Explanation:

There is no known insulator for magnetic flux.
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.
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 against magnetic flux, it is necessary to employ some means of directing the flux around the instrument.
This is accomplished by placing a soft-iron 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.