An ammeter is an instrument used to measure the electric current in a circuit, which is measured in amperes (A) . It is connected in series with the circuit in which the current is to be measured/20%3A_Circuits_and_Direct_Currents/20.4%3A_Voltmeters_and_Ammeters). An ammeter usually has low resistance so that it does not cause a significant voltage drop in the circuit being measured. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters. The device has an inbuilt fuse that protects the device from heavy current. If substantial current flows through the ammeter, the fuse will break. The ammeter cannot measure the current until the new fuse is replaced. Ammeters vary in their operating principles and accuracies. The D’Arsonval-movement ammeter measures direct current flowing through a coil suspended between the poles of a magnet with accuracies of from 0.1 to 2.0 percent. The electrodynamic ammeter uses a moving coil rotating in the field produced by a fixed coil. It measures direct and alternating current (by using a rectifier to convert the AC to DC) with accuracies of 0.1 to 0.25 percent. The thermal (or hot-wire) ammeter, used primarily to measure AC with accuracies of 0.5 to 3 percent, heats a piece of wire, and the current is indicated by how much the wire expands. Digital ammeters, with no moving parts, use a circuit such as the dual slope integrator to convert a measured analog (continuous) current to its digital equivalent. Many digital ammeters have accuracies better than 0.1 percent. In a circuit, the ammeter is represented by the letter “A” .