IIT JEE , AIEEE Forum - Physics , General

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When current is passed through the tangent galvanometer a magnetic field is created at its corners given by $B={\mu_0 nI\over 2r}$ where I is the current in ampere, n is the number of turns of the coil and r is the radius of the coil.

If the TG is set such that the plane of the coil is along the magnetic meridian i.e. B is perpendicular to $B H$ ( $B H$ is the horizontal component of the Earth's magnetic field), the needle rests along the resultant. From tangent law, $B = B H T a n θ$ , i.e.

${\mu_0 nI\over 2r} = B_H tan\theta$

or

$I=({2rB_H \over {\mu_0 n}})Tan\theta$

or $I = K T a n θ$ , where K is called the Reduction Factor of the tangent galvanometer.

The value of $θ$ is taken at 45 degrees for maximum accuracy.

Therefore, the answer is London, as it is closest to latitude of 45 degrees north (or south, if it was appropriate).

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