The Royal Institution have kindly posted many of their Christmas Lectures online as free webcasts. The lectures have been taking place since 1825, and on tv since 1966; the collection on the RI site is not complete but there are plenty to watch.
For convenience we’ve collected links to the episodes that we could find, along with alternative sources where available. Enjoy :)
(BTW, if you like this kind of thing, series 3 of Richard Hammond’s Engineering Connections is currently on BBC2/iPlayer.)