Re: how does a magnetic strip on a credit card work

The magnetic strip on a credit card works with the same physics as 
computer hard drives and video tapes.   Let me explain the physics in a 
different way.  

If you have been to a beach or played in a sandbox, you might have written 
your name in the sand.  The process you followed was probably smoothing 
out a section of sand, and then using either your finger or a stick to 
make indentations in the smooth sand.  You eye detects these indentations 
by the variations in the light waves as they reflect off the sand.  Your 
brain translates the signals from the eye and deciphers the letters that 
make up your name.

This process is very much like what is happening on the credit card 
strip.  Instead of a beach or sandbox, the credit card uses a plastic 
tape.  And while the beach has trillions of grains of sand, the plastic 
tape has trillions of electrons sitting on the tape.  The credit card 
company uses a focused magnetic field to move the electrons into 
indentations on the tape to write the cards account number, just as you 
used a finger to focus pressure on the sand to write your name.   The 
magnet on your refrigerator exerts pressure on the electrons on your 
fridge door in the same way as the credit card writer.  Helpful hint, keep 
the refrigerator magnet away from the credit card strip, it is like 
rolling over your sandbox with a steam roller!

Credit card readers use magnetic waves to detect the indentations in the 
electrons (very similar to your eyes using light waves). There is a 
processor in the card readers that translate the magnetic wave patterns 
from the indentions into credit card account numbers.  Again, like your 
brain interprets your name in the sand from the patterns of light waves.  
By the way, these card readers speak binary, not English, the indentions 
in the cards electrons are all 0’s and 1’s.

So next time you are on the beach, take a moment to appreciate the marvels 
of the tiny brown tape on the back of a credit card.

[Moderator Note:  They're not quite "indentations", although that is a
helpful metaphor.  Please see this previous answer for more links to the 
physics of magnetic stripes. -- RJS]