Answer: Radioactive Paper

Today they’re ubiquitous: automatic teller machines (ATMs) that dispense cash to you with the swipe of a bank card and the input of your personal identification number (PIN). Stroll back through history a scant fifty years or so, however, and both inventors and banks alike were wrangling with how to implement the now commonplace technology.

While ATMs and the bank cards we use in them are everywhere now, when the original ATM was invented by British inventor John Shepherd-Barron back in the 1960s, there was one critical missing component: the cards. When you create an automatic teller in an age without bank cards, how do you verify the users are who they say they are? Radioactive paper, naturally.

The work around that Shepherd-Barron came up with was to issue paper checks imbued with very mildly radioactive carbon-14. The early ATMs could detect the radioactive signature of the carbon-14 and that, in combination with a user supplied PIN, were enough to verify the user and allow for the withdrawal of up to 10 pounds worth of cash.

Although 10 pounds doesn’t seem like much today, in 1967 (when the first ATM was activated) it had the purchasing power of approximately 163 pounds (or approximately 248 U.S. dollars), which would have been more than enough to tide you over until the bank was open for business.

The idea of ATMs caught on and now, roughly half a century later, there are over 2.2 million ATMs around the world.