Measuring Wallets' Contents with Metal Detectors

Turns out it may be more than RFID protection you may need for your wallet. New Scientist reports how some academics at University of Washington - Seattle can exploit some of the metallic features of dollar bills to count the money in your wallet. An excerpt:

They found an ordinary handheld metal detector was able to pick up a dollar bill from 3 centimetres away, and placing the notes behind plastic, cardboard and cloth did little to block the signal. Adding further bills in $5 increments increased the strength of the signal, making it is possible to count the number of bills, though converting this into an actual dollar value would be difficult as notes of different denominations contain the same amount of magnetic ink.

Using larger metal detectors such as those found in airports should also increase the range of sensing, though detecting banknotes in such situations would be trickier as many other sources could interfere with the signal.