A tangent galvanometer is an instrument used to measure the intensity of an electric current. Its essential component is a ring with one or more coils through which the current flows. At the geometric center of the ring is a magnetic needle. The needle rotates due to the interaction between Earth’s magnetic field and the magnetic field generated by the current flowing through the ring. When the instrument is positioned so that the two fields are perpendicular to each other, the tangent of the needle’s angle is proportional to the ratio of their intensities. Additionally, the tangent galvanometer could be used to measure the horizontal component of Earth’s magnetic field.

This type of instrument was first described in 1837 by Claude Pouillet (1790–1868).

Maciej Kluza, PhD (Museu da Universidade Jaguelônica, Collegium Maius)