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A galvanometer has a wide range of applications as well as use. But what is a galvanometer? A device used to detect a small electric current and its magnitude is a galvanometer. The intensity of the current is indicated by the movement of the magnetic needle, which forms an important part of any galvanometer.
There are different kinds of galvanometers. These types are astatic galvanometers, tangent galvanometers, ballistic galvanometers, mirror galvanometers, etc. The galvanometer widely used today is the moving coil galvanometer.
Moving coil galvanometer
A galvanometer used to measure any electric current is known as a moving coil galvanometer. It is an electromagnetic device that helps detect low currents even if they are present in little microamperes.
There are two types of moving coil galvanometers:
- Weston galvanometer or a pivoted-coil galvanometer
- Suspended coil galvanometer
Principle of a moving coil galvanometer
If a coil that carries current is placed in an external magnetic field, it experiences a magnetic torque. The angle by which the coil gets deflected after getting affected by the magnetic torque is directly proportional to the current magnitude present in the coil.
Materials required for moving coil galvanometer construction
There are some basic requirements to construct a moving coil galvanometer. A moving coil, a permanent magnet, and a suspension form the main components of a moving coil galvanometer.
The core of a moving coil galvanometer can be made from soft iron. This is because soft iron initially has low hysteresis loss and high permeability. A horseshoe magnet is one of the best options for a permanent magnet.
Moving coil galvanometer construction
A moving coil galvanometer is constructed of a rectangular coil. This coil has many turns of insulated thin copper wires. It is wounded over a metallic frame. The lower end of the rectangular coil is a phosphor bronze hairspring that has only a few turns.
The hairspring’s other side is connected to a screw. A radial magnetic field is produced by the magnetic poles. Here, the magnetic field is parallel to the plane of the coil. A soft iron bar or cylinder is placed inside the coil symmetrically. A small-sized plane mirror is connected to the wire with a scale arrangement to measure the coil’s deflection.
Working of moving coil galvanometer
Let PQRS represent any single turn of the rectangular coil. Let current I flow through the rectangular coil. As the magnetic field is radial, the coil plane is parallel to the magnetic field. This shows that side QR and SP are always parallel to the magnetic field. They do not go through any other force. The side PQ and side RS are always perpendicular to the magnetic field.
Let the length of the coil be l = PQ = RS. Let the breadth of the coil be b = PS = QR. Let the force on side PQ be F = BI, PQ = BI. The force F is perpendicular to the rectangular coil plane, acting outwards, as per Fleming’s left-hand rule.
The force on RS, F = BI * RS = BI. This force is also perpendicular to the plane of the rectangular coil. This force acts inwards. The deflection of the coil is caused due to these two opposite parallel forces. Let the turns in the rectangular coil be ‘n’, then the moment of deflecting the couple be = nBI-b.
The moment of a couple of deflections will be = nBIA.
When the rectangular coil starts deflecting, the wire starts to twist. A restoring couple is also formed in the wire. The restoring couple is directly proportional to the twist.
Let Ө represent the angular twist, and then the couple’s moment will be = CӨ. C represents the restoring couple per unit of the twist.
Hence, it can be written as nBIA=CӨ.
So, I = (C-nBA) * Ө. Here C represents the torsional constant of the spring. It can also be known as restoring torque per unit twist. A pointer is attached to the spring. This pointer indicates the deflection on the measurement scale.
Advantages of moving coil galvanometer
- The sensitivity of the moving coil galvanometer increases if the values of B, A and n increase and if the value of k decreases.
- The coil can be brought to rest quickly as well as easily by eddy currents. Eddy currents are produced by wounding coil over a metallic frame.
Disadvantages of moving coil galvanometer
- The sensitivity of a moving coil galvanometer can not be changed.
- Any galvanometer can be damaged due to overloading.
Conclusion
An electromagnetic instrument that can measure small values of current is known as a moving coil galvanometer. The main components of a moving coil galvanometer are coil, permanent magnets like a horseshoe magnet, soft iron core, non-metallic frame, pivoted spring, pointer, measurement scale, etc.
The principle on which the moving coil galvanometer operates is very simple and basic. The moving coil galvanometer’s principle states that its rectangular coil’s deflection is directly proportional to the current flowing in the coil. A phosphor-bronze wire is used in a suspension wire for moving coil galvanometers. This is because it has high tensile strength. Johann Schweigger invented the moving coil galvanometer on 16 September 1820.
