Deep underground in a vault beneath Paris lives the most important lump of metal in the world - Le Grand K. Created in the 19th century, it's the world's master kilogramme, the weight on which every other weight is based. But there is a problem with Le Grand K - it is losing weight. Professor Marcus du Sautoy explores the history of this strange object and the astonishing modern day race to replace it.

From lightning bolts and Watt engines to electromagnetic waves and single electrons, Professor Marcus du Sautoy continues his journey into the world of measurement as he reveals how we came to measure and harness the power of heat, light, and electricity. It's a journey that has involved the greatest minds in science and, today, is getting down to the very building blocks of atoms.