Question

In a moving coil instrument, the coil is suspended in a radial magnetic field instead of a uniform magnetic field. This is done to:
A. Increase the sensitivity of the instrument.
B. increase the accuracy of the instrument.
C. make the instrument compact and portable.
D. make its deflection proportional to current through it.

Answer 1

A moving coil instrument is an instrument in which current or voltage is determined by the couple created by curves with poles having radial magnetic fields.

Step by step solution:
The moving coil instrument can be used for measuring the value of current or voltage, as per requirement.
Working of moving coil instrument,
The working of a moving coil instrument depends on the principle that when a current carrying coil is placed in a magnetic field then a force acts on the coil as per Fleming's left hand rule.
When a current carrying coil is placed in between the permanent magnet the magnet applies a magnetic field on the current carrying coil and the coil will experience a force as per Fleming’s left hand rule. This is the basic working of moving coil instrument of the

In a moving coil instrument a coil is placed between the horseshoe magnet which has cylindrically concave north and south pole and produces magnetic field in the radial direction which is parallel to the plane of motion of the coil, when current passes from the coil placed in between the magnet the magnet applies a force on coil and the coil deflects. So, the coil is suspended in a radial magnetic field in order to increase the accuracy of the instrument, so option (B) is correct.

Note: The moving coil instrument can work on d.c (direct current) as well as a.c (alternate current), the moving coil instrument can be converted from a.c to d.c source with the help of a rectifier network. It is important to observe that moving coil instrument follows Fleming’s left hand rule in which the applied magnetic field, current and the force are mutually perpendicular to each other, so when a current passes through a coil in the influence of magnetic field the coil will experience force which is perpendicular to the magnetic field as well as the direction of current.