Moving coil galvanometer is an instrument used of detection and measurement of small electric currents.
Principle its working is based on the fact that when a current carrying coil is paved in a magnetic field it experiences a torque.
Construction: It consists of a coil PQRS1 having large number of turns of insulated copper wire the coil is wound over a non-magnetic metallic frame (usually brass) which may be rectangular or circular in shape. The coil is suspended from a movable torsion head H by means of phosphor bronze strip in a uniform magnetic field produced by two strong cylindrical magnetic pole pieces N and S.
The lower end of the coil is connected to one end of a hair spring S of quartz or phosphor bronze. The other end of this highly elastic spring S is connected to a terminal T2 L is soft iron core which may be spherical if the coil is circular and cylindrical if the coil is rectangular it is so held within the coil that the coil can rotate freely without touching the iron core or pole pieces. This makes the magnetic field linked with coil to be radial field the plane of the coil in all position remains parallel to the direction of magnetic field M is concave mirror attached to the phosphor bronze strip this helps us to not the arrangement the whole arrangement is enclosed in a non - metallic case to avoid disturbance due to air etc. the case is provided with leveling screws at the base. The torsion head is connected to terminal T1. Th egalvaomter can be connected to the circuit through terminal T1 and T2.
Suppose the coil PQRS is suspended freely in the magnetic field.
Let I = length PQ or RS of the coil,
B = breadth QR or S1P1 of the coil,
N = number of turns in the coil.
Area of each turn of the coil A = I X b
Let B = strength of the magnetic field in which coil is suspended.
I = current passing through the coil in the direction PQRD
Let at any instant a be the angle which the normal drawn on the plane of the coil makes with the direction of magnetic field.
As already discussed the rectangular coil carrying current when played in the magnetic field experiences a torque who semagntidue is given by
τ = n / BA sin a.
If the magnetic field is radial the plane of the coil is parallel to the direction of the magnetic field then a = 90** and sin a = 1 τ = n/BA
Due to this torque the coil rotates. The phosphor bronze strip gets twisted. As a result of it's a restoring torque comes into play in the phosphor bronze strip which would try to restore the coil back to its original position.
Let θ be the twist produced in the phosphor bronze strip due to rotation of the coil and k be the restoring torque per unit twist of the phosphor bronze strip then
Total restoring torque produced = k θ
In equilibrium position of the coil,
Deflecting torque = restoring torque
∴ n/BA = k θ
Or I = K / nBA θ or I = Gθ
Where k / nBA = G = a constant for a galvanometer.
It is known as galvanometer constant.
Hence I ∝θ
It means the deflection produce is proportional ot the current flowing through the galvanometer. Such a galvanometer has a linear scale.
Current sensitivity of a galvanometer is defined as the deflection produce in the galvanometer when a unit current flows through it.
If θ is the deflection in the galvanometer when current I is passed through it then
I = θ / I = nBA / k
(∴ I = k / nBA θ)
The unit of current sensitivity is rad A or div. A-1
Voltage sensitivity of a galvanometer is defined as the deflection produced in the galvanometer when a unit voltage is applied across the two terminals of the galvanometer.
Let V = voltage applied across the two terminals of the galvanometer,
θ - Deflection produced in the galvanometer
Then voltage sensitivity,
V1 = θ/V
Fi R = resistance of the galvanometer
I = current through it.
Then V = IR
∴ Voltage sensitivity
V = θ IR = nBA / kR
The unit of V is rad V -1 or div V -1
Conditions for a sensitive galvanometer,
A galvanometer is said to be very sensitive if it shows large deflection even when a small current is passed through it.
From the theory of galvanometer
θ = nBA / k I
For a given value of I. θ will be large if nBA/k is large it is so if
N is large
B is large
A is large and
K is small
The value of n cannot be increased beyond a certain limit because it results in an increase of the resistance of the galvanometer and salsa makes the galvanometer bulky. This tends to decrease the sensitivity. Hence n cannot be increased beyond a limit.
The value of B can be increased by using a strong horse shoe magnet.
The value of cannot be increased beyond a limit because in that case the coil will not be in a galvanometer bulky and unmanageable.
The value of k can be decreased. The value of k depends upon the nature of the material used as suspension strip. The value of k is very small for quartz or phosphor bronze. The is why in sensitive galvanometer quartz or phosphor bronze strip is used as a suspension strip.
ExpertsMind.com - Moving Coil Galvanometer Assignment Help, Moving Coil Galvanometer Homework Help, Moving Coil Galvanometer Assignment Tutors, Moving Coil Galvanometer Solutions, Moving Coil Galvanometer Answers, Electromagnetism Assignment Tutors