In case of tangent galvanometer
A) The galvanometer reduction factor depends on earth’s magnetic field
B) External fields have effect on T.G  (Tangent Galvanometer) and therefore T.G cannot be used in mines.

For a tangent galvanometer, 

$\mathrm{i}=\mathrm{K} \mathrm{tan\theta }$

Where, K is reduction factor of tangent galvanometer, which is given as,

$\mathrm{K}=\frac{2{\mathrm{rB}}_{\mathrm{H}}}{{\mathrm{\mu }}_0\mathrm{n}}$

$\Rightarrow \mathrm{K}\propto {\mathrm{B}}_{\mathrm{H}}$

In tangent galvanometer, net magnetic field of coil and earth's horizontal component is used. So, magnetic field other than these can affect the measurement of the instrument. Therefore, tangent galvanometer cannot be used in mines.