(a) Importance and production of radial magnetic field : In a radial magnetic field, magnetic torque remains maximum for all positions of the coils.
It is produced due to cylindrical pole pieces and soft iron core.
(b) Reason :
Voltmeter : This ensures that a very low current passes through the voltmeter and hence does not change (much) the original potential difference to be measured.
Ammeter : This ensures that the total resistance of the circuit does not change much and the current flowing remains (almost) at its original value.