Contactless bank cards

Tom Thomson colinthomson1 at
Thu Nov 18 00:46:53 GMT 2010

> I've never actually found a machine to try my card out upon. But the
> Oyster pads in London require the card to be pretty much out of a wallet
> and touching the surface. Similarly the RFID cards used on the buses in
> Nottingham. The technology is dozens of order of magnitude away from
> scanning the bus pass in the passenger's pocket as he gets on board.
> --
> Roland Perry

Well Roland, I usually understand "orders of magnitude" as decimal ones,  But even if I were to assume that you meant binary orders of magnitude (which would usually suggest to me an intention to mislead, but let us assume that although you meant binary you had no such intention) "dozens" means a factor of at least 2 to the 24th, or something a bit bigger that 2 times 10 to the 7. If I guess that there's an inverse square law in there somewhere, I get something over 1.4 times 10 cubed on the distance - and I have happily waved my oyster card at more than a centimetre from the reader on buses in London (it seemed to need to be closer on tubes, I guess because the S/N ration is lower in tube stations). So I guess that dozens of orders of magnitude technology change suggests at readability at something like 14 metres, which is quite a bit more than the distance from my shirt pocked to the reader when I board a bus.  If your orders of magnitude are honest decimal ones, and not binary ones as I have cynically assumed, it comes to such an enormous distance that I don't want to contemplate it.  So I think you are probably wrong in your assertion.


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