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A moving-coil galvanometer (resistance $G$, full-scale deflection current $I_g$) can be converted into an ammeter or voltmeter:

Ammeter (measures current, connected in series): Add a low-resistance shunt $S$ in parallel with the galvanometer. $S = GI_g/(I - I_g)$ where $I$ is the desired full-scale range. The shunt carries most of the current, protecting the galvanometer. Effective resistance = $GS/(G+S) \approx S$ (very low, minimizing circuit disturbance).

Voltmeter (measures voltage, connected in parallel): Add a high resistance $R_s$ in series. $R_s = V/I_g - G$ where $V$ is the desired full-scale range. Effective resistance = $G + R_s \approx R_s$ (very high, drawing minimal current).

An ideal ammeter has zero resistance (no voltage drop across it). An ideal voltmeter has infinite resistance (draws no current). Real instruments are compromises. Galvanometer sensitivity depends on: number of turns, magnetic field strength, area of the coil, and torsional constant of the suspension.

For multi-range ammeters and voltmeters, different shunts or series resistors are switched in. The ammeter reading is always slightly less than the true current (voltage drop across it), and the voltmeter reading is slightly less than the true voltage (it draws some current).