Tangent galvanometer
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A 'tangent galvanometer' is a measuring instrument used for the measurement of current (electricity). It works on the basis of tangent law of magnetism.
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[edit] Construction
A TG consists of a circular coil of insulated copper wire wound on a circular non magnetic frame. The frame is mounted vertically on a horizontal base provided with levelling screws on the base. The coil can be rotated on a vertical axis passing through its centre. A compass box is mounted horizontally at the centre of a circular scale. The compass box is circular in shape. It consists of a tiny, powerful magnetic needle pivoted at the centre 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 aluminium pointer is attached to the needle at its centre and at right angle to it. To avoid errors due to parallax a plane mirror is mounted below the compass needle.
[edit] Theory
When current is passed through the TG a magnetic field is created at its corners given by 
where I is the current in ampere, n is the number of turns of the coil and r is the radius of the coil.
If the TG is set such that the plane of the coil is along the magnetic meridian i.e. B is perpendicular to BH (BH is the horizontal component of the Earths magnetic field), the needle rests along the resultant. From tangent law, B = BHTanθ, i.e.
or
or I = KTanθ, where K is called the Reduction Factor of the TG.
[edit] Important things to remember while performing experiment
The value of θ is to be taken at around 45 degrees for maximum accuracy. The wire connected to the Tangent Galvanometer has to be wound, other wise the field due to the wire will affect the deflection and hence wrong reading will be obtained.
[edit] Geomagnetic field measurement
A tangent galvanometer can also be used to measure the magnitude of the horizontal component of the geomagnetic field. When used in this way, a low-voltage power source, such as a battery, is connected in series with a rheostat, the galvanometer, and an ammeter. The galvanometer is first aligned so that the coil is parallel to the geomagnetic field, whose direction is indicated by the compass when there is no current through the coils. The battery is then connected and the rheostat is adjusted until the compass needle deflects 45 degrees from the geomagnetic field, indicating that the magnitude of the magnetoc field at the center of the coil is the same as that of the horizontal component of the geomagnetic field. This field strength can be calculated from the current as measured by the ammeter, the numer of turns of the coil, and the radius of the coils.
