Inventor of ATM proposed 6 digit bank PIN but switched it to 4 digits

John Adrian Shepherd -Barron was a Scottish inventor, who pioneered the development of the cash machine, sometimes referred to as the Automated Teller Machine or ATM.

Shepherd-Barron joined De La Rue Instruments in the 1960s and came up with the concept of a self-service machine which would dispense paper currency with 24/7 availability. This was the Automated Teller Machine (ATM). The first machine was established outside an Enfield, north London, branch of Barclays Bank in June 1967, when he was Managing Director of De La Rue Instruments. According to the ATM Industry Association, there are now more than 1.7 million installed worldwide. He received the Order of the British Empire in the 2005 New Year’s Honours list for services to banking as “inventor of the automatic cash dispenser”. Shepherd-Barron told the BBC that he was inspired by chocolate vending machines.

There is still some controversy over the invention. James Goodfellow developed an alternative ATM design, using PIN technology (which he invented), resembling modern ATMs more than Shepherd-Barron’s machine. However, Shepherd-Barron’s machine (the idea for which he had in the bath, after having been locked out of his bank) was the first to be installed.

The Shepherd-Barron dispenser actually predated the introduction of the plastic card with its magnetic strip: the machines used special cheques which had been impregnated with a radioactive compound of carbon-14, which was detected and matched against the personal identification number (PIN) entered on a keypad. The short-range beta emission from carbon-14 could be easily detected, and he determined that the radiation hazard was acceptable as “you would have to eat 136,000 such cheques for it to have any effect on you”. A proposed PIN length of 6 digits was rejected and 4 digits chosen instead, because it was the longest string of numbers that his wife could remember.