When a bar magnet is suspended in two Magnetic fields B and B_{h}, it comes to rest making an angle θ with the direction of B_{h}.

figure (1) figure (2)

From figure (1) B=B_{h} tanθ. This is known as tangent law of magnetism.

Theory:

Tangent galvanometer is an early measuring instrument for small electric currents. It consists of a coil of insulated copper wire wound on a circular non-magnetic frame. Its working is based on the principle of the tangent law of magnetism. When a current is passed through the circular coil, a magnetic field (B) is produced at the center of the coil in a direction perpendicular to the plane of the coil. The TG is arranged in such a way that the horizontal component of earth’s magnetic field (B_{h}) is in the direction of the plane of the coil. The magnetic needle is then under the action of two mutually perpendicular fields. If θ is the deflection of the needle, then according to tangent law,

Let I is the current passing through the coil of radius a with n turns, then the field generated by the current carrying circular coil is,

Equating (1) and (2), we get,

The left hand side of equation (4) is a constant and is called the reduction factor (K) of the given tangent galvanometer.

Now from equation (3) & (5), the horizontal intensity of earth’s magnetic field Bh is,

Applications:

1. T.G can be used to measure the magnitude of the horizontal component of the geomagnetic field.

2. The principle can be used to compare the galvanometer constants.

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Amrita University, India 2009 - 2015
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