Ammeter is an instrument which can measure a particular quantity. As we know, the unit of current is Ampere. Ammeter means Ampere-meter which measures ampere value. Ampere is the unit of current so an ammeter is a meter or an instrument which measures current.
Working Principle of Ammeter
The main principle of ammeter is that it must have a very low resistance and also inductive reactance. Now, why do we need this? Can’t we connect an ammeter in parallel? The answer to this question is that it has very low impedance because it must have very low amount of voltage drop across it and must be connected in series connection because current is same in the series circuit.
Also due to very low impedance the power loss will be low and if it is connect in parallel it becomes almost a short circuit path and all the current will flow through ampere meter as a result of high current the instrument may burn.
So due to this reason it must be connect in series. For an ideal ammeter, it must have zero impedance so that it has zero voltage drop across it so the power loss in the instrument is zero. But the ideal is not achievable practically.
The majority of ammeters are either connected in series with the circuit carrying the current to be measured (for small fractional amperes), or have their shunt resistors connected similarly in series. In either case, the current passes through the meter or (mostly) through its shunt. Ammeters must not be connected directly across a voltage source since their internal resistance is very low and excess current would flow.
Ammeters are design for a low voltage drop across their terminals, much less than one volt; the extra circuit losses produce by the ammeter are call its “burden” on the measure circuit.