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.