A coil of $N$ turns, area $A$, in radial magnetic field $B$: torque $\tau = NIAB\sin\theta$. With concentric pole pieces creating a radial field: $\sin\theta = 1$ always, so $\tau = NIAB$. At equilibrium: $NIAB = k\phi$ where $k$ is the torsional constant and $\phi$ is the deflection. Current sensitivity $= \phi/I = NAB/k$. Voltage sensitivity $= \phi/V = NAB/(kR)$. High sensitivity requires: large $N$, large $A$, strong $B$, and small $k$ (soft suspension).