Lab Notes: Why a metre is a metre long

The next time you pick up a bag of spuds from the supermarket or fill up the car with petrol, you can thank the Treaty of the Metre for the metric system that underpins daily life. The treaty was signed exactly 150 years ago, when delegates from 17 countries gathered on a Parisian spring day to establish a new and standardised way of measuring the world around us. But the metre's inception predates the treaty that bears its name by nearly 100 years. So how did it come about, and how has its definition changed over the centuries?