Radial Field (Top view of the Galvanometer)
A moving coil galvanometer is an instrument used for detection and measurement of small electric currents.
Principle
A current carrying conductor placed in a magnetic field experiences a torque.
Construction
It consists of N turns of coil PQRS wound over a non-magnetic metallic frame usually rectangular or circular. The coil is suspended from a torsion head H by means of phosphor bronze strip in a magnetic pole by pieces N and S. A mirror M is attached to the phosphor bronze strip. The other end of the coil is connected to a hairspring, which in turn is connected to an external terminal.
The concave or cylindrical pole pieces ensure that the plane of coil is parallel to B thus (cosq = 1 and sina = 1) in the expression t = NIBA cosq and t = NIBA sina .The whole arrangement is enclosed in non-magnetic case, which is provided with leveling screws. The torsion head is connected to terminal T1. The galvanometer can be connected to the circuit through terminals T1 and T2.
Working
When a current is passed through the coil in the direction PQRS, then the coil experiences a torque
t = NIBA sina
Since the magnified field is radial the plane of the coil is parallel to the magnetic field 
such that a = 90o and hence t = NIBA.
The coil rotates and the phosphor bronze strip gets twisted. As a result a restoring torque comes into play trying to restore the coil back to original position.
If f be the twist produced in the strip and C be the restoring torque per unit twist then the restoring torque = Cf.
In equilibrium,
where G is the galvanometer constant.
This shows that galvanometer has a linear scale and detects the presence of current.
While designing the instrument one should ensure that
1) The moving coil should have large number of turns and large area as this increases the turning effect. The magnets should be strong and soft iron cylinder increases the field and changes the shape of the field. This radial field as said earlier renders a uniform scale on the instrument.
The following diagram shows two spiral springs which control how far the coil turns and are made from very fine hair springs which have a weak unwinding effect. The pointer sometimes moves over a scale with evenly spaced graduations or the scale is replaced by a small mirror which deflects a ray of light falling on it.
Note:
Current sensitivity of galvanometer is the deflection produced for a unit current flowing through it. i.e.,
A sensitive galvanometer should have a long deflection for small current through it.
Voltage sensitivity is the deflection produced for a unit voltage applied across the two terminals of the galvanometer
Conversion of Galvanometer into Ammeter
A galvanometer can be converted into an ammeter (device measuring the current flowing through a conductor) by connecting a low resistance (called shunt resistance) in parallel to the galvanometer as shown in the figure.
Let Rg represent the resistance of the galvanometer, ig the current which produces full scale deflection in the galvanometer. Since the shunt is connected in parallel to the galvanometer, the potential difference across galvanometer = potential difference across shunt.
Where as S is the shunt resistance.
This works as an ammeter of range 0 to iA. The effective resistance of shunt and galvanometer is
It is to be noted that as the value of S is low, the parallel combination has a much lower resistance. It is for this reason that ammeter have very low resistance.
'Ideal Ammeters have zero resistance'.
Conversion of Galvanometer to Voltmeter
A Galvanometer can be converted into a voltmeter by connecting a high resistance in series with a galvanometer as shown.
In series connection the current through the galvanometer is same as that due to the resistance.
The total resistance of voltmeter
'Ideal Voltmeter has infinite resistance'.
