A) Tangent galvanometer is a moving magnet type galvanometer
B) Tangent galvanometer works on tangent law

In tangent galvanometer, a small magnetic needle is pivoted at the centre of this magnetic box. Aluminum pointer is attached at right angles to the needle. It shows deflection on circular scale which is marked from 0⁰  to 90⁰ . 

Tangent Law states that when a bar magnet is left suspended in the influence of a magnetic field at right angle with the Earth's Magnetic Field's Horizontal Component, it comes to rest at an angle such that $\tan \mathrm{\theta }=\mathrm{B}/{\mathrm{B}}_{\mathrm{H}}$. A tangent galvanometer works on tangent law.