A tangent galvanometer is a device used to measure the strength and direction of a magnetic field. It consists of a circular coil of wire, mounted on a horizontal axis, with a small magnetic needle placed at the center of the coil. When a current flows through the coil, it produces a magnetic field that deflects the needle.

The angle of deflection of the needle is proportional to the strength of the magnetic field. The device is called a tangent galvanometer because the angle of deflection is determined by the tangent of the angle between the needle and the magnetic field lines. The tangent galvanometer was commonly used in the 19th century as a tool for measuring electric currents and for demonstrating the principles of electromagnetism.