Ammeters, as well as voltmeters and ohmmeters, are designed with the use of a sensitive current detector such as a galvanometer. An ammeter is placed in series with a circuit element to measure the electric current flow through it. The meter must be designed offer very little resistance to the current so that it does not appreciably change the circuit it is measuring. To accomplish this, a small resistor is placed in parallel with the galvanometer to shunt most of the current around the galvanometer. Its value is chosen so that when the design current flows through the meter it will deflect to its full-scale reading. A galvanometer full-scale current is very small: on the order of milliamperes.
Ammeters are non-mechanical, or digital, and use an analog to digital converter (ADC) to measure the voltage across the shunt resistor. The ADC is read by a microcomputer that performs the calculations to display the current through the resistor.
One problem with ammeters is the need for these meters to be inserted into the circuit and become part of it. Mistakenly placing ammeters in parallel with a circuit will blow the fuse, possibly damaging the ammeter and causing injury.
An ideal ammeter has zero resistance so that the the circuit in which it has been placed is not disturbed. An ideal ammeter is a short circuit. However, as with the voltmeter, no ammeter can ever be ideal, and therefore all ammeters have some ( hopefully) small internal resistance.