A tangent galvanometer is used for detection and measurement of low electric currents. It is based on tangent law in magnetism, according to which $F=G \tan \theta$, where $\theta$ is angle with $H$ made by a magnet suspended freely under the combined effect of $H$ and $(F \perp H) .$ Now, $F=\frac{\mu_{0}}{4 \pi} \frac{2 \pi n I}{r}$, where $n$ is number of turns in the coil of radius, $r$ earrying current $I$. From, $F=H \tan \theta$, we get $$ I=\frac{2 r H}{\mu_{0} n} \tan \theta=K \tan \theta $$ where $K$ is called reduction factor of tangent galvanometer.
In the use of tangent galvanometer, $H$ is
(a) earth's magnetic ficld
(b) horizontal component of earth's magnetic field
(c) vertical component of earth's magnetic ficld
(d) None of the above