Question:
Maam can u please add in the topic moving coil galvanometer on youtube
Answer:
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 cosqand 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 torquet = 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.
Not what you are looking for? Go ahead and submit the question, we will get back to you.
