Video series: How we measure things

How we measure things is a series of 7 videos explaining the basic units of measurement. Australia’s Chief Metrologist, Dr Bruce Warrington from the National Measurement Institute, explains each unit simply for kids.
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These videos are part of our outreach to improve science, technology, engineering and mathematics (STEM) education. They aim to make learning about measurement fun, so please share them far and wide, especially with any teachers you know!

Videos

How we measure time looks at the second

Hello. I’m Doctor Bruce Warrington – Australia’s Chief Metrologist.

There are 7 basic units from which every measurement is made. Today I want to talk to you about the second which measures time.

A hundred years ago we told time by the swing of a pendulum, accurate to about a second a year. Then we used quartz crystals, over a thousand times more accurate. Today here at the National Measurement Institute we use atomic clocks, which tune in to a kind of heartbeat inside a cesium atom. They will take tens of thousands of years to gain or lose a single second, and they help keep Australia on time with the rest of the world.

Global positioning systems need clocks a bit like these but made to orbit in satellites. Measuring time to billionths of a second lets us know where we are to better than a metre, and makes things like driverless vehicles possible.

Every day thousands of computers connect to our clocks to get accurate time, and use it for telecommunications, transport, financial markets and many more.

Does being responsible for these clocks make me a Time Lord?

Thanks for listening, and stay tuned to hear about the other basic units of measurement!

How we measure distance looks at the metre

Hello again. I’m Doctor Bruce Warrington, Australia’s Chief Metrologist.

Today I’m talking about the metre, one of the seven basic units of measurement. It took a long time to get an agreed measure of length.

The Egyptian cubit was the length of a man’s arm –depending on the man. The English inch was three pieces of barley end to end. And in the 1700’s every town in France measured length differently, a real problem for trade between towns and between countries.

France proposed a new metre tied to the Earth, one ten-millionth of the distance from the North Pole to the Equator. In 1875 seventeen countries adopted the metre, and made a special bar made out of platinum alloy to keep as the international reference standard.

Today pretty much the whole world uses the metre, but since 1983 it is defined as the distance light travels in 1/299 792 458 of a second. This definition uses the speed of light, a fixed constant of nature, and lets us measure precisely from the very big to the very small.

Here at the National Measurement Institute we can measure down to a nanometre, that’s a billionth of a metre, almost a hundred thousand times smaller than a human hair. This means we can measure nanomaterials, and help Australian manufacturers make new nanotechnology products.

Thanks for listening, and stay tuned to hear about the other units of measurement!

How we measure weight looks at the kilogram

Hello again. I’m Doctor Bruce Warrington, Australia’s Chief Metrologist.

Today I want to talk about the kilogram – the unit for mass (or weight), which has a long history.

One of the earliest units of measure, from over 2000 years ago, was based on a grain – a single barley seed - still used today to measure the mass of bullets, arrows and some medicines.

In 1824 the English Imperial pound was defined as 7000 grains exactly (interestingly that’s about 70 jelly babies). Around the same time in France, the new metric system proposed the kilogram, based on the mass of a cube of pure water 10cm on each side.

In 1875 17 countries signed a treaty to adopt the metre and kilogram. A special kilogram was made out of a platinum alloy to be the international reference, known as ‘Big K’, and copies were distributed to countries using the metric system. Australia joined formally in 1947, and here is our copy of ‘Big K’ kept at the National Measurement Institute in Sydney.

Just last year in 2018 the nations of the world agreed to a new definition for the kilogram, based on a fixed constant of nature – Planck’s constant. The best measurements of mass can now be made using an electromagnetic Kibble balance, and we no longer risk ‘Big K’ being lost or damaged.

Thanks for listening, and stay tuned for more on the basic units of measurement.

How we measure temperature looks at the kelvin

Hello again, I’m Doctor Bruce Warrington, Australia’s Chief Metrologist.

Today I’m talking about how we measure temperature. The unit of temperature is actually the Kelvin; zero Kelvin is ‘absolute zero’, and zero degrees Celsius is 273.15 Kelvin. In space the temperature ranges from 2.7 kelvin – the background glow after the Big Bang - to tens of thousands of kelvin at the surface of a star.

In the 1600s Galileo made a device called a thermoscope to measure relative changes in temperature. The first mercury thermometer was invented in the early 1700s.

Today the International Temperature Scale is based on defined temperatures for the melting and freezing points of a set of pure materials, covering 14 to 1400 Kelvin or -260 to 1100 degrees Celsius. This wide temperature range is needed for everything from cryo-storage of tissue samples to heat treatment of turbine blades. 

Here at the National Measurement Institute we can measure temperature to a thousandth of a degree, important for critical applications like accurately monitoring the temperature of the ocean.

Just last year in 2018 the nations of the world agreed to a new definition of the kelvin, based on a fixed constant of nature – the Boltzmann constant. This new definition opens up new ways of measuring temperature today, and will enable more accurate measurements into the future.

Thanks for listening, and stay tuned to hear about the other units of measurement!

How we measure brightness looks at the candela

Hello again, I’m Doctor Bruce Warrington, Australia’s Chief Metrologist.

I’ve been talking about the seven basic units from which every measurement is made. And today it’s the candela, which measures the intensity or brightness of light.

Up to the middle of last century, many countries had their own standards for light output, often based on the brightness of a standard candle or ‘candlepower’. In 1948 the international community adopted a new unit, based on the light glow from molten platinum, and called it the candela.

A common candle emits about 1 candela; the light on your mobile phone about 10; a 40 Watt light bulb around 100; and a lighthouse thousands or even millions of candela. These measurements are of the total light output; the apparent brightness also depends on the size of the source.

This is some of the equipment we use at the National Measurement Institute for measuring and characterising light. Measurements like this are important for a whole range of applications, from the safety of lasers and ultraviolet lamps through to making sure that traffic lights can be seen on a bright, sunny day.

I hope this has been illuminating – and stay tuned for more on the other units of measurement!

How we measure electricity looks at the ampere

Hello again, I’m Doctor Bruce Warrington, Australia’s Chief Metrologist.

Today I want to talk to you about how we measure electricity.

The basic electrical unit in our international system is the ampere, the unit of current or how much charge is flowing each second. It’s closely connected to the volt, for voltage, and the ohm, for resistance, because these three physical quantities are related through a famous equation called Ohm’s Law. All three units are named after scientists who helped develop our understanding of electricity.

Today our best electrical standards are quantum standards. It turns out that under just the right conditions, voltage and resistance are quantised – they have a kind of ruler of fixed steps, where the step size is set by fundamental constants and is always the same.

We can scale these measurements up and down to go from billionths of a volt - to millions of volts! This facility at the National Measurement Institute makes lightning to test parts of our electricity supply grid to make sure they are safe.

I think it’s fascinating that the electricity we use every day is ultimately measured using the quantum properties of nature – and I hope you don’t find that too shocking!

How we measure substance looks at the mole

Hello again, I’m Doctor Bruce Warrington, Australia’s Chief Metrologist.

I’ve been talking about the base units of measurement, beginning with the second, metre and kilogram. The others are the kelvin for temperature, the candela for luminous intensity, and the ampere for electric current.

The last is the mole, which is a measure of stuff—of how many atoms or molecules you have of a particular substance, such as water, or gold, or DNA. For example, when we get our blood tested our glucose levels are measured in moles per litre.

Following an international decision last year, from 20 May 2019 the number of molecules in one mole will be 6 point 022 140 76 times ten to the power twenty-three. This number is the Avogadro constant. Even though it’s huge – twenty four digits long – a mole of water molecules is roughly a spoonful.

The mole is arguably the unit with the biggest reach, spanning all of chemistry and biology. Here at the National Measurement Institute we make an enormous range of measurements that depend on it, from levels of vitamins and pesticides in food, to the makeup of drugs and pharmaceuticals, to testing for environmental contaminants in soil, air and water.

It’s time for me to go now – I hope you have enjoyed this series on the base units of measurement, and see you in the future!