Ampèremeter

An ammeter is an instrument used to measure the current values in a circuit. It is connected in series with the element across which current must be measured.

An ideal ammeter has zero resistance; however, real ammeters have some finite but negligible resistance.

If the resistance is not negligible, the ammeter in the circuit would change the circuit's equivalent resistance and modify the current being measured.

Any galvanometer can be used to measure current by adding a shunt resistance in parallel with it.

The parallel connected shunt resistor divides the current and hence allows the galvanometer to measure DC currents several times greater than its full-scale deflection current.

The shunt resistance needed is determined using the known values of the galvanometer's full-scale deflection current and coil resistance.

This is done by equating the voltage drops across the galvanometer and the shunt resistance.

Here, I_a is the total current passing through the parallel combination, and I_a minus I_fs is the current passing through the shunt resistor.