Alan Davies and Marcus Du Sautoy get to grips with the problem of measuring the exact length of a piece of string. They visit the National Physics Laboratory, look at how a metre is defined now, and explore the history of measurement and standardisation along the way.

First broadcast:
17 November 2009

Before showing the clip, ask students to explain the meaning of a metre? Who decides what a metre is? What other measurements do we use? After the clip, students could draw a timeline outlining the different technologies used to measure length.