How do we measure brightness, electricity and substance?

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!

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!

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!