Moving coil galvanometer

A device used to measure electric currents even as low as a few microamperes is known as a galvanometer. Generally, there are two major types of galvanometer – one with suspended coil and another with pivoting coil or a Weston galvanometer. When an electric current is made to pass via a conductor, a deflection in the magnetic needle indicates current. Direction of the current is specified by direction of the shift. Let’s know more about galvanometers as well as moving-coil galvanometers. 

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What Is a Moving Coil Galvanometer? 

A type of electromagnetic device capable of measuring even the smallest amounts of currents is known as a moving coil galvanometer. Some of the major parts of the moving coil galvanometer are permanent horseshoe magnets, pivoted spring, non-metallic frame, coil, scale, soft iron core and pointer.

Principle Of Moving Coil Galvanometer 

When a current-carrying coil is made to put in an external magnetic field, it experiences a magnetic torque. Because of magnetic torque in the current-carrying loop or coil, it starts to rotate. Hence, the deflection of the coil due to the magnetic torque becomes proportional to the current magnitude of the coil or the current flowing through the coil. 

Galvanometer Construction And Diagram 

A rectangular coil with multiple turns wrapped around a metallic frame is used to construct a galvanometer. A thin insulated or, most precisely, a fine copper wire is used to construct it. By using a fine phosphor–bronze strip, the coil is suspended. It is usually suspended from a movable torsion head between the poles of a horseshoe magnet. The coil’s low end is connected to fine phosphor – bronze’s hairspring. In this case, there are just a few turns in the hairspring of fine phosphor – bronze strips. The upper end of the coil connects the binding screw. On the inside of the coil, we can find an asymmetrically placed soft iron cylinder. The two magnetic poles of hemispherical shape are known to produce a radial magnetic field. This is done to ensure that Torque is maximised. To measure the deflection of the coil, a small plane mirror, a lamp and scale arrangement is used. 

Working Of Moving Coil Galvanometer 

Let us consider a rectangular shape, PQRS with a single-coil turn and the current, I, always flow through the coil. The coil’s plane and magnetic field are parallel to each other, only in the presence of a radial magnetic field. The two sides, PQ and RS, are always at right angles to the direction of the magnetic field, whereas the two sides, QR and SP, are always parallel to the direction of the magnetic field. 

Length of the coil, PQ = RS = L

Breadth of the coil, PS = QR = b

Let’s find the force acting on the four sides of the coil. 

Force on PQ, F = BI (PQ) = BIL

Fleming’s left-hand rule is used to find the direction of the magnetic force. As per the rule, the force acts in the normal direction to the coil’s plane and acts in the outward direction. 

Force on RS, F = BI (RS) = BIL. 

Though the force on RS is equal to the force on PQ, they differ in direction. They always constitute a couple with a different line of action, and the coil is deflected. Suppose when there is n, a number of turns in the coil, the moment of the deflecting couple is expressed by nBIl – b. 

A twisting force is set up to restore the twist of the suspension wire, which is known as the moment of the restoring couple = Cθ. 

When the state of equilibrium is reached, the resorting couple becomes, 

nBIA = Cθ

Where  C = torsional constant of the spring or the restoring torque per unit twist. 

The current of the moving coil galvanometer is represented by I = (C/nBA) × θ. 

Factors Affecting The Sensitivity Of The Moving Coil Galvanometer

Certain factors affect the sensitivity of the moving coil galvanometer, but before we move to the factors, here is the definition of the sensitivity of the moving coil galvanometer. A ratio to the alternation of deflection change to the current change is known as the sensitivity of the galvanometer. 

Sensitivity = dθ/di=nBA/C

Here are the factors that affect the sensitivity of the moving coil galvanometer. 

• The number of turns of the coil, n, and the area of the coil, A. 

• Magnetic induction, B

• A couple per unit twist, k/nAB

Remember, the sensitivity increases with the increase in the first two factors, but with the increase in a couple per unit twist, the sensitivity of the galvanometer decreases. 

Advantages Of The Moving Coil Galvanometer 

Here are a few advantages that a moving coil galvanometer has to offer. 

• With the increase in the number of turns of the coil, n, the magnetic field, B and the area, A, the sensitivity value also increases. 

• The produced eddy currents help the coil to reach the resting stage easily and quickly. The reason is that the coil is wrapped around the light metallic frame. 

• Known to produce a high sensitivity 

• Higher ratio of torque-to-weight

• High dependability and precision 

• Little to no effect of stray magnetic fields

Drawbacks Of Moving Coil Galvanometer 

Aside from the positive sides of the galvanometer, there are a few disadvantages of moving coil galvanometers. 

• If there is overloading, the galvanometer will be damaged immediately. 

• Alternation in the sensitivity of the galvanometer isn’t possible. 

• No other current rather than direct currents are measured. 

Conclusion  

Any circuit in which the current is present can use a moving coil galvanometer. It is a sensitive instrument used in Wheatstone’s bridge circuit to detect and measure the current in the circuit. The pointers in the moving coil galvanometer are used to detect the direction of the flow of the current.