From: SFPhysics@aol.com
Date: Mon Feb 18 2002 - 18:17:03 PST
From: SFPhysics@aol.com Message-ID: <168.90c1153.29a30f9f@aol.com> Date: Mon, 18 Feb 2002 21:17:03 EST Subject: Magnetic Card Readers...
> Subject: Credit cards and grocery stores
> From: "Tom Woosnam" <twoos@csus.com>
> Date: Fri, 15 Feb 2002 07:58:39 -0800
> You swipe your credit card through the reader at a grocery store. It
> doesn't work. The clerk takes the card, puts it in a plastic bag and
> swipes the card and bag together through the reader. It works. If the
> stripe contains information in 'magnetic form' why would what appears to
> be a static electricity phenomenonmake make any difference? Many thanks
> for any light y'all can shed.
> Tom Woosnam
>>
Greetings Tom:
Electrostatics are not involved. The reading head of a magnetic card reader
is a coil of wire wound around a set of magnetic laminations with a gap in
the center. As each of the magnetic fields cross the gap a voltage is
induced in the coil around the laminations. The electronic signal from the
coil goes to a self-leveling operational amplifier. Newer units use a "Hall"
device which is a semiconductor that varies the current with the magnetic
field using the Hall Effect (ask a Physics teacher). Either way when cards
have the magnetic domains smeared or diminished the opamp tries to average
out the level to achieve a digital output of 1s and 0s for the computer to
decode. If the magnetic field is weak the opamp ups the amplification.
Likewise if the opamp senses that 1s and 0s are not being generated then it
makes an adjustment to change the amplitude of amplification. A smeared card
where the domains have blurred will present a signal that the automatic
circuitry will try to decode but cannot. Putting the plastic bag over the
smeared magnetic domain areas will make the 0s where the field is smeared go
back down to a lower level as if the domain was not smeared and the 1s become
more distinct. In essence you are decoupling the magnetic circuit to get a
better contrast on the reading. The 1s won't be a high in amplitude but the
0s will be back down below a threshold level so they are again recognized as
0s.
Sometimes dirt or other materials get deposited on a card surface which will
cause the reader to give indecipherable output. We have all seen the
checkers take a card and rub it on some fabric. This does not change the
magnetic domains recorded on the card strip but does clean off the material
so a clean swipe can be done.
If you could see the magnetic domains on a card stretched out they might look
like:
II IIII III I III IIII III III II
now imagine that you saw this:
II¦iII||ii|||IiIIIIi||IIIIiII|IIIiII
as if the first code smeared magnetically (don't know if the server will take
the extended character set and alter them)
It would be essentially unreadable and the card would be unusable. Decouple
the card and the 1s should stand out against the 0s once again.
On the older readers there is also the factor of how fast you put the card
through the reader. A fast card swipe will give a higher induced voltage in
the sensing coil so this is why the card might read after several different
passes were done at various velocities. (See Faraday's experiments proving
magnitude of induced currents are a function of flux density of the field
being cut by a conductor divided by delta t.)
Hope that helps explain what is going on.
Best to everyone on the list,
Al Sefl
It isn't magic - IT'S PHYSICS - the next best thing.
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