Measuring instruments

Operating principle of analogue ammeters and voltmeters are almost identical. In an ammeter, torque production is by the current to be measured and in voltmeter; torque production is by a current which is proportional to the voltage to be measured. That is why we used to say that both voltmeter and ammeter are current measuring devices. The commonly used instruments as ammeters and voltmeters are PMMC (for DC measurements only), Moving Iron, Electrodynamometer, Hot wire, Thermocouple, Induction type (for AC measurements only), Electrostatic and Rectifier. Moving coil and moving iron type are the most common and cheaper variety. Accurate measurements of both AC and DC currents/voltages possible with this type. Electrodynamometer readings are independent of the frequency. Electrostatic instruments consume less energy. The induction type is commonly used in watt-hour meters.