An ammeter is an instrument used to measure the flow of electricity in the form of current in a circuit. It measures the electric current flow through it by being placed in series with a circuit element. Ammeters are designed to offer very low resistance to the current, so that it does not appreciably change the circuit it is measuring. The SI unit of measuring electric current is ampere, hence the name ammeter. An ammeter usually has low resistance so that it does not cause a significant voltage drop in the circuit being measured. Ammeters vary in their operating principles and accuracies. They can measure either direct (DC) or alternating (AC) electric current, in amperes. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters. Ammeters must not be connected directly across a voltage source since their internal resistance is very low and excess current would flow.