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• Measurement is the process of obtaining the magnitude of a quantity relative to an agreed standard. The science of weights and measures is called metrology. The Measurement Standards Laboratory of New Zealand (MSL) is our national metrology institute. MSL ensures that New Zealand’s units of measurement are consistent with the International System of Units (Système International d’Unités) or SI.

Rights: Public domain

Measurement

Leonardo Da Vinci’s Vitruvian Man is based on the relationship between ‘ideal’ human proportions and geometry as described by the ancient Roman architect Vitruvius.

### SI units

Measurement of any quantity involves comparison with some precisely defined unit value of the quantity. Standard units of measure need to be identified and defined as accurately as possible. All of the SI units used in scientific measurements can be derived from just seven fundamental standards called base units. Each of these units has a definition based on a physical constant – an unchanging property of nature – such as the speed of light. SI derived units come from multiplying, dividing or powering the base units in various combinations. A significant number of SI derived units have been named in honour of individuals who did ground-breaking work in science.

The International Bureau of Weights and Measures (BIPM) has guidelines on how to properly use and apply the SI units. The article Expressing quantities describes how unit symbols and names should be written and used and how the values of quantities should be expressed. Powers of 10 explains the prefix names and symbols for decimal multiples and submultiples of SI units.

Rights: Greg L, CC BY-SA 3.0

International prototype kilogram

The international prototype kilogram (IPK) made of a platinum/iridium alloy. Its height and diameter are the same at 39.17 mm. It is kept at the International Bureau of Weights and Measures in Sèvres on the outskirts of Paris.

The IPK was retired on 20 May 2019, when the kilogram was formally redefined in terms of Planck's constant.

### Revising the SI

The SI was revised in May 2019. Four of the base units – kilogram, ampere, kelvin and mole – were redefined. They now join the second, metre and candela as being defined by physical constants. This event likely happened without much notice from the general public – there was no disruption to our daily lives. For metrologists, however, it was an unprecedented event. Never before have so many base units been revised at one time.

Advances in technology (like GPS) require increasingly precise and accurate measurements. Up until May 2019, mass was defined by a material artefact – a metal cylinder made in the 1880s – known as the international prototype kilogram (IPK). This caused scientific uncertainty. Redefining the kilogram in terms of Planck’s constant meant that the IPK could be retired and scientific certitude established.

Changing the mass standard from an artefact to a fundamental physical constant may seem relatively simple in principle, but it was technically challenging. Metrologists had to demonstrate that the alternatives are accurate and reliable. MSL scientists Dr Laurie Christian and Dr Chris Sutton developed a novel research approach called the ‘Kiwi Kibble balance’.

The SI – past and present

The International System of Units – the SI – was created to meet the needs of growing globalisation.

## The history of measurement

The SI system standardised international measurement systems. Read about earlier measurement systems – from the Babylonians to more recent European systems. An interactive timeline will take you through many of the significant milestones. Māori had their own measurement standards for building, carving and weaving.

## Take up the challenge

The student activities covering measurement range from quirky to precise. Measurements, weird and wonderful is a collection of unusual measurement units – such as a moment (1/40 of an hour). Follow this up with Cubits, spans and digits to reinforce the degree of uncertainty when using non-SI measurements.

Measuring foot pressure provides practice using SI units, derived units and prefixes. Precision and accuracy provides various datasets for students to judge precision and accuracy in scientific settings. How long is it? is a collection of length measurements found within the Science Learning Hub. Lengths range from the very small to the very big, helping students develop an understanding of the decimal system as applied to length measurement.

## Nature of Technology

Metrologists are able to measure a kilogram mass to one part in 10 billion – a resolution of just 0.1 millionths of a gram (micrograms). This is useful for scientists who work in pharmaceuticals or experimental physics. Using the new mass standard removes the uncertainty presented when scaling down from the physical IPK artefact.

## Useful links

The International Bureau of Weights and Measures (BIPM) website has information on the revision of SI units.

Check out this PDF Te reo Māori guide to the International System of Units (the SI) from the Measurement Standards Laboratory.

MSL has produced an interactive map that shows how we use measurement in daily life.

## Acknowledgement

This resource has been updated with the assistance of the Measurement Standards Laboratory of New Zealand.

Rights: Measurement Standards Laboratory

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The Measurement Standards Laboratory of New Zealand (MSL) is New Zealand’s national metrology institute. It ensures that New Zealand’s units of measurement are consistent with the SI, the international system of units.

Published 17 August 2011, Updated 19 August 2019 Referencing Hub articles
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