Moving coil Galvanometer

Principle

Magnetic torque is experienced when a current-carrying coil is put in an external magnetic field. The angle through which the coil is deflected due to the magnetic torque effect is proportional to the coil’s current magnitude.

Construction 

The moving coil galvanometer is formed out of a rectangular coil with numerous turns wound on a metallic frame, commonly made of thinly insulated or fine copper wire. A fixed axis allows the coil to freely revolve. A phosphor-bronze strip linked to an adjustable torsion head suspends the coil in a uniform radial magnetic field.

The material used to suspend the coil must have both conductivity and a low torsional constant. A cylindrical soft iron core is symmetrically positioned inside the coil to improve the magnetic field’s intensity and make it radial. The lower part of the coil is attached to a phosphor-bronze spring with a small number of turns. Binding screws are used to secure the spring’s other end.

The spring is used to create a counter-torque that balances the magnetic torque and results in a uniform angular deflection. A plane mirror connected to the suspension wire, and also a lamp and scale setup, are used to measure the coil’s deflection. The zero points on the scale are in the center.

Working

Permit me to pass through a rectangular coil with m turns and a cross-sectional area of K. The coil gets torque when it is put in a uniform radial magnetic field B. Consider a single-turn PQRS rectangular coil with a length of l and width of Q. The coil is suspended in a B-field magnetic field, with the coil’s plane parallel to the field. Because the sides PQ and SR are parallel to the field’s direction, these are not subjected to any magnetic field’s effective force.

The sides PS and QR, which are perpendicular to the field’s direction, are subjected to an equivalent force F, which is defined by,

                                                      F=BI

Using Fleming’s left-hand rule, we can determine that the forces acting on PS and QR are in opposite directions. Torque is produced when equal and opposite forces F, known as the pair, act on the coil. The torque causes the coil to deflect.

We know that the torque is the product of force and perpendicular distance

                                   T= force × perpendicular distance

                                   T= F× b

By using the value of F we can calculate the value of the torque acting on the single loop PQRS which is given by

                                   T= B l×b

Where l×b is the area of the coil

Hence the torque in m turns is given by

                                       T=mIAB

As a result of the magnetic torque, the coil rotates and the phosphor bronze strip twists. The counter torque or restoring torque k is provided by the spring S connected to the coil, resulting in a constant angular deflection.

                                    gθ = nIAB

The letter g stands for the spring’s torsional constant (restoring couple per unit twist). A pointer attached to the suspension wire indicates a value on a scale, which is used to measure the deflection or twist.

                                   θ = (nAB / g) I

Sensitivity of Moving Coil Galvanometer

The ratio of the change in deflection of the galvanometer to the change in current is the sensitivity of a Moving Coil Galvanometer. As a result, Sensitivity = d/di is written. A sensitive galvanometer is one that has a larger deflection for a small current. I = (g/nBA) × θ is the current in a Moving Coil galvanometer.

                     Therefore 

                             θ = (nBA/g) × I

                   Differentiating both sides we get

                             dθ/di = (nBA/g)

To sum up, the sensitivity of the Moving Coil Galvanometer increases by increasing the number of turns and the coil’s area, as well as the magnetic induction and the couple per unit twist of the suspension fiber.

Advantage of moving coil galvanometer

• The sensitivity of the moving coil galvanometer increases as the value of n, B, A increases.
• The eddy current produced in the frame of the moving coil galvanometer brings the coil to the rest because the coil is wound to the metallic frame. 

The disadvantage of moving coil galvanometer

• We cannot change the sensitivity of the moving coil galvanometer at will.
• Loading over a certain value can damage any type of galvanometer.

Conclusion

The moving coil galvanometer is an electrical device that is used for the measurement of the electrical current by connecting parallel low resistance and voltage by connecting high resistance in series. It is based on the principle that torque is experienced in the current-carrying conductor in the external magnetic field. Permanent horseshoe magnets, coil, soft iron core, pivoting spring, non-metallic frame, scale, and pointer make up a moving coil galvanometer.