The Amp Meter or Ammeter measures the current that flows through an electrical circuit. The circuit is ‘opened’ and the meter (containing two input terminals, leads, or connectors) is inserted in the opening and the current flowing though the meter presented on an analogue or digital display. Inside the meter, a very low resistance is connected across the input terminals and a high impedance input voltmeter measures the potential across this resistance, which (according to Ohms Law) is directly proportional to the current flowing through it. The resistance, known as the shunt resistance, must be very small so that its impedance does not affect the performance of the rest of the circuit. The measuring unit that determines the potential across the shunt resistance (a volt meter) usually involves a high input impedance amplifier such as a field effect transistor (FET) or operational amplifier, which drives the display. The range of current that is measured can be changed by choosing the pertinent shunt resistor to suit the current range required. The shunt resistance can usually be chosen by a suitable switch.

Further Information on this Topic can be found in the books of R.P.W. Scott by following the listed links