# The Hall Effect (Warning: Physics Geek Mode Required)

Rob Winchell wrote:

>One thing I know is the Hall Effect Sensor is in the dist on the 20v. I have
>no idea what the Hall Effect is, or what the sensor does, but I know it's
> in there and must be affecting something.

Since the List seems to be having a little physics fetish lately, I decided
why not...

<GEEK MODE ON>

The Hall Effect sensor in the distributor is a non-contact position sensor
that lets the ECU control the precise timing of the spark.

Here it is from Nasa's thesaurus:
http://www.sti.nasa.gov/thesaurus/H/word6482.html

HALL EFFECT:

The electrical polarization of a horizontal conducting sheet of limited
extent, when that sheet moves laterally through a magnetic field having a
component vertical to the sheet. The Hall effect is important in determining
the behavior of the electrical currents generated by winds in the lower
atmosphere. Used for Hall coefficient and Hall currents.

Here's a more relevant description from:

http://vanosf.physto.se/~kennet/QHE.html

The classical Hall effect was discovered by the American physicist E. C.
Hall (1855-1929) in 1879, he observed that when a metal plate in the
xy-plane was placed in a perpendicular magnetic field B (pointing in the z
direction) a potential difference appeared between the opposite edges of
the plate (in the y direction) when a current passes in the x direction. This
effect can be explained in terms of electromagnetism, the Lorentz force
that acts on a moving electron is perpendicular to the velocity and
the magnetic field, the electrons will therefore be accelerated in the
direction perpendicular to the current. This results in a potential difference
between the two edges of the sample, balancing the forces yields
equilibrium. The potential difference is then equal to the Hall voltage, call
the corresponding electric field strength Ey. The above analyse gives
jx=(nec/B)*Ey and thus defining the Hall conductivity as nec/B with n the
carrier density, e the electric charge and c the speed of light. We see that
the Hall resistivity varies linearly with the applied magnetic field.

You don't need a metal plate - semiconductors work, too.  Check out this
site for an unusually thorough description of the theory of operation of
some Hall-Effect position sensors:

http://www.designinfo.com/elweco/html/TechBrief.html

<GEEK MODE OFF>

There you go! :-)

Best Wishes,
Alex