The tangent galvanometer was one of the most frequently used mobile magnetic needle galvanometers in the middle of the 19th century. It was invented in 1837 by the French physicist Claude Servais Mathias Pouillet (1791 1868), together with the sine galvanometer. The two instruments are so called because the current intensity is proportional to the tangent and sine of the needle's rotation angle respectively. In its traditional form it is composed of a small magnetic needle suspended from a thread having a negligible elastic moment of torsion, in the centre of a circular vertical coil through which the current to be measured passes. The same thread holds an index that allows reading of the rotation angles on a fairly large graduated scale (radius 10÷15 cm).
The instrument is prepared for measurements by rotating it around a vertical axis until the magnetic needle, which without current is subject to the earth's magnetic field only, lies on the plane of the coil. Then, on applying the current to be measured to the instrument, the magnetic needle becomes subject to the simultaneous action of the horizontal component of the earth's magnetic field, which lies on the plane of the coil, and the field created by the coil, which is directed perpendicularly to that plane. ...continues