amounts of mass. They are not large enough amounts that you would be able to feel it if you held them in your hand, but when very accurate measurements are needed for science and engineering tiny differences can create big problems.

For this reason, scientists have found a way to define the kilogram using some of the same universal constants used to define the other 6 SI units. This will have an impact on other SI units such as the mole, which use the kilogram as part of their definitions.

The definition of the Ampere and the Kelvin are also going to be changed to make them more accurate and easy to measure.

These new definitions are expected to come into force on 20th May 2019 (World Metrology Day). This will make the SI units of measurement even more accurate and standardised across the world. We’ve come a long way from everyone using the length of their own forearm!

 

A replica of the prototype of the kilogram at the Cité des Sciences et de l’Industrie, Paris, France. 15 February 2009 Credit: Japs 88

 

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