From: sg@cunixa.cc.columbia.edu (Scott Genzer)
Subject: Re: Computer desiphering
Reply-To: sg@cunixa.cc.columbia.edu (Scott Genzer)
Organization: Columbia University
Date: Wed, 23 Dec 1992 01:02:57 GMT

Subject: Re: Computer desiphering

>>     I was saving a message on a 3.5" disk and wondered how a
>>floppy disk is made. Is it like a compact disk or more like a
>>record. Is the information retrieved with a laser or a needle? How
>>does the computer decipher all of the little holes or dots? I was
>>also surprised how much information fits onto a High Density disk.

>Mike,

>A floppy disk is just like a cassette tape that has been flattened.  A magnetic
>read/write head accesses the info, just like the play and record heads on a 
>cassette tape deck or VCR.  In fact, the early PCs (including the original
>IBM PC) allowed cassette players to be used as data storage.  

>---Bill VanHorne

I don't know whether this is too techincal for you, and if it is, I
apologize.  But for those interested, here's a bit more exact
definition copied w/o permission from "Elements of Engineering
Electromagnetics" by N.N. Rao:

  The floppy disk consists of a coating of ferrite material applied
over a thin flexible nonmagnetic substrate for physical support.
Ferrites are a class of magnetic materials characterized by almost
rectangular-shaped hysteresis loops so that the two remanent states
are well-defined.  The disk is divided into many circular tracks, and
each track is subdivided into regions called sectors.  To access a
sector, an electromagnetic read/write head moves across the spinning
disk to the appropriate track and waits for the correct sector to
rotate beneath it.  The head consists of a ferrite core around which a
coil is wound and with a gap at the bottom.  Writing of data on the
disk is done by passing current through the coil.  The current
generates a magnetic field which in the core confines essentially to
the material but in the air gap spreads out into the magnetic material
below it, thereby magnetizing the region to represent the '0' state.
To store the '1' state in a region, the curent in the coil is reversed
to magnetize the medium in the reverse direction.  Reading of data
from the disk is accomplished by the changing magnetic field from the
magnetized regions on the disk inducing a voltage in the coil in the
head, as the disk rotates under the head.  The voltage is induced in
accordance with Faraday's Law whenever there is a change in magnetic
flux linked by the coil.

I realize this is a bit more than you asked for, but I think it is
interesting.  In more normal language, there is a wire wrapped a lot
of times around a piece of iron-like material (ferrite) and this
produces a magnetic field (ever seen an electromagnet?).  This field
spreads at the "air gap" of the iron and affects the disk, which is
also made of very thin iron-like material, such that it changes the
field of the disk.  It's very similar to having an electromagnet and
having a north and a south, and then, by having the current go the
other way, the poles change sides.  It's called Faraday's Law (the
relationship between current and magnetic fields) and you can read
about it in any basic physics book.  Anyway, I hope this helps a
little bit for further clarification and interest.  BTW, this is NOT
to say Bill's definition was wrong!  I am just a simple electrical 
engineering student who finds this stuff neat and if there are others
like me, well, I hope this satisfied their curiosity a bit more.
Anyway, e-mail me if you want more info about electromagnetics or
such.

Scott Genzer

Columbia University - School of Engineering and Applied Science
Electrical Engineering major
<sg@cunixa.cc.columbia.edu>