Tangent galvanometer by Elliott Bros.

Tangent galvanometer by Elliott Bros., Helmholtz type with pivoted needle. Helmholtz refined the tangent galvanometer (first described 1837) in 1849, adding a second outer coil in order to make the magnetic field between the coils, where the galvanometer needle sits, essentially uniform. When at rest, the galvanometer needle and coils are parallel to each other and to the Earth's magnetic field. When current flows, the needle deflects, and the amount of current flowing is exactly proportional to the trigonometric tangent of the angle through which the needle has turned.