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Moving Coil Galvanometer

Moving Coil Galvanometer

Edited By Vishal kumar | Updated on Jul 02, 2025 05:54 PM IST

A moving coil galvanometer is an essential instrument in the field of physics, designed to measure small electric currents with high precision. It operates on the principle that a current-carrying coil, placed within a magnetic field, experiences a torque proportional to the current. This device is a cornerstone in electrical measurements, transforming minute currents into observable mechanical deflections. In real life, the moving coil galvanometer finds its applications in various fields, including electronics and instrumentation. For instance, in a household analogue multimeter, the galvanometer serves as a core component, enabling users to measure current, voltage, and resistance accurately. Furthermore, it plays a crucial role in scientific research and education, helping students and professionals understand and analyze the behaviour of electric circuits. The device's sensitivity and reliability make it indispensable in developing and testing new electronic devices, ensuring their proper functionality and safety. In this article, we will discuss the Moving Coil Galvanometer and provide examples for better understanding.

This Story also Contains
  1. What is a Moving Coil Galvanometer?
  2. Solved Examples Based on Moving coil Galvanometer
  3. Example 1: An ammeter reads up to 1 ampere. Its internal resistance is 0.81 ohm. To increase the range to 10 A, the value of the required shunt is (in $\Omega$)
  4. Summary
Moving Coil Galvanometer
Moving Coil Galvanometer

What is a Moving Coil Galvanometer?

A moving coil galvanometer is an electromagnetic device which is used to measure small values of current. It consists of a permanent horse-shoe magnet, coil, soft iron core, pivoted spring, non-metallic frame, scale, and pointer as shown in the figure.

As we have studied the torque acts on a current-carrying coil suspended in the uniform magnetic field. Due to this, the coil rotates. Hence, the deflection in the coil of a moving coil galvanometer is directly proportional to the current flowing in the coil.

In this, the coil is suspended between the pole pieces of a strong horse-shoe magnet. The magnetic field is made radial and for this, the pole pieces are made cylindrical and a soft iron cylindrical core is placed within the coil without touching it. The benefit of this type of field is that the plane of the coil always remains parallel to the field. Therefore θ=90o and the deflecting torque always has the maximum value.

$\tau_{\text {deflection }}=N B i A$

Now if the coil deflects, a restoring torque is set up in the pivoted spring. If $\alpha$ is the angle of twist, the restoring torque is

$\tau_{\text {restoring }}=C \alpha$

where C is the torsional constant of the fibre.

When the coil is in equilibrium, then

$
N B i A=C \alpha \text {, }
$

So, $\quad i=\frac{C}{N B A} \cdot \alpha \Rightarrow i=K \alpha$
where $K=\frac{C}{N B A}$ is the galvanometer constant.

This linear relationship between i and $\alpha$ makes the moving coil galvanometer useful for current measurement and detection.

Here we will discuss two important terminologies

1. Current sensitivity (Si): The current sensitivity of a galvanometer is defined as the deflection produced in the galvanometer per unit of current flowing through it. So it can be written as

$S_i=\frac{\alpha}{i}=\frac{N B A}{C}$

2. Voltage sensitivity (SV): The voltage sensitivity of a galvanometer is defined as the deflection produced in the galvanometer per unit voltage applied to it. So it can be written as -

$
S_V=\frac{\alpha}{V}=\frac{\alpha}{i R}=\frac{S_i}{R}=\frac{N B A}{R C}
$
(By using Ohm's law)

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Solved Examples Based on Moving coil Galvanometer

Example 1: An ammeter reads up to 1 ampere. Its internal resistance is 0.81 ohm. To increase the range to 10 A, the value of the required shunt is (in $\Omega$)

1) 0.09

2) 0.3

3) 0.9

4) 0.03

Solution:

Moving coil galvanometer

It is a very sensitive instrument used to measure the small current. The galvanometer gives the deflection which is proportional to the electric current flowing through it.

$\begin{aligned} & \frac{S}{S+G}=\frac{I_g}{I} \Rightarrow S=\frac{I_g G}{I-I_g} \\ & \therefore \quad S=\frac{1 \times 0.81}{10-1}=\frac{0.81}{9}=0.09 \Omega \text { in parallel. }\end{aligned}$

Hence, the answer is the option (1).

Example 2: A moving coil galvanometer has 150 equal divisions. Its current sensitivity is 10 divisions per milliampere and voltage sensitivity is 2 divisions per millivolt, so that each division reads 1 volt, the resistance in ohms needed to be connected in series with the coil will be

1) 9995

2) 99995

3) 103

4) 105

Solution:

Moving coil galvanometer

It is a very sensitive instrument used to measure the small current. The galvanometer gives the deflection which is proportional to the electric current flowing through it.

$
\begin{aligned}
& V_{\max }=\frac{150}{2}=75 \mathrm{mV} \\
& I_{\max }=\frac{150}{10}=15 \mathrm{~mA}=I_g
\end{aligned}
$

Resistance of galvanometer $G=75 / 15=5 \Omega$

For conversion into a voltmeter, a high resistance should be connected in series with the galvanometer

$\begin{aligned} & V=I_g(G+R)=\frac{15}{1000}(5+R) \Rightarrow 150=15 \frac{(5+R)}{1000} \\ & \text { or } 5+R=\frac{150 \times 1000}{15}=10000 \\ & \therefore \quad R=9995 \Omega\end{aligned}$

Hence, the answer is the option (1).

Example 3: When a current of 5 mA is passed through a galvanometer having a coil of resistance 15 $\Omega$ , it shows full-scale deflection. The value of the resistance to be put in series with the galvanometer to convert it into a voltmeter of range 0−10 V is

1) $1.985 \times 10^3 \Omega$

2) $2.045 \times 10^3 \Omega$

3) $2.535 \times 10^3 \Omega$

4) $4.005 \times 10^3 \Omega$

Solution:

Given,
$
\begin{aligned}
& \mathrm{I}_{\mathrm{g} \text { (Galvanometer Current) }}=5 \mathrm{~mA} \\
& \mathrm{G}=\text { Resistance of Galvanometer }=15 \Omega
\end{aligned}
$
$R=$ The value of the resistance to be put in series with the galvanometer
We know,
$
\begin{aligned}
& V=I_g(R+G) \\
& 10=5 \times 10^{-3}(R+15) \\
& 2000=R+15
\end{aligned}
$
$
\Rightarrow R=1985 \Omega=1.985 \times 10^3 \Omega
$

Hence, the answer is the option (1).

Example 4: A $50 \Omega$ resistance is connected to a battery of 5 V. A galvanometer of resistance $50 \Omega$ is to be used as an ammeter to measure the current through the resistance, for this, a resistance rs is connected to the galvanometer. Which of the following connections should be employed if the measured current is within 1% of the current without the ammeter in the circuit?

1) $r_{\mathrm{s}}=0.5 \Omega$ in parallel with the galvanometer
2) $r_{\mathrm{s}}=0.5$ in series with the galvanometer
3) $r_{\mathrm{s}}=1 \Omega$ in series with the galvanometer
4) $r_{\mathrm{s}}=1 \Omega \mathrm{n}$ parallel with the galvanometer

Solution:

Moving coil galvanometer

It is a very sensitive instrument used to measure the small current. The galvanometer gives the deflection which is proportional to the electric current flowing through it.

Current in the circuit without an ammeter

$I=\frac{V}{R}=\frac{5 v}{50 \Omega}=0.1 A$

Allowed current with an ammeter

$\begin{aligned} & I^{\prime}=0.099 A \\ & I^{\prime}=\frac{V}{R_{e q}} \\ & \text { where } R_{e q}=50+\frac{100 r_s}{100+r_s} \\ & \therefore 0.099=\frac{5}{50+\frac{100 r_s}{100+r_s}} \\ & \Rightarrow 50+\frac{100}{100+r_s}=\frac{5}{0.099} \\ & \Rightarrow 100 r_s=50+0.5 r_s \\ & \Rightarrow r_s=0.5 \Omega\end{aligned}$

Hence, the answer is the option (1).

Example 5: The sensitivity of the galvanometer is increased by :

1) Increasing no. of turns

2) Decreasing the Area of the coil

3) Increasing current

4) Introducing soft Iron core

Solution:

Current sensitivity

It is defined as the deflection produced in the galvanometer per unit current.

$S_i=\frac{N B A}{K} \Rightarrow S_i \propto N$

Therefore the sensitivity of the galvanometer is directly proportional to the number of turns.

Hence, the answer is the option (1).

For more information, below video can be referred to :

Summary

A moving coil galvanometer is a precise instrument used to measure small electric currents by converting them into mechanical deflections. It operates based on the torque experienced by a current-carrying coil in a magnetic field. Key parameters like current sensitivity and voltage sensitivity are crucial for its function. Practical applications include its use in analogue multimeters and its importance in scientific research, making it an indispensable tool in electronics and education.

Frequently Asked Questions (FAQs)

1. How can a moving coil galvanometer be converted into an ammeter?
A moving coil galvanometer can be converted into an ammeter by connecting a low-resistance shunt in parallel with the galvanometer. The shunt diverts most of the current around the galvanometer, allowing it to measure larger currents without damage. The value of the shunt resistance is chosen based on the desired current range and the galvanometer's internal resistance.
2. How does the resistance of the galvanometer coil affect its use in different circuits?
The resistance of the galvanometer coil is an important consideration when using the instrument in different circuits:
3. Why is the "Ayrton shunt" preferred over a simple shunt in some galvanometer applications?
The Ayrton shunt, also known as a universal shunt, offers several advantages:
4. What is the difference between a galvanometer and an ammeter?
A galvanometer is designed to detect and measure very small currents, typically in the microampere or milliampere range. An ammeter, on the other hand, is designed to measure larger currents, often up to several amperes. While a galvanometer has high sensitivity but a limited range, an ammeter sacrifices some sensitivity to achieve a broader measurement range.
5. How does the "flux-meter" application of a galvanometer differ from its use as a current measuring device?
In a flux-meter application, the galvanometer is used to measure changes in magnetic flux:
6. What is a moving coil galvanometer and how does it work?
A moving coil galvanometer is a sensitive instrument used to detect and measure small electric currents. It works on the principle of electromagnetic interaction between a current-carrying coil and a permanent magnet. When current flows through the coil, it experiences a torque due to the magnetic field, causing it to rotate. The rotation is proportional to the current, allowing for precise measurements.
7. Why is the coil in a galvanometer suspended between the poles of a strong horseshoe magnet?
The coil is suspended between the poles of a strong horseshoe magnet to create a uniform magnetic field. This arrangement ensures that the torque experienced by the coil is consistent and proportional to the current flowing through it, allowing for accurate measurements across the entire range of the instrument.
8. What is the purpose of the soft iron core in a moving coil galvanometer?
The soft iron core serves to concentrate and strengthen the magnetic field in the region where the coil rotates. This increases the sensitivity of the galvanometer, allowing it to detect and measure even smaller currents. The soft iron core also helps to make the magnetic field more uniform in the gap between the magnet poles.
9. How does the spring in a galvanometer affect its operation?
The spring in a galvanometer provides a restoring torque that opposes the electromagnetic torque acting on the coil. This restoring force ensures that the coil returns to its original position when no current is flowing. The spring also helps to control the movement of the coil, preventing excessive rotation and allowing for a stable, readable deflection proportional to the current.
10. What factors determine the sensitivity of a moving coil galvanometer?
The sensitivity of a moving coil galvanometer depends on several factors:
11. What is the significance of the "ballistic constant" in a ballistic galvanometer?
The ballistic constant relates the total charge passed through the galvanometer to the maximum deflection observed. It's typically expressed in coulombs per division. This constant is crucial for quantitative measurements with a ballistic galvanometer, allowing the user to calculate the total charge from the observed deflection. The ballistic constant depends on the galvanometer's construction and must be determined through calibration.
12. Why is a moving coil galvanometer considered a "dead-beat" instrument?
A moving coil galvanometer is considered "dead-beat" because its pointer quickly comes to rest at the correct position without oscillating. This is achieved through careful design, including the use of a damping mechanism (often an aluminum frame on which the coil is wound). The damping effect is caused by eddy currents induced in the frame as it moves through the magnetic field, quickly bringing the pointer to a stable position.
13. How can a moving coil galvanometer be converted into a voltmeter?
A moving coil galvanometer can be converted into a voltmeter by connecting a high-resistance multiplier in series with it. The multiplier limits the current flowing through the galvanometer to a safe level while allowing it to measure voltage. The value of the multiplier resistance is chosen based on the desired voltage range and the galvanometer's internal resistance.
14. Why is the coil in a galvanometer typically made of many turns of fine wire?
The coil in a galvanometer is made of many turns of fine wire to increase its sensitivity. More turns mean that a given current will produce a stronger magnetic field around the coil, resulting in a larger torque and greater deflection. Fine wire is used to keep the coil lightweight and reduce its moment of inertia, allowing for quicker response to changes in current.
15. What is the function of the mirror attached to the coil in some galvanometers?
The mirror attached to the coil in some galvanometers is used to enhance the accuracy of readings. A beam of light is reflected off the mirror onto a scale, effectively amplifying the movement of the coil. This arrangement, known as a mirror galvanometer, allows for the detection of extremely small currents by observing the movement of the light spot on the scale.
16. How does temperature affect the performance of a moving coil galvanometer?
Temperature changes can affect a galvanometer's performance in several ways:
17. What is the significance of the "figure of merit" for a galvanometer?
The figure of merit for a galvanometer is a measure of its sensitivity, defined as the current required to produce a standard deflection (usually 1 scale division). A lower figure of merit indicates a more sensitive instrument. It's typically expressed in amperes per division and helps in comparing the sensitivity of different galvanometers or in selecting the appropriate instrument for a specific measurement task.
18. Why is a moving coil galvanometer not suitable for AC measurements?
A moving coil galvanometer is not suitable for AC measurements because it responds to the average value of the current over time. In an AC circuit, the current constantly changes direction, resulting in a net zero average current over a complete cycle. This would cause the galvanometer's pointer to remain at zero or vibrate slightly, providing no useful information about the AC signal's characteristics.
19. How does the shape of the pole pieces in a galvanometer affect its operation?
The pole pieces in a galvanometer are typically shaped to create a radial magnetic field. This ensures that the torque on the coil remains constant regardless of its angular position, resulting in a linear relationship between current and deflection. The curved shape of the pole pieces helps maintain a uniform magnetic field strength across the entire range of coil movement, improving the instrument's accuracy and linearity.
20. What is the purpose of the aluminum frame on which the coil is wound in many galvanometers?
The aluminum frame serves two primary purposes:
21. How does the moment of inertia of the coil affect the galvanometer's performance?
The moment of inertia of the coil affects the galvanometer's response time and sensitivity. A lower moment of inertia allows the coil to respond more quickly to changes in current, improving the instrument's speed and accuracy. However, it also makes the coil more susceptible to external vibrations. Designers must balance these factors to achieve optimal performance for the intended application.
22. What is the role of the control magnet in some galvanometer designs?
The control magnet, typically located above the main magnet, serves to adjust the zero position of the galvanometer. By rotating this magnet, the user can fine-tune the magnetic field acting on the coil, compensating for any slight misalignments or changes in the Earth's magnetic field. This allows for precise zeroing of the instrument before taking measurements.
23. Why are galvanometers often used as null detectors in bridge circuits?
Galvanometers are ideal as null detectors in bridge circuits due to their high sensitivity to small currents. In a balanced bridge, the current through the galvanometer should be zero. Any small imbalance will cause a detectable deflection, allowing for precise measurements. The galvanometer's ability to detect tiny currents makes it possible to achieve high accuracy in bridge measurements without drawing significant current from the circuit under test.
24. How does the suspension system of a galvanometer affect its performance?
The suspension system, typically consisting of fine metal ribbons or threads, plays a crucial role in galvanometer performance:
25. What is the significance of the "current sensitivity" of a galvanometer?
Current sensitivity is a measure of how much the galvanometer's pointer deflects for a given current. It's typically expressed in divisions per microampere or a similar unit. Higher current sensitivity means the instrument can detect and measure smaller currents. This parameter is crucial for selecting the appropriate galvanometer for specific applications, especially when dealing with very low currents.
26. How does the strength of the permanent magnet in a galvanometer affect its performance over time?
The strength of the permanent magnet can gradually decrease over time due to factors like temperature fluctuations, mechanical shocks, or exposure to strong external magnetic fields. This gradual weakening can affect the galvanometer's calibration and sensitivity. High-quality galvanometers use magnets made from materials with excellent long-term stability to minimize this effect, but periodic recalibration may still be necessary for precise measurements.
27. Why is it important to level a galvanometer before use?
Leveling a galvanometer is crucial because:
28. How does the scale design of a galvanometer affect its readability and accuracy?
The scale design is critical for a galvanometer's usability and accuracy:
29. What is the purpose of the "swamping resistor" sometimes used with galvanometers?
A swamping resistor is a high-value resistor connected in series with the galvanometer coil. It serves several purposes:
30. How does the presence of nearby ferromagnetic materials affect a galvanometer's readings?
Nearby ferromagnetic materials can distort the magnetic field around the galvanometer, leading to inaccurate readings. These materials can:
31. What is the difference between a tangent galvanometer and a moving coil galvanometer?
A tangent galvanometer and a moving coil galvanometer operate on different principles:
32. How does the concept of "critical damping" apply to galvanometers?
Critical damping in galvanometers refers to the optimal level of damping that allows the pointer to reach its final position quickly without overshooting or oscillating. It's achieved by balancing the mechanical and electromagnetic forces acting on the coil. Critical damping ensures:
33. Why are some galvanometers designed with a "light-beam" pointer instead of a physical pointer?
Light-beam pointers offer several advantages over physical pointers:
34. How does the "ballistic galvanometer" differ from a standard moving coil galvanometer?
A ballistic galvanometer is designed to measure the total charge in a short pulse of current, rather than continuous current. Key differences include:
35. How does the "figure of eight" coil winding in some galvanometers improve performance?
The "figure of eight" coil winding offers several benefits:

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