Magnetic Shielding

Electronic Principles
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magnet, all the nails will fall. The reason for this is that

each nail becomes a temporary magnet, and as soon as the

magnetizing force is removed, their domains once again assume a

random distribution.

Magnetic induction will always produce a pole polarity on the

material being magnetized opposite that of the adjacent pole of

the magnetizing force. It is sometimes possible to bring a weak

north pole of a magnet near a strong magnetic north pole and

note the attraction between the poles. The weak magnet, when

placed within the magnetic field of the strong magnet, has its

magnetic polarity reversed by the field of the stronger magnet.

Therefore, it is attracted to the opposite pole. For this

reason, it is important to keep a very weak magnet, such as a

compass needle, away from a very strong magnet.

(5) Magnetic Shielding. There is no known INSULATOR for

magnetic flux. If a nonmagnetic material is placed in a

magnetic field, there is no appreciable change in the magnetic

flux; that is, the flux penetrates the nonmagnetic material.

For example, a glass plate placed between the poles of a

horseshoe shaped magnet. will have no appreciable effect on the

field, although glass itself is a good insulator in an electric

circuit. If a magnetic material (for example, soft iron) is

placed in a magnetic field, the flux may be redirected to take

advantage of the greater permeability of the magnetic material,

as shown in figure 7 on the following page. Permeability is the

quality of a substance which determines the ease with which it

can be magnetized.

The sensitive mechanisms of electric instruments and meters can

be influenced by stray magnetic fields, which will cause errors

in their readings. Because instrument mechanisms cannot be

insulated from magnetic flux it is necessary to employ some

means of directing the flux around the instrument. This is

accomplished by placing a softiron case, called a MAGNETIC

SCREEN or SHIELD, about the instrument. Because the flux is

established more readily through the iron (even though the path

is longer) than through the air inside the case, the instrument

is effectively shielded, as shown in figure 8 on the following

page.