Question

Why are shields made of iron usually provided around precision instruments?
(A) For protection against the effect of moisture in the air.
(B) For guiding the instrument against unauthorised handling.
(C) for protection against the effect of external magnetic fields.
(D) for absorbing heat generated during the functioning of the instruments.

Answer 1

Precision instruments have only one use to make the measurements more accurate. This accuracy can sometimes be hindered by the effect of magnetic or electric field. So, here the main purpose is to make the measurement of the instrument more precise by eliminating any of the external affecting forces.

Complete Step By Step Answer:
Metallic spears or any closed Metallic body that surrounds any object prevents or stops the electric and magnetic field from entering in the inside of the spear.
So, in order to stop the forces of outside areas from hindering the precise measurement of the instrument, we shield it from a Metallic shield.
Metal like iron being very ferromagnetic is found to be very useful for this purpose, it prevents the magnetic field lines from entering inside thus any kind of metal needle or anything doesn’t get deflected by the action of these field lines.

The above image shows a hollow spear placed near a positive charge. The field in the inside becomes zero as the field from the outside charge and the induced charge balances in such a way that the field strength in the inside becomes zero.

Note:
The metal shield of iron also protects the instrument from rusting but nowadays precision instruments are usually stainless. The option that says metal absorbs heat from the instrument is also wrong as when the instrument is heated it expands and the precision changes vigorously, making this also incorrect as the metals of the instruments have low expansion and heat absorption nature.