Moving coil Galvanometer
Moving coil Galvanometer:- m.imp
A Galvanometer is a device which is used to detect the current in a circuit.
Principle:-
The Galvanometer is based on the principal that a current carrying coil placed in external magnetic field experience a torque.
Construction:- –
A moving coil Galvanometer consist of
- A coil PQRS consist of a large number of turns of fine insulated copper wire wounded over a non magnetic metallic frame. The coil is suspended from a movable torsion head H by mean of a fine phosphor bronze strip. The lower end of coil is connected to fine spiral spring S’. An arrangement of strong north & South Pole is fixed on both sides of the coil.
- A soft iron core L of spherical or cylindrical shape is placed between the coils.
- A circular Mirror M is attached to the Phosphor bronze strip, to measure the deflection of the coil.
- The torsion head is connected to a binding terminal T1 & spring S’ is connected to another binding terminal T2.
Theory:-
Let N is number turns in coil, A is area of coil, B is magnetic field I is the amount of current following through the coil, then torque acting on the coil is
Here θ is angle between normal to the plane of coil & applied magnetic field.
Since the field is radial
i,eθ = 900 
Due to this torque the phosphor bronze wire suffer twist, if θ is angle of twist & k is moment of restoring couple per unit angular twist then moment of restoring couple = k θ
In equilibrium
Or
Here 
= K = a another constant
SoI 
θ
Hence current through the coil is directly proportional to the deflection of the coil.
Sensitivity of a moving coil Galvanometer
A galvanometer is said to be more sensitive if it show large deflection even for a small change in current is passed through it.
- Current sensitivity: – it is defined as the deflection produced in the galvanometer when a unit current is passed through it.
I,e. Current sensitivity
- Voltage sensitivity: – It is defined as the deflection produced in the galvanometer when a unit potential difference is applied across it.
Voltage sensitivity Vs=
=
=
Clearly voltage sensitivity =
The sensitivity of a moving coil galvanometer can be increased by
By increasing number of turns in the coil.
By increasing the magnetic field.
By increasing the area of the oil.
By decreasing the value of torsion constant R
Advantage of moving coil Galvanometer:-
- It tells about the deflection due to current in a circuit.
Disadvantage of a moving coil Galvanometer:
- Its sensitivity cannot be changed at will.
- It is damaged by overloading.
Q16.What is the importance of a radial magnetic field and how is it produced?
In a radial magnetic field magnetic torque remains maximum for all positions of the coils. It is produced due to cylindrical pole pieces and soft iron core.
24 Conversion of a Galvanometer into an ammeter m.imp:-
(An ammeter is a device which is used to measure the current in a circuit, as current flow in wire so it is connected in series. An ammeter is formed by connecting a low resistance (shunt) in parallel to Galvanometer. The value of shunt depends upon the current which is to be measured.)
As shunt & Galvanometer are connected in parallel
So
G = (I – ) S
Or
So by connecting a shunt across galvanometer, we get an ammeter of desired range,
Moreover
= 
As the deflection in galvanometer is directly proportional to & hence perpendicular so the current can be measured. Also the effective resistance is
R – 
< S
- An ideal ammeter has zero resistance.(imp:-MCQ)
- Higher the range of ammeter to be prepared lower should be the value of shunt.
- The range of ammeter can be increased but cannot be decreased.
Q17. A galvanometer has a resistance of 30Ω. It gives full scale deflection with a current of 2 mA. Calculate the value of the resistance needed to convert it into an ammeter of range 0-0.3 A.
Ans: As 
Substitution so 
25 Conversion of a galvanometer into a Voltmeter:- m.imp
(Voltmeter is a device which is used to measure the potential difference between two points in a circuit so connected in parallel. A voltmeter is formed by connecting a high resistance in series a Galvanometer.)
The total resistance in the Circuit is 
Now by ohm’s Law
= 
Or
So by connecting a high resistance in Series with G, we can get a voltmeter of desire range. As deflection is proportional to & hence V. So the scale can be arranged to measure potential difference. Hence a voltmeter is a high resistance galvanometer.
Its effective resistance is 
- An ideal voltmeter should have infinite resistance.(imp:-MCQ)
Why is it that while using a moving coil galvanometer as a voltmeter a high resistance in series is required whereas in an ammeter a shunt is used?
Reason:
Voltmeter: Thisensuresthataverylowcurrentpassesthroughthevoltmeterandhencedoesnotchange(much)the original potential difference to be measured.
Ammeter: This ensures that the total resistance of the circuit does not change much and the current flowing remains (almost) at its original value.
Example8. In the circuit the current is to be measured. What is the value of the current if the ammeter shown (a) is a galvanometer with a resistance 
; (b) is a galvanometer described in (a) but converted to an ammeter by a shunt resistance r s = 0.02Ω; (c) is an ideal ammeter with zero resistance?
Solution (a) Total resistance in the circuit is
.
Hence
- Resistance of the galvanometer converted to an ammeter is,
Total resistance in the circuit is, 
Hence,
- For the ideal ammeter with zero resistance,
