The Hall Effect

In 1879, E.H. Hall observed that a small voltage is generated across a conductor carrying current in an external magnetic field. The Hall voltage was very small with typical conductors, and little use was made of this effect. However, with the development of semiconductors, lager values of Hall voltage can be generated. The semiconductor material indium arsenide (In As) is generally used. As illustrated in Fig.13-14, the InAs element inserted in the magnetic field can generate 60 mV with B equal to 10 KG and an I of 100 mA. The applied flux must be perpendicular to the direction of current. With current in the direction of the length of conductor, the generated voltage is developed across the which.

The amount of Hall voltage v/H is directly proportional to the value of flux density B. This means that gauss meter in Fig.13-15 uses an InAs probe in the magnetic field to generate a proportional Hall voltage v/H. This value of v/H is then read by the meter, which is calibrated in gauss. The original calibration is made in terms of a reference magnet with a specified flux density.

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Magnetic Shielding

The idea of preventing one component from affecting another through their common electric or magnetic field is called shielding. Examples are the braided copper wire shield around the inner conductor of a coaxial cable, metal shield can that encloses an RF coil, or a shield of magnetic material enclosing a cathode-ray tube.
The problem in shielding is to prevent one component from inducing an effect in the shielded component. The shield material are always metals, but there is a difference between using good conductors with low resistance like copper and aluminum and using good magnetic materials like soft iron.













A good conductor is best for two shielding function. One is to prevent induction of static electric charges. The other is to shield against the induction of a varying magnetic field. For static charges, the shield provides opposite induced charges, which prevent induction inside the shield. For a varying magnetic field, the shield has induced to produce induction inside the shield.
The best shield for a steady magnetic field is a good magnetic material of high permeability. A steady field is produced by a permanent magnetic, a coil with steady direct current, or the earth’s magnetic field. A magnetic shield of high permeability concentrates to magnetic flux. Then there is little flux to induce poles in a component inside shield. The shield can be considered as a short circuit for the lines of magnetic flux. Read More!

Magnetic Flux

The entire group of magnetic field lines, which can be considered to flow outward from the north pole of magnet, is called magnetic flux. Its symbol is the Greek letter (phi). A strong magnetic field has more lines of force and more flux than a week magnetic field.
THE MAXWELL
One Maxwell (Mx) unit equals one magnetic field line. In Fig.13-5, as an example, the flux illustrated is 6 Mx because there are 6 field lines flowing in or out for each pole. A 1-1b magnet can provide a magnetic flux of about 5000 Mx. This unit is named for James Clerk Maxwell (1831-1879), an important Scottish mathematical physicist who contributed much to electrical and field theory. Read More!