I totally get with the prepaid oyster that when it’s topped up I’m physically touching it to the contact so the data is transferred, but with a debit card (especially if I’ve say paid for something online) how does it know what’s on the card ?)
In: Technology
The same way card machines in shops know, they talk to the bank. It’s really the same stuff, just with ‘working out the price’ coming after tapping instead of before.
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