A moving coil galvanometer is a device used in electromagnetism to calculate the values of small currents. The galvanometer can measure the strength, presence, and direction of current in a conductor. The galvanometer contains a needle that deflects upon identifying the lines of magnetic field. A galvanometer works on a specific principle, and various factors govern the working of a galvanometer. Moreover, the factors that affect the sensitivity of a moving coil galvanometer are very important.

The Principle and Mechanism of a Moving Coil Galvanometer

In electromagnetism, the current passing through a coil creates a magnetic field. The coil behaves like a bar magnet. Similarly, when current is passed through the copper wire coiling a soft iron core, the soft iron core gains temporary magnetism. We call those temporary magnets, electromagnets.

The principle of a moving coil galvanometer works similarly- current is passed through a coil suspended in a uniform magnetic field. As a result, a torque acts on the coil, making it move. The movement of the coil is directly proportional to the supplied current. 

Construction

The construction of a moving coil galvanometer is simple. Thin insulated copper wires are coiled rectangularly around a soft iron core. Then the rectangular coil is suspended in the middle of the poles of a horseshoe magnet. The suspension used is a fibre of phosphorus-bronze. The poles of the horseshoe magnet are cylindrically concave. Coiled springs or suspensions are attached to the top and bottom of the rectangular coil. 

Working 

Let us point out two things that mainly govern the working of the moving coil galvanometer:

• Coiled springs are attached at the rectangular coil’s top and bottom, producing a counter torque. This counter-torque helps the needle stop and return to its original place when the current is not passed.
• The poles of the horseshoe magnet are made cylindrically concave to produce a radial magnetic field. Due to this, the coil stays parallel to the magnetic field lines. The angle between the pointer and the magnetic field lines stays constant (90 degrees) even after deflection.

The current is passed through the coil. The current enters through the fibre and leaves through the spring. The soft iron core is magnetised, and the rectangular coil rotates due to the torque. One length of the coil rotates outward according to Fleming’s left-hand rule. Similarly, another side of the rectangular coil turns inwards. A counter-torque is applied through the coiled springs. At equilibrium, the deflecting torque and the restoring torque become equal. Hence the formula arises:

T (counter) = T(deflection)

Now, Cϕ [ ϕ is the angle of deflection] = MBsinΘ

Or, Cϕ = NIABsinΘ

Or, ϕ ∝ I [as all other factors are constant]

Here, 

• N- Number of times the coil is turned.
• I- Current 
• A- area of the coil
• B- uniform magnetic field
• Θ- constant[90 degrees] 

This proves that the deflection angle is directly proportional to the current applied. 

What are the Common Factors of a Moving Coil Galvanometer?

Several factors determine the sensitivity of a moving coil galvanometer. But let us understand what the sensitivity of a moving coil galvanometer is.

Current sensitivity

We know , T(counter) ∝ Θ [ angle of rotation of the coil ]

Or, T(counter) = kΘ [ k= twisting constant]

Or, NIAB = kΘ

Or, Θ/I = k/NIAB

Now, this Θ/I is called current sensitivity(Is). The unit is Radian A-1.

Voltage sensitivity

Vs is the voltage sensitivity. The unit is Radian V-1. The formula is Θ/IR

These two constitute the sensitivity of a moving coil galvanometer.

Factors of the Sensitivity of a Moving Coil Galvanometer

• Sensitivity increases with the coil’s number of turns(N). But there is a catch: if we keep increasing the number of turns, the coil might become too heavy, and the system will fall apart. We can only increase the number of turns of the coil up to a certain limit.
• Sensitivity increases with the increase in the uniform magnetic field. This is either by including a soft iron core at the core of the coil or by making the poles of the horseshoe magnet cylindrically concave. 
• Sensitivity will increase with the increase in the area of the coil. But we can only increase the area up to a certain limit keeping in mind the weight of the coil.
• Sensitivity will increase by decreasing the twisting constant (k). It can be done by using a phosphor-bronze-made suspension.

What is the Reduction Factor of a Moving Coil Galvanometer?

The reduction factor of a moving coil galvanometer is the particular ampere of current passed through the coil to maintain the angle of deflection ( ϕ ) under 45 degrees. The unit of the reduction factor is ampere.

Conclusion

This chapter focuses more on informative procedures and the moving coil galvanometer construction. One should get a good grasp of how a galvanometer works and its limitations. Practicals on the mechanism of moving coil galvanometers are a must to understand this topic fully. Some major points of this topic include the principle of moving coil galvanometer and the construction and working formula of moving coil galvanometer—the sensitivity of the moving coil galvanometer and the factors that control it. One should also go through how galvanometers are converted into ammeters and voltmeters.