The coil 2

The coil of a moving coil voltmeter is 40 mm long, 30 mm wide, and has 100 turns. The control spring exerts a torque of 240 x 10-6 N-m when the deflection is 100 divisions on a full scale if the flux density of the magnetic field in the air gap is 1.0 Wb/m². How can we modify the coil arrangement to give one volt per division? The resistance of the voltmeter coil may be neglected.

Correct answer:

L =  1 H

Step-by-step explanation:

$$\begin{gathered} a=40\text{mm}\to \text{m}=40:1000\text{m}=0.04\text{m} \\ b=30\text{mm}\to \text{m}=30:1000\text{m}=0.03\text{m} \\ N=100 \\ M=240\cdot 10^{-6}\doteq 0.0002\text{Nm} \\ \Phi =1.0{\text{Wb/m}}_2 \\ L=1=1\text{H} \end{gathered}$$

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