ammeter Instrument for measuring current. The main types are moving-coil, moving iron, and thermoammeter.

Moving coil ammeter: Schematic diagram of the ammeter is shown infig. When current is passed through the coil, kept in a magnetic field, the coil experiences a torque. The pole pieces provide the magnetic field. These are made semi-circular so that the radial field is produced. The restoring couple provided by suspension wires balances the torque produced. The rotation produced in the coil is proportional to the current. By calibrating the rotation unknown current can be measured. It can only be used for dc, but can be adopted for ac by using a rectifier*.

Moving-iron ammeter: A piece of soft iron moves in a magnetic field created when the current to be measured flows through a fixed coil. They can be used for ac and dc, but are less accurate. However they are more robust.

Thermoammeter: It can be used for ac and dc. The current is passed through a resistor. The temperature of the resistor rises as a result of Joule’s heating. A thermocouple is kept in contact with the resistor. The emf developed in the thermocouple is measured using a galvanometer. The emf developed is calibrated to give directly the value of the current.