The units of the SI

The International System of Units (the SI) is made up of seven universally recognised units of measurement. Some of the units – the kilogram and the metre – we use every day. Other measurements are useful for industrial and scientific needs. On 20 May 2019, four of the base units were redefined based on fixed values for natural constants.

Discussion point: Why do you think the measurements are referred to as base units?

Transcript

PETER SAUNDERS

Originally, the SI just included these six physical base units – so we have the candela, the kilogram, the metre, the second, the ampere and the kelvin, and a few years later in 1971, they added the mole.

And so most people will be familiar with at least the first three of these units. The kilogram is the unit for mass. The metre is the unit for distance or length. The second is the unit for time. The ampere is the unit of electric current, so that would be familiar with people that work in electronics or electricity. The kelvin is the unit for temperature. People will be more familiar with the °C unit, but there is a simple relationship between kelvin and °C. So a temperature in kelvin is just a °C temperature plus 273.15 so it’s just a slight shift, so water which freezes at 0°C on the Celsius scale freezes at 273.15 on the Kelvin scale. The mole is the unit for amount of substance, so this is used by chemists when they are analysing chemical reactions etc. And the candela is the unit for luminous intensity. That tells how bright light sources appear to the human eye, so it’s is a very human-centric unit that one. There’s nothing particularly special about which units were chosen here and even the number of them – seven of them – which are called base units.

All of the other units can be derived from these base units. So for example, the unit of force, which is the newton, is a combination of these. So a newton is a kilogram times a metre divided by a second squared.

So all of the other physical units can be related to these. So there’s nothing special about them, they just happen to be useful to serve our everyday industrial needs and scientific needs etc. And there is also nothing particularly special about the size of the units. They are designed to be suitable for human purpose. So for example, 1 metre is about the length of a human arm, a kilogram is about as much as you can eat in one sitting and a second is probably about as long as you can stay awake in a talk like this.

Acknowledgements

This video clip is from a recording of a presentation by the Measurement Standards Laboratory of New Zealand (MSL) in celebration of the redefinition of the International System of Units (SI), which happened on 20 May 2019. The presentation by Peter Saunders and Farzana Masouleh of MSL was filmed at Unleash Space, Faculty of Engineering, Auckland University.