Mohammed Z. Tahar, an assistant professor of physics at the State University of New York College at Brockport, responds:

HALL EFFECT An applied magnetic field deflects the charge carriers in a material, causing a difference in electrical potential--the Hall effect--across the side of the material that is transverse to the magnetic field and the current's direction. Above, both positive and negative charges are deflected and in both pictures, the current is up and the magnetic field is into the page. The side of the material that becomes more positive, though, depends on the sign of the charge carrier.

In its most common application, a Hall effect transducer serves to measure a magnetic field and convert that measurement into voltage. To understand how it does so, you have to know a little about the Hall effect itself--a phenomenon named after the physicist Edwin Hall, who first observed and reported it in 1879.

As you probably already know, when an electric field exists in a metal it sets up an electric charge. The electric field exerts a force on the charge that makes a current move from one end of the conducting metal to the other. Now imagine a flat, thin strip of metal with a current running through it from left to right. If you set up a magnetic field perpendicular to this metal strip, some interesting things start to happen to the current-carrying particles.

The magnetic field either pushes the positive charges toward the top edge of the strip and the negative charges toward the bottom edge, or if the current is produced by negatively charged carriers, sends them in the opposite directions. In either case, a measurable electric field, called the Hall potential, is established between the two charged areas. And the sign of the potential difference between points on the top and bottom of the strip, known as the Hall effect, determines if the charge carriers are positive or negative.

In the semiconductor field, the Hall effect is most helpful in determining the appropriate doping and polarity of semiconductor materials. The strength of the Hall potential also is proportional to the strength of the magnetic field applied to the metal strip, which is known as a Hall Probe.

A change in the magnetic field around the Hall probe produces a corresponding change in the Hall potential. Both of these parameters, the Hall potential and the strength of the magnetic field, can be measured by a Hall effect transducer or sensor. The instrument readily detects changes and sends signals to a monitoring device.

Many common applications rely on the Hall effect. For instance, some computer keyboards employ a small magnet and a Hall probe to record when a key is pressed. Antilock brakes use Hall effect transducers to detect changes in a car wheel's angular velocity, which can then be used calculate the appropriate braking pressure on each wheel. And Hall probes can be used to measure very small and slow fluctuations in a magnetic field, down to a hundredth of a gauss (the unit of measurement for magnetic field strength, named for mathematician Carl Friedrich Gauss). In my own work, which involves magnetic materials and their characterization in magnetic fields, I use a Hall probe to determine the strength and direction of magnetic fields.