From: SFPhysics@aol.com
Date: Wed Nov 29 2000 - 01:51:53 PST
From: SFPhysics@aol.com Message-ID: <a8.da2b8be.27562bb9@aol.com> Date: Wed, 29 Nov 2000 04:51:53 EST Subject: Debit Card Reader
> I am one of those people who have a finicky debit card. So when I go to =
> Safeway, I have about a 50/50 chance of it working. Fortunately, the =
> checkers have a fool proof method which will make my card work. They =
> wrap my card in one of the plastic bags and swipe it. Low and behold, =
> the machine reads my card and we proceed as normal.
> Why does putting my ATM in a plastic bag make the machine read it?
> sally
The magnetic strip on your card has a binary encoded series of magnetic
fields. If the magnetic reading head in the swipe slot or the amplifier of
the electronic signal is set too high then the individual electronic pulses
get pushed together and the decoder cannot read the card. By putting the
plastic bag around the card the magnetic gap is increased and the pulses
become more individually discernible by the card reader. Rubbing a card is
to clean it in case the magnetic field is too low and any dirt will create
more of a magnetic gap. Your card is the opposite with too strong a magnetic
signature and the bag just opens the gap a bit for it to read well. The
speed of the swipe also effects the results. The faster you run the card
through the slot the higher the voltage induced in the coils of the reading
head by the magnetic fields going by, except where Hall Effect devices are
used in place of a reading head. Hall Effect sensors have no velocity
parameters.
Al Sefl
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