A confusion with tangent galvanometer

[Rishavutkarsh]

When no current is passed through the coil it shows no deflection i.e. B is in the direction shown. But according to theory both fields should be perpendicular (Earth's horizontal field and the one due to coil).
But I am confused because the coil would ideally create a magnetic field that is either parallel or anti-parallel to Earth's Horizontal magnetic field. Is there somewhere I made a blunder or am I missing something?

 

[mfb]

But I am confused because the coil would ideally create a magnetic field that is either parallel or anti-parallel to Earth's Horizontal magnetic field.

If that happens, rotate the device by 90 degrees, then they are perpendicular to each other. The orientation of the magnet without current helps to find the right orientation.

 

[Philip Wood]

What do you mean by "ideally" in this case? I simply don't know why you say that the parallel fields or antiparallel fields configurations are "ideal".

 

[Rishavutkarsh]

If that happens, rotate the device by 90 degrees, then they are perpendicular to each other. The orientation of the magnet without current helps to find the right orientation.

Yeah that's what I thought too but as far as I know this is the way this device is used (Without rotating it).

What do you mean by "ideally" in this case? I simply don't know why you say that the parallel fields or antiparallel fields configurations are "ideal".

I mean the field lines created by a coil are exactly perpendicular to it's plane 'ideally'.

 

[Philip Wood]

I mean the field lines created by a coil are exactly perpendicular to it's plane 'ideally'.

That's correct, but it's not what you said before!
You orientate the coil with its plane in the Earth's magnetic North-South meridian plane. So the coil's field (at its centre) is in the East-West direction, that is at right angles to the Earth's field. With no current through the coil a compass (or magnetometer) magnet placed at the centre of the coil will point magnetic North. As soon as you have a current through the coil it will deflect from North.

 

[Rishavutkarsh]

I got it that's correct; Thank you. I got confused a bit by that compass and magnet.