Tangent galvanometer
= Tangent galvanometer
{title2=$\tan\theta=B_c/B_H$}
A current-measuring coil and compass arranged so that the coil's central <magnetic field> is perpendicular to the horizontal terrestrial field. With unchanged coil geometry and terrestrial field, the <electric current> ratio is the ratio of the tangents of the needle deflections. A small needle and controlled alignment make the central-field approximation useful.