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  4. A moving coil galvanometer has a coil with 175 turns and area 1 cm2. It uses a torsion band of torsion constant 10-6 N-m/rad. The coil is placed in a magnetic field B parallel to its plane. The coil deflects by 1° for a current of 1 mA. The value of B (in Tesla) is approximately :

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Q. A moving coil galvanometer has a coil with $175$ turns and area $1\, cm^2$. It uses a torsion band of torsion constant $10^{-6}\, N-m/rad$. The coil is placed in a magnetic field $B$ parallel to its plane. The coil deflects by $1^{\circ}$ for a current of $1\, mA$. The value of $B$ (in Tesla) is approximately :

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Solution:

$\tau =\vec{M} \times\vec{B} $
$ C \theta =i NAB $
$ 10^{-6} \times\frac{\pi}{180} = 10^{-3} \times10^{-4} \times175 \times B $
$ B =10^{-3} $ Tesla.