The sensitivity of the galvanometer

Question

I came across this problem in my text book:

And the solution in my book is like this:

sensitivity= θ/i = 60/30 = 2 deg/mA

I don't understand why it used the angle 60 (the angle between the coil face and the field lines) instead of the angle 30 (the angle between the normal to the coil and the field lines). My book defines sensitivity as:

The sacale deflection per unit current intensity passes through the coil of the galvanometer.

But still I don't understand what is the relation between the angle 60 degrees and the scale deflection, could you explain it for me?

Answer 1

You first have to decide which position is the current equals zero position.
Is it position A or position B?

Well it has to be position A otherwise when a current flows through the coil there will be no torque acting on the coil to make it rotate.

If is is position A then the angle of rotation is the angle the face of the coil makes with the magnetic field lines.