What is autoradiography and how did John Cairns use it?

Autoradiography shows where radioactive atoms decay, and it can be used to show the position and length of DNA. John Cairns cultivated E. coli in radioactive thymine. He placed them on a membrane digested their cell walls so their DNA would spill into the membrane. He coated the membrane with photographic film and left it for two months. He was then able to see where its DNA was and he noticed it was circular