What does the world's largest machine do?

In 1967, Homer Loutzenheuser flipped a switch and connected the power grids of the United States, forming one interconnected machine. Today, the US power grid is the world's largest machine, containing more than 7,300 electricity-generating plants. So how exactly do these power plants work? Henry Richardson digs into the delicate balancing act of how power grids supply us energy. [Directed by Anna Benner, narrated by Jack Cutmore-Scott, music by Raphael Tschernuth].