My old billfold was worn out over the years. I planned to retire it when I noticed in back was a coil of 10 revolutions (6 cm x 4.8 cm) with a capacitor marked 47 soldered to the ends, welded in plastic, the size of a credit card. I had never noticed this. Could it be a security feature to prevent devices from getting to my credit card? How would it function? Just curious. A picture of it is shown above.
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What your wallet had was an antitheft tag. See www.highlight86.com/blog/high-quality-highlight-coil-supermarket-store-retail-security-antitheft-tag-ferrite-supplier.html for a photo of a similar device. The device is simply a tuned circuit that can be disabled at the point of sale, such as by burning out an internal fuse. Occasionally, the circuit is not burned out (or someone tries to shoplift) and in passing through the detector at the store entrance, an alarm is set off.
What you have is an LC parallel resonant circuit in which the inductor (L) is a loop antenna similar to the ones found in old AM radios, and 47 picofarad capacitor (C) tuned to resonate at the 13,56 MHz signal frequency used by credit card RFID chip readers. At this resonant frequency, the parallel LC circuit has a very high impedance and effectively absorbs most of the RF energy from a hackers reader to protect the information on your credit card chip. I bought a couple of wallets that protect RFID chips, which have a layer of aluminum foil sandwiched between the layers of leather and cloth in the wallet. It makes a Faraday cage which prevents the hackers RF reader from stealing my card info.
It looks like you have the remains of a security device that was intended to prevent shoplifting the wallet when it was new. Checkpoint Systems or something like it. Someone forgot to remove it when the wallet was sold?