The tangent galvanometer was first described in an 1837 paper by Claude-Servais-Mathias Pouillet (1790-1868), who later employed this sensitive form of galvanometer to verify Ohm's law. To use the galvanometer, it is first set up on a level surface and the coil aligned with the magnetic north-south direction. This means that the compass needle at the middle of the coil is parallel with the plane of the coil when it carries no current. The current to be measured is now sent through the coil, and produces a magnetic field, perpendicular to the plane of the coil, and directly proportional to the current. The magnitude of the magnetic field produced by the coil is B; the magnitude of the horizontal component the earth's magnetic field is B'. The compass needle aligns itself along the vector sum of B and B' after rotating through an angle Ø from its original orientation. The vector diagram shows that tan Ø = B/B'. Since the magnetic field of the earth is constant, and B depends directly on the current, the current is thus proportional to the tangent of the angle through which the needle has turned.