Question

The use of Helmholtz coil is to produce
(A) uniform magnetic field
(B) non-uniform magnetic field
(C) varying magnetic field
(D) zero magnetic field

Answer 1

Hint
Helmholtz coil is a set up where there are two identical electromagnets placed coaxially. The direction of the current is also the same in both of them. We can see that this setup ensures production of a uniform magnetic field near the axis.

Complete step by step answer
We know that the magnitude of the field becomes maximum at the centre of the circle and it decreases along the axis, on either side. As a result, no uniform magnetic field is obtained at any point of the axis. So, a problem arises while constructing electrical instruments with circular coils. This problem can be removed by using a Helmholtz double coil.
Basically, it is the combination of two current carrying rings. They both carry equal current.
Two circular coils having the same radii (radius = r) are placed coaxially at a distance equal to their radii.
If a direct current is passed through them in the same direction, the magnetic fields generated between them will have the same direction. Thus, the resultant magnetic field remains almost uniform in between the coils.

It can be seen from the figure; the magnetic field is constant between the coils. It is used to produce a uniform magnetic field between two circular coils.

Note
The magnetic field generated in the Helmholtz coil is not completely uniform. There are some variations of magnetic fields about up to $7\% $ . But it is mainly used to produce uniform magnetic fields so option b is not correct. Only option a will be correct.