Question

A pole piece of magnet used in a pivoted coil galvanometer are:
A. Plane surface of a bar magnet
B. Plane surface of a horse-shoe magnet
C. Cylindrical surface of a bar magnet
D. Cylindrical surface of a horse-shoe magnet

Answer 1

The changes in a pivoted type galvanometer is made with respect to a suspended type galvanometer. The construction of a suspended type galvanometer is compared with the construction of a pivoted type galvanometer. The changes brought upon the suspended type galvanometer need to be analyzed in-order to differentiate between the two types of galvanometers and find out which type of magnet is used for pivoted type galvanometers.

Complete answer:
A moving coil galvanometer is a device used to detect and measure small amounts of direct current. There are two types of moving coil galvanometers, namely, the suspended type galvanometer, pivoted type galvanometer. The purpose of both of these galvanometers are the same but however their principle may be different.

In-order to determine which type of magnet is used in the pivoted type galvanometer we first need to know the concept and constructions behind the suspended type galvanometer since the pivoted coil galvanometer was made in an attempt to overcome the disadvantages of a suspended type galvanometer. So we can say that the pivoted type galvanometer is just the modernized version of the suspended type galvanometer.

Let us first look at the concept and construction behind the suspended type galvanometer. The suspended type galvanometer uses a soft iron core placed inside a coil that is suspended in between the pole pieces of the magnet which rotates about its axis to cut the magnetic field lines of force to produce current.

One main disadvantage is that since the soft iron core is suspended freely there is a tendency of the soft iron core to get attached to one side of the magnet if not handled properly and hence it becomes very difficult to remove it from the magnet. Hence in order to avoid this problem the pivoted type galvanometer was made and as the name suggests it ensures that the iron core is pivoted and kept steady while the coil rotates.

Due to this construction of the suspended type galvanometer the current could not be directly measured. The amount of current is said to be detected and measured directly only when the galvanometer is made sensitive. The deflection angle produced must be directly proportional to the current. This is nothing but sensitivity. This condition can only be satisfied when the magnetic field produced is radial, that is, parallel to the plane of the coil and at points when it rotates.

This can only be achieved when the poles of magnetic used in the galvanometer are cylindrical in shape and when the soft iron core in the coil is placed symmetrically between the strong permanent cylindrical horseshoe magnets. This is the main reason for the use of cylindrical pole horseshoe magnets in pivoted type galvanometers. Hence the pole pieces of magnets used in a pivoted type galvanometer is a cylindrical horse shoe magnet which is our required answer.

Therefore, the correct option is option D.

Additional information: A galvanometer is designed to handle very small amounts of current because if there is a splurge in the amount current the galvanometer, the coils may melt and this may result in the breakdown of the device as it will stop working. The moving coil galvanometer used a linear scale in order to measure current which means that it uses a circular scale with equal divisions. The current sensitivity is said to be the amount of large scale deflection even when there is a small amount of current produced. The formula for sensitivity is as follows:
${I_s} = \dfrac{\alpha }{I}$
Where, $\alpha$ is the deflection produced and $I$ is the current produced.

Note: The suspended type galvanometer used a mirror to measure the current produced. A mirror suspended along with a lamp and a scale was used to detect the current but there was no mirror used in construction of a pivoted type galvanometer which used a pointer directly attached to a scale. The principle of both remains the same. The soft iron core used in the construction of a suspended galvanometer is really delicate and should be handled with care.