Draw a labelled diagram of a moving coil galvanometer. State the principle on which it works.
Deduce an expression for the torque acting on a rectangular current carrying loop kept in a uniform magnetic field.Write two factors on which the current sensitivity of a moving coil galvanometer depends.
Principle: A current carrying coil suspended in a magnetic field experiences a torque.
![image](https://ask.learncbse.in/uploads/db3785/original/3X/9/f/9f40d75c7f54047258c519b55a7041ad03d7f998.png)
Let I be the current through the loop PQRS.
a, b = Sides of the rectangular loop
A = ab = Area of the loop
N = Number of turns in the loop
![image](https://ask.learncbse.in/uploads/db3785/original/3X/9/3/93fe2f8aecafcc6fe5ed08bf934e25b85d9c0b92.png)
According to Fleming’s left hand rule, the side AB experiences a normal inward force, F1= IaB and side CD experiences a normal outward force, F2 = IaB. These two equal and opposite forces form a couple, which exerts a torque given by,
= Force × Perpendicular distance
= IaB × b sin![](http://images.topperlearning.com/topper/questions/220188_image245.gif)
= IBab sin![](http://images.topperlearning.com/topper/questions/220188_image245.gif)
τ = IBA sin![](http://images.topperlearning.com/topper/questions/220188_image245.gif)
For N turns,
τ = NIBA sin![](http://images.topperlearning.com/topper/questions/220188_image245.gif)
The current sensitivity of a moving coil galvanometer depends on
(i) number of turns in the galvanometer coil
(ii) torsional constant of the suspension fibre.