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$$\text{FIGURE VII.5}$$
The current is led into the coil of $$N$$ turns through a spiral spring of torsion constant $$c$$. The coil is between two poles of a specially-shaped magnet, and there is an iron cylinder inside the coil. This ensures that the magnetic field is everywhere parallel to the plane of the coil; that is, at right angles to its magnetic moment vector. This ensures that the deflection of the coil increases linearly with current, for there is no $$\sin \theta$$ factor. When a current flows through the coil, the torque on it is $$NABI$$, and this in counteracted by the torque $$c\theta$$ of the holding springs. Thus the current and deflection are related by
$NABI = c\theta.$