What is a Hall Effect Sensor.

[Dennis]

Hi Everyone,
I'm kind of new here but I have been reading a lot of the posts and I would
like to ask a question or two. :) A long time ago I read someone say that
you can pick up the magnet in a speedometer if you have a hall effect
sensor. I am trying to make something of a wheel possition sensor for class
to measure the rotating speed of the wheel. I wanted to make it easy to
install on different vehicles. I would like to know if you can buy a hall
effect sensor or what is the particular design so that I might build one. 

As I understand it, they are just magnetic coils set up in a daisy fassion
around a magnet rotating on the end of a circular object. The coils generate
a positive or negative voltage due to the magnetic field generated, and it
is the voltage of zero that is measured that tells just when the magnet
passes the coil. Is this right? 

And does the magnet in a speedometer turn at 1:1 ratio with the drive wheel? 

Thanks in advance,

Dennis Espinoza(Grad Student at California State University Northridge)