01-06-2017, 09:29 AM
Tangent galvanometer
A tangent galvanometer is an early measuring instrument used for measuring electrical current. It works by using a compass needle to compare a magnetic field generated by the unknown current to the Earth's magnetic field. It gets its name from its working principle, the tangent law of magnetism, which states that the tangent of the angle made by a compass needle is proportional to the ratio of the forces of the two perpendicular magnetic fields. It was first described by Claude Pouillet in 1837.
A tangent galvanometer consists of a coil of insulated copper wire wound in a non-magnetic circular frame. The frame is mounted vertically on a horizontal base provided with leveling screws. The coil can rotate on a vertical axis passing through its center. A compass box is mounted horizontally in the center of a circular scale. It consists of a tiny and powerful magnetic needle pivoted in the center of the coil. The magnetic needle is free to rotate in the horizontal plane. The circular scale is divided into four quadrants. Each quadrant is graduated from 0 ° to 90 °. A long thin aluminum pointer attaches to the needle at its center and at right angles to it. To avoid errors due to parallax, a flat mirror is mounted under the compass needle.
In operation, the instrument is rotated first until the Earth's magnetic field, indicated by the compass needle, is parallel to the plane of the coil. Then the unknown current is applied to the coil. This creates a second magnetic field on the axis of the coil, perpendicular to the Earth's magnetic field. The compass needle responds to the vector sum of the two fields, and deviates by an angle equal to the tangent of the relationship of the two fields. From the reading angle of the compass scale, the current can be found in a table. [2] Current power cables have to be wound up in a small propeller, like the tail of a pig, otherwise the field due to the wire will affect the compass needle and you will get an incorrect reading.
