Question

What do we shield the machinery of a wrist watch with to make it anti magnetic ?
A) Plastic sheets
B) A metal of high conductivity
C) A magnetic substance of low permeability
D) A magnetic substance of high permeability

Answer 1

Hint:A machinery can be made antimagnetic if the materials which have high permeability and high response to the magnetic field are covered all around the machinery . It means we have to provide the machinery with good metallic shielding.

Step by step solution :-
Wrist watches are made anti-magnetic by the phenomenon of shielding their machinery with a magnetic substance of high permeability.

Since the materials which have high permeability have a high response to the magnetic field and restrict the field to penetrate through them , hence they serve as a good magnetic shielding.

Hence option ( D ) is the correct answer.

Permeability refers to the ability of the material to attract and conduct the magnetic lines of flux through itself. Usually the more conductive is the material the more is the permeability of the material.
Copper and Gold are the most permeable materials as known .

Option (A) is wrong because plastic can-not provide magnetic shielding.

Option (B) is wrong because it is not necessary that a conducting material provides a better magnetic shielding.

Option ( C ) is wrong because a low permeable material also does not provide a magnetic shielding.

Note:- Electromagnetic interference enclosures ( EMI enclosures )
When we want to shield a device or machinery from electromagnetic interference ( EMI ), especially at low frequencies where the skin depth is greater , a high permeable metal will kill off the fields before they enter into your module. High permeable materials gives an ability to absorb the magnetic fields or magnetic energy.
Mumetal ( moo-metal or mew – metal ) and permalloy are the trade names of materials that are designed for high permeability. They are generally the alloys of iron and nickel. Permeability is as much a property of the material’s grain structure as it is a bulky property.
An important step of making a highly permeable material is annealing.