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18. MOVING-COIL GALVANOMETER:

Moving Coil Galvanometer | Physics Class 12 - NEETFig: Moving coil galvanometer

The main parts of a moving-coil galvanometer are shown in figure. A rectangular coil of several turns is wound over a soft-iron core. The wire of the coil is coated with an insulating material so that each turn is insulated from the other and from the iron core. The coil is suspended between the two pole pieces of a strong permanent magnet.

The current to be measured is passed through the galvanometer. As the coil is in the magnetic field Moving Coil Galvanometer | Physics Class 12 - NEET of the permanent magnet, a torque Moving Coil Galvanometer | Physics Class 12 - NEET acts on the coil. Here n = number of turns, i = current in the coil Moving Coil Galvanometer | Physics Class 12 - NEET = area-vector of the coil and Moving Coil Galvanometer | Physics Class 12 - NEET = magnetic field at the site of the coil. This torque deflects the coil from its equilibrium position.

Fig: Radial magnetic fieldFig: Radial magnetic field

The pole pieces are made cylindrical. As a result, the magnetic field at the arms of the coil remains parallel to the plane of the coil everywhere even as the coil rotates. The deflecting torque is then Moving Coil Galvanometer | Physics Class 12 - NEET. As the upper end of the suspension strip W is fixed, the strip gets twisted when the coil rotates. 

This produces a restoring torque acting on the coil. If the deflection of the coil is q and the torsional constant of the suspension strip is k, the restoring torque is kq. The coil will stay at a deflection q where:

niAB = kθ or, Moving Coil Galvanometer | Physics Class 12 - NEET

Hence, the current is proportional to the deflection. The constant Moving Coil Galvanometer | Physics Class 12 - NEET is called the galvanometer constant, and may be found by passing a known current, measuring the deflection q and putting these values in equation.

Sensitivity:

The sensitivity of a moving-coil galvanometer is defined as θ/i. From equation, the sensitivity is Moving Coil Galvanometer | Physics Class 12 - NEET. For large sensitivity, the field B should be large. The presence of soft-core increases the magnetic field.

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FAQs on Moving Coil Galvanometer - Physics Class 12 - NEET

1. What is a moving coil galvanometer?
Ans. A moving coil galvanometer is a device that measures small electric currents. It consists of a coil of wire suspended in a magnetic field, which is deflected when a current passes through it.
2. How does a moving coil galvanometer work?
Ans. The working principle of a moving coil galvanometer is based on the interaction between the magnetic field produced by the current passing through the coil and the external magnetic field. When a current flows through the coil, it experiences a torque, causing it to rotate. The deflection of the coil is proportional to the current passing through it.
3. What are the applications of a moving coil galvanometer?
Ans. Moving coil galvanometers have various applications, including: - Measurement of small electric currents in scientific experiments. - Detection of current flow in electrical circuits. - Monitoring and control of current in industrial processes. - Use as a sensitive indicator in electronic instruments. - Testing and calibration of other electrical measuring devices.
4. What are the advantages of a moving coil galvanometer?
Ans. Some of the advantages of a moving coil galvanometer include: - High sensitivity and accuracy in measuring small currents. - Wide dynamic range, allowing the measurement of both low and high currents. - Compact size and portability. - Low power consumption. - Reliability and durability.
5. How can a moving coil galvanometer be calibrated?
Ans. To calibrate a moving coil galvanometer, a known current is passed through the coil and the corresponding deflection is measured. By comparing the measured deflection with the known current, a calibration curve can be established. This curve can then be used to determine the current for any future deflection of the galvanometer. Calibration can be done using a standard resistor or by comparing with a reference galvanometer.
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