(Wikipedia excerpt:…”…An ammeter (A) ….. is a measuring instrument used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the name. The (A) is usually connected in series with the circuit in which the current is to be measured. An (A) 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 milli (A) or micro (A) . Early (A) were laboratory instruments that relied on the Earth’s magnetic field for operation. By the late 19th century, improved instruments were designed which could be mounted in any position and allowed accurate measurements in electric power systems. It is generally represented by letter ‘A’ in a circuit.”)