Sensitivity of Moving Coil Galvanometer

The galvanometer was discovered around the 1800s, after which many changes were made in its design. Some major types of galvanometers can be seen today: tangent, static, and ballistic galvanometers. D’Arsonval type galvanometers are the important galvanometers widely used in many fields. We call them the Weston type or moving coil type galvanometer. 

A Galvanometer is a mechanical device used to find the electric current flowing in a working volume. Or this instrument is used to measure the magnitude of the current flowing through it, which we also know as a moving-coil electric current detector. It consists of a magnetic needle, mainly used to measure intensity. It is mainly used to tell the position of the magnetic field; we can also call it the central violence of this device.

What is a Moving Coil Galvanometer?

It is a device used as an electromagnetic device, which is used to measure or quantify electric currents in working quantities. The most important thing about this device is that it is more accurate than microampere and helps measure electric current in working amounts. Coils, permanent horseshoe magnets, malleable iron cores, pointers, spindle springs and non-metal frames are used to manufacture this device. We also know the galvanometer as an ammeter; here, when the current passes through the circuit, its needle is deflected towards right angles; we will talk about this in detail later. 

Principle 

The principle work of galvanometers is to convert electrical energy into mechanical energy. The flowing current passes through its circuit, creating a magnetic field because a magnetic force is acting here. 

• The inner part of the galvanometer consists of a coil, where two wires are mounted on a copper plate and a horseshoe magnet mounted on top of it. 
• The plate attached to it is connected with voltage, which creates a magnetic field in the middle of the coil, whose job is to attract the magnet. 
• In this way, we create a magnetic field in the galvanometer.

Construction

It is a device made of rectangular coils, insulated with thin or fine copper wire, wound on a metal frame. The coil is free to rotate freely in its fixed axis. Here, a phosphor bronze bar is attached mainly to a movable torsion head, mainly used to suspend the coil in a radial magnetic field. 

It is mainly characterised by low-value properties of conductivity and torsion constant.

• Here one can see a cylindrical malleable iron core, symmetrically located inside the coil, whose primary function is to improve the strength of the magnetic field and make the field radial. 
• The coil has two ends, in which the bottom or part of the coil is attached to the phosphor bronze, and the other end is attached to the binding screw. 

• The spring helps to generate the counter-torque here, which is a magnetic torque.
• Its job is to produce a constant angular deflection.
• In addition, a plane mirror is used, whose primary role is to measure the coil’s deflection. It is primarily coupled to the suspension wire with the lamp and scale arrangement.

Working Principle

We’ll use a coil with a length of l and a width of b for this example. A current flows through a rectangular coil, creating a magnetic field in a permanent horseshoe magnet. The coil will always be parallel to the magnetic field in a radial magnetic field, mainly because QR and SP are always parallel to the field, which does not feel any force and PQ and RS forces on the magnetic field.

PQ=RS= l (where l is the length of the rectangular coil)

PS = QR = b (where b is the breadth of the rectangular coil)

 F=BII ( Each side’s force is different.)

Fleming’s left-hand rule states that forces are equal in magnitude but opposite in direction, work in a plane and act externally. Torque is created when these forces are equal and opposite.

Torque = Force × upright distance between the forces

τ = F × b

τ = BI l × b

τ = BI A ( as lb = A area of the coil)

τ = n BIA (If the coil is made up of ‘n’ turns)

When the torque is applied, the coil rotates at an angle. The twist produced by the coil’s rotation provides a proportional torque to the deflection.

τ = θ

τ = k θ (The restoring torque per unit twist is denoted by k.)

The restoring torque will balance the deflecting torque after the coil has reached equilibrium.

Deflecting torque = restoring torque

n BIA = k θ

I = ( k / NBA) θ

I = θ

As a result, the current in a moving coil galvanometer is proportional to the angle of the coil’s deflection.

Sensitivity of Moving Coil Galvanometer

The ratio of the change in deflection of the galvanometer to the change in current in the coil is used to calculate the sensitivity of a moving coil galvanometer.

S = dθ/dI

The galvanometer’s sensitivity is high, and the instrument shows a large deflection for a small amount of current. Current sensitivity and voltage sensitivity are the two most common types of this sensitivity.

• Current Sensitivity- The current deflection per unit deflection Current sensitivity/I is used to describe how sensitive a device is to current.

θ/I = nAB/k

• Voltage Sensitivity- Voltage sensitivity /V is the amount of deflection per unit voltage. In the equation = (nAB / k)I, divide both sides by V.

θ/V= (nAB /V k)I = (nAB / k)(I/V) = (nAB /k)(1/R)

The letter R denotes the effective resistance in the circuit.

It’s worth remembering that voltage sensitivity equals current sensitivity/coil resistance.

As a result, assuming R remains constant, voltage sensitivity equals current sensitivity.

Conclusion

The galvanometer, whose main job is to measure the current flowing through it, is well known for its sensitivity. It also incorporates a Wheatstone bridge circuit for better current measurement and zero deflection of the pointer. It is mostly used to assess the current value by connecting it in series with low resistance and parallel with high resistance. So that was all about the sensitivity of the moving coil galvanometer.