what is moving coil galvanometer
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Galvanometer
A galvanometer is a device that is used to detect small electric current or measure its magnitude. The current and its intensity is usually indicated by a magnetic needle’s movement or that of a coil in a magnetic field that is an important part of a galvanometer.
Since its discovery in the 1800s, galvanometer has seen many iterations. Some of the different types of galvanometer include Tangent galvanometer, Astatic galvanometer, Mirror galvanometer and Ballistic galvanometer. However, today the main type of galvanometer type that is used widely is the D’Arsonval/Weston type or the moving coil type. A galvanometer is basically a historical name that has been given to a moving coil electric current detector.
What is a Moving Coil Galvanometer?
A moving coil galvanometer is an instrument which is used to measure electric currents. It is a sensitive electromagnetic device which can measure low currents even of the order of a few microamperes.
Moving-coil galvanometers are mainly divided into two types:
Suspended coil galvanometer
Pivoted-coil or Weston galvanometer
Moving Coil Galvanometer Principle
A current-carrying coil when placed in an external magnetic field experiences magnetic torque. The angle through which the coil is deflected due to the effect of the magnetic torque is proportional to the magnitude of current in the coil.
Moving Coil Galvanometer Construction And Diagram
The moving coil galvanometer is made up of a rectangular coil that has many turns and it is usually made of thinly insulated or fine copper wire that is wounded on a metallic frame. The coil is free to rotate about a fixed axis. A phosphor-bronze strip that is connected to a movable torsion head is used to suspend the coil in a uniform radial magnetic field.
Essential properties of the material used for suspension of the coil are conductivity and a low value of the torsional constant. A cylindrical soft iron core is symmetrically positioned inside the coil to improve the strength of the magnetic field and to make the field radial. The lower part of the coil is attached to a phosphor-bronze spring having a small number of turns. The other end of the spring is connected to binding screws.
Also Read: Alternating Current
The spring is used to produce a counter torque which balances the magnetic torque and hence help in producing a steady angular deflection. A plane mirror which is attached to the suspension wire, along with a lamp and scale arrangement is used to measure the deflection of the coil. Zero-point of the scale is at the centre.
Working of Moving Coil Galvanometer
Let a current I flow through the rectangular coil of n number of turns and a cross-sectional area A. When this coil is placed in a uniform radial magnetic field B, the coil experiences a torque τ.
Let us first consider a single turn ABCD of the rectangular coil having a length l and breadth b. This is suspended in a magnetic field of strength B such that the plane of the coil is parallel to the magnetic field. Since the sides AB and DC are parallel to the direction of the magnetic field, they do not experience any effective force due to the magnetic field. The sides AD and BC being perpendicular to the direction of field experience an effective force F given by F = BIl
Moving Coil Galvanometer
- The principle of a moving coil galvanometer is that it experiences a torque after placing a current-carrying coil in a magnetic field.
- The coil springs along with the radial field ensure that the strength of the current is proportionate to the deflection. This is the reason why it is called a moving coil galvanometer.
- The use of moving coil galvanometer-
It helps to measure the value of the current and the voltage in the circuit by connecting it in parallel to low resistance and in series with high resistance respectively.