An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes (A), hence the name.
Neither type of basic dc PMMC meter movement will correctly indicate an alternating current. There are, however, four basic types of meter movement that will indicate ac values:-
An electrodynamic meter uses an electromagnet instead of the permanent magnet. This instrument can respond to both alternating and direct current and also indicates true RMS for AC.
Principle of working
The operation is basically based on the mechanical interaction between two conductors in which an electric current is present. It consists of a measuring transducer, which converts the quantity being measured into an alternating or direct current, and a measuring mechanism of an electrodynamic system.
Electrodynamic meters have a fixed coil, inside of which is a movable coil and a pointer on a spindle. A torque is developed around the spindle by the interaction of the currents in the two coils and is proportional to the product of the effective values of the currents. A counter torque is created by a spring connected to the spindle. The pointer comes to rest when the two torques are equal.
Thermocouple ammeter or RF meter uses thermocouple for measuring current. It is commonly used to measure current at high and ultra-high frequencies or radio frequencies.
Principle of working:
A thermocouple ammeter basically uses the phenomena used in a thermocouple to measure the current flowing in the circuit. The current, to be measured, when flows through the wire, heats it up, this temperature is then compared with the temperature of the resistance wire. The difference of the temperature induces an