Describe Theory of Tangent Galvanometer - QS Study
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Theory of Tangent Galvanometer: When the plane of the coil is placed parallel to the horizontal component of Earth’s magnetic induction (Bh) and a current is passed through the coil, there will be two magnetic fields acting perpendicular to each other: (1) the magnetic induction (B) due to the current in the coil acting normal to the plane of the coil and (2) the horizontal component of Earth’s magnetic induction (Bh) (Figure).

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Due to these two crossed fields, the pivoted magnetic needle is deflected through an angle θ. According to tangent Law,

B = Bh tan θ … 1)

If a current I passes through the coil of n turns and of radius a, the magnetic induction at the centre of the coil is

B = (μ0 NI / 2α) … (2)

Substituting equation (2) in equation (1)

μ0 NI / 2α = Bh tan θ

so, I = (2a Bh / μ0 N) tan θ

I = K tan θ

where, K = 2a Bh / μ0 N; is called the reduction factor of the tangent galvanometer. It is a constant at a place. Using this equation, current in the circuit can be determined.

Since the tangent galvanometer is most sensitive at a deflection of 450, the deflection has to be adjusted to be between 300 and 600.