Measuring Instruments

PMMC, moving iron and digital instruments — how analog and digital meters work.

Easy

PMMC (permanent magnet moving coil) — accurate DC measurement, linear scale; with a rectifier for AC.
Moving iron — rugged, works on AC and DC, non-linear scale; panel meters.
Electrodynamometer — true RMS; used in wattmeters.
Digital meters — ADC-based; high accuracy and input impedance (typically 10 MΩ).

Range extension: a shunt resistor for ammeters, $R_{sh} = \dfrac{I_m R_m}{I - I_m}$ ; a multiplier resistor for voltmeters, $R_s = \dfrac{V}{I_m} - R_m$ .

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