An ammeter is an instrument used to measure electric current in an electric circuit.
A ammeter is a modified form of a galvanometer for measuring current in the circuit we require a device of a very low or practically zero resistance.
Conversion of Galvanometer into Ammeter :
"A Galvanometer can be converted into an ammeter by connecting a low resistance ( called shunt) parallel to the galvanometer."
A small resistance S (shunt resistance) connected in parallel to galvanometer (G) is shown in figure.
Let G and S be the resistance of a galvanometer and shunt respectively. Let I be the total current to be measured by an ammeter in the circuit.
Let $I_{g}$ be the current flowing through the galvanometer. The remaining current $(I-I_{g})$ flows through the shunt resistance.
Since G and S are parallel, the potential difference across them is same.
$$(I-I_{g}) S=I_{g} G$$
$$S = \left (\frac{I_{g} }{I-I_{g} } \right )G$$
This is the required value of shunt resistance to convert a galvanometer into an ammeter of range 0-I ampere.
Effective resistance of ammeter :
The total effective resistance $R_{eff}$ of an ammeter is given by
$$\frac{1}{R_{eff}} = \frac{1}{G} + \frac{1}{S}$$
$$\frac{1}{R_{eff}} = \frac{G+S}{GS}$$
$$R_{eff} = \frac{GS}{G+S}$$
$$Since \ G>>S, $$
$$so \ (G+S) \simeq G$$
Hence,
$$R_{eff} = \frac{GS}{G}$$
$$R_{eff} = S$$
Thus, an ammeter is a low resistance device. Resistance of an ideal ammeter is Zero.
Note : an ammeter is always connected in series in the circuit in which current is to be measured.